Colloid-probe AFM studies of the interaction forces of proteins adsorbed on colloidal crystals.

PubMed

Singh, Gurvinder; Bremmell, Kristen E; Griesser, Hans J; Kingshott, Peter

2015-04-28

In recent years, colloid-probe AFM has been used to measure the direct interaction forces between colloidal particles of different size or surface functionality in aqueous media, as one can study different forces in symmerical systems (i.e., sphere-sphere geometry). The present study investigates the interaction between protein coatings on colloid probes and hydrophilic surfaces decorated with hexagonally close packed single particle layers that are either uncoated or coated with proteins. Controlled solvent evaporation from aqueous suspensions of colloidal particles (coated with or without lysozyme and albumin) produces single layers of close-packed colloidal crystals over large areas on a solid support. The measurements have been carried out in an aqueous medium at different salt concentrations and pH values. The results show changes in the interaction forces as the surface charge of the unmodified or modified particles, and ionic strength or pH of the solution is altered. At high ionic strength or pH, electrostatic interactions are screened, and a strong repulsive force at short separation below 5 nm dominates, suggesting structural changes in the absorbed protein layer on the particles. We also study the force of adhesion, which decreases with an increment in the salt concentration, and the interaction between two different proteins indicating a repulsive interaction on approach and adhesion on retraction.

Effective interactions between inclusions in an active bath

NASA Astrophysics Data System (ADS)

Zaeifi Yamchi, Mahdi; Naji, Ali

2017-11-01

We study effective two- and three-body interactions between non-active colloidal inclusions in an active bath of chiral or non-chiral particles, using Brownian dynamics simulations within a standard, two-dimensional model of disk-shaped inclusions and active particles. In a non-chiral active bath, we first corroborate previous findings on effective two-body repulsion mediated between the inclusions by elucidating the detailed non-monotonic features of the two-body force profiles, including a primary maximum and a secondary hump at larger separations that was not previously reported. We then show that these features arise directly from the formation, and sequential overlaps, of circular layers (or "rings") of active particles around the inclusions, as the latter are brought to small surface separations. These rings extend to radial distances of a few active-particle radii from the surface of inclusions, giving the hard-core inclusions relatively thick, soft, repulsive "shoulders," whose multiple overlaps then enable significant (non-pairwise) three-body forces in both non-chiral and chiral active baths. The resulting three-body forces can even exceed the two-body forces in magnitude and display distinct repulsive and attractive regimes at intermediate to large self-propulsion strengths. In a chiral active bath, we show that, while active particles still tend to accumulate at the immediate vicinity of the inclusions, they exhibit strong depletion from the intervening region between the inclusions and partial depletion from relatively thick, circular zones further away from the inclusions. In this case, the effective, predominantly repulsive interactions between the inclusions turn to active, chirality-induced, depletion-type attractions, acting over an extended range of separations.

Experimental comparison of forces resisting viral DNA packaging and driving DNA ejection

PubMed Central

Keller, Nicholas; Berndsen, Zachary T.; Jardine, Paul J.; Smith, Douglas E.

2018-01-01

We compare forces resisting DNA packaging in bacteriophage phi29 inferred from optical tweezers studies with forces driving DNA ejection inferred from osmotic pressure studies. Ejection forces from 0–80% filling are consistent with a model that assumes a repulsive DNA-DNA interaction potential derived from DNA condensation studies and predicts an inverse spool DNA conformation. Forces resisting packaging from ~80–100% filling are also consistent with this model. However, that electron microscopy does not reveal a spool conformation suggests that this model overestimates bending rigidity and underestimates repulsion. Below 80% filling, inferred ejection forces are higher than those resisting packaging. Although unexpected, this suggests that most force that builds during packaging is available to drive DNA ejection. PMID:28618627

Forces between Two Glass Surfaces with Adsorbed Hexadecyltrimethylammonium Salicylate.

PubMed

Imae, T; Kato, M; Rutland, M

2000-02-22

Forces have been measured for hexadecyltrimethylammonium salicylate (C(16)TASal) layers on glass beads. During the inward process, hydrophobic attraction occurred at lower adsorption of C(16)TASal and electrostatic repulsion interactions happened at higher adsorption. While the jump-in phenomenon was observed for solutions of concentrations below the critical micelle concentration (cmc = 0.15 mM), the step-in phenomenon was characteristic for solutions at the cmc and above the cmc, suggesting the push-out of adsorbed C(16)TASal layers and/or inserted micelles. The remarkable pull-off phenomenon on the outward process occurred for all solutions, indicating a strong interaction between C(16)TASal molecules. For aqueous 0.15 mM C(16)TASal solutions of various NaSal concentrations, on the inward process, the electrostatic repulsive interaction decreased with adding NaSal. This is due to the electrostatic shielding by salt excess. The height of the force wall on the inward process reached a maximum at 0.01 M NaSal, but the interlocking between molecules on two surfaces during the outward process was minimized at 0.1 M NaSal. These tendencies, which are different from that of the electrostatic repulsion interaction, imply the strong cohesion between adsorbed C(16)TASal layers.

Surface Forces Apparatus Measurements of Interactions between Rough and Reactive Calcite Surfaces.

PubMed

Dziadkowiec, Joanna; Javadi, Shaghayegh; Bratvold, Jon E; Nilsen, Ola; Røyne, Anja

2018-06-26

nm-Range forces acting between calcite surfaces in water affect macroscopic properties of carbonate rocks and calcite-based granular materials and are significantly influenced by calcite surface recrystallization. We suggest that the repulsive mechanical effects related to nm-scale surface recrystallization of calcite in water could be partially responsible for the observed decrease of cohesion in calcitic rocks saturated with water. Using the surface forces apparatus, we simultaneously followed the calcite reactivity and measured the forces in water in two surface configurations: between two rough calcite surfaces (CC) and between rough calcite and a smooth mica surface (CM). We used nm-scale rough, polycrystalline calcite films prepared by atomic layer deposition. We measured only repulsive forces in CC in CaCO 3 -saturated water, which was related to roughness and possibly to repulsive hydration effects. Adhesive or repulsive forces were measured in CM in CaCO 3 -saturated water depending on calcite roughness, and the adhesion was likely enhanced by electrostatic effects. The pull-off adhesive force in CM became stronger with time, and this increase was correlated with a decrease of roughness at contacts, the parameter which could be estimated from the measured force-distance curves. That suggested a progressive increase of real contact areas between the surfaces, caused by gradual pressure-driven deformation of calcite surface asperities during repeated loading-unloading cycles. Reactivity of calcite was affected by mass transport across nm- to μm-thick gaps between the surfaces. Major roughening was observed only for the smoothest calcite films, where gaps between two opposing surfaces were nm-thick over μm-sized areas and led to force of crystallization that could overcome confining pressures of the order of MPa. Any substantial roughening of calcite caused a significant increase of the repulsive mechanical force contribution.

Polymers with nearest- and next nearest-neighbor interactions on the Husimi lattice

NASA Astrophysics Data System (ADS)

Oliveira, Tiago J.

2016-04-01

The exact grand-canonical solution of a generalized interacting self-avoid walk (ISAW) model, placed on a Husimi lattice built with squares, is presented. In this model, beyond the traditional interaction {ω }1={{{e}}}{ɛ 1/{k}BT} between (nonconsecutive) monomers on nearest-neighbor (NN) sites, an additional energy {ɛ }2 is associated to next-NN (NNN) monomers. Three definitions of NNN sites/interactions are considered, where each monomer can have, effectively, at most two, four, or six NNN monomers on the Husimi lattice. The phase diagrams found in all cases have (qualitatively) the same thermodynamic properties: a non-polymerized (NP) and a polymerized (P) phase separated by a critical and a coexistence surface that meet at a tricritical (θ-) line. This θ-line is found even when one of the interactions is repulsive, existing for {ω }1 in the range [0,∞ ), i.e., for {ɛ }1/{k}BT in the range [-∞ ,∞ ). Thus, counterintuitively, a θ-point exists even for an infinite repulsion between NN monomers ({ω }1=0), being associated to a coil-‘soft globule’ transition. In the limit of an infinite repulsive force between NNN monomers, however, the coil-globule transition disappears, and only NP-P continuous transition is observed. This particular case, with {ω }2=0, is also solved exactly on the square lattice, using a transfer matrix calculation where a discontinuous NP-P transition is found. For attractive and repulsive forces between NN and NNN monomers, respectively, the model becomes quite similar to the semiflexible-ISAW one, whose crystalline phase is not observed here, as a consequence of the frustration due to competing NN and NNN forces. The mapping of the phase diagrams in canonical ones is discussed and compared with recent results from Monte Carlo simulations on the square lattice.

Separating the effects of repulsive and attractive forces on the phase diagram, interfacial, and critical properties of simple fluids

NASA Astrophysics Data System (ADS)

Fuentes-Herrera, M.; Moreno-Razo, J. A.; Guzmán, O.; López-Lemus, J.; Ibarra-Tandi, B.

2016-06-01

Molecular simulations in the canonical and isothermal-isobaric ensembles were performed to study the effect of varying the shape of the intermolecular potential on the phase diagram, critical, and interfacial properties of model fluids. The molecular interactions were modeled by the Approximate Non-Conformal (ANC) theory potentials. Unlike the Lennard-Jones or Morse potentials, the ANC interactions incorporate parameters (called softnesses) that modulate the steepness of the potential in their repulsive and attractive parts independently. This feature allowed us to separate unambiguously the role of each region of the potential on setting the thermophysical properties. In particular, we found positive linear correlation between all critical coordinates and the attractive and repulsive softness, except for the critical density and the attractive softness which are negatively correlated. Moreover, we found that the physical properties related to phase coexistence (such as span of the liquid phase between the critical and triple points, variations in the P-T vaporization curve, interface width, and surface tension) are more sensitive to changes in the attractive softness than to the repulsive one. Understanding the different roles of attractive and repulsive forces on phase coexistence may contribute to developing more accurate models of liquids and their mixtures.

Separating the effects of repulsive and attractive forces on the phase diagram, interfacial, and critical properties of simple fluids.

PubMed

Fuentes-Herrera, M; Moreno-Razo, J A; Guzmán, O; López-Lemus, J; Ibarra-Tandi, B

2016-06-07

Molecular simulations in the canonical and isothermal-isobaric ensembles were performed to study the effect of varying the shape of the intermolecular potential on the phase diagram, critical, and interfacial properties of model fluids. The molecular interactions were modeled by the Approximate Non-Conformal (ANC) theory potentials. Unlike the Lennard-Jones or Morse potentials, the ANC interactions incorporate parameters (called softnesses) that modulate the steepness of the potential in their repulsive and attractive parts independently. This feature allowed us to separate unambiguously the role of each region of the potential on setting the thermophysical properties. In particular, we found positive linear correlation between all critical coordinates and the attractive and repulsive softness, except for the critical density and the attractive softness which are negatively correlated. Moreover, we found that the physical properties related to phase coexistence (such as span of the liquid phase between the critical and triple points, variations in the P-T vaporization curve, interface width, and surface tension) are more sensitive to changes in the attractive softness than to the repulsive one. Understanding the different roles of attractive and repulsive forces on phase coexistence may contribute to developing more accurate models of liquids and their mixtures.

Modelling and simulation of particle-particle interaction in a magnetophoretic bio-separation chip

NASA Astrophysics Data System (ADS)

Alam, Manjurul; Golozar, Matin; Darabi, Jeff

2018-04-01

A Lagrangian particle trajectory model is developed to predict the interaction between cell-bead particle complexes and to track their trajectories in a magnetophoretic bio-separation chip. Magnetic flux gradients are simulated in the OpenFOAM CFD software and imported into MATLAB to obtain the trapping lengths and trajectories of the particles. A connector vector is introduced to calculate the interaction force between cell-bead complexes as they flow through a microfluidic device. The interaction force calculations are performed for cases where the connector vector is parallel, perpendicular, and at an angle of 45° with the applied magnetic field. The trajectories of the particles are simulated by solving a system of eight ordinary differential equations using a fourth order Runge-Kutta method. The model is then used to study the effects of geometric positions and angles of the connector vector between the particles as well as the cell size, number of beads per cell, and flow rate on the interaction force and trajectories of the particles. The results show that the interaction forces may be attractive or repulsive, depending on the orientation of the connector vector distance between the particle complexes and the applied magnetic field. When the interaction force is attractive, the particles are observed to merge and trap sooner than a single particle, whereas a repulsive interaction force has little or no effect on the trapping length.

Toward the description of electrostatic interactions between globular proteins: potential of mean force in the primitive model.

PubMed

Dahirel, Vincent; Jardat, Marie; Dufrêche, Jean-François; Turq, Pierre

2007-09-07

Monte Carlo simulations are used to calculate the exact potential of mean force between charged globular proteins in aqueous solution. The aim of the present paper is to study the influence of the ions of the added salt on the effective interaction between these nanoparticles. The charges of the model proteins, either identical or opposite, are either central or distributed on a discrete pattern. Contrarily to Poisson-Boltzmann predictions, attractive, and repulsive direct forces between proteins are not screened similarly. Moreover, it has been shown that the relative orientations of the charge patterns strongly influence salt-mediated interactions. More precisely, for short distances between the proteins, ions enhance the difference of the effective forces between (i) like-charged and oppositely charged proteins, (ii) attractive and repulsive relative orientations of the proteins, which may affect the selectivity of protein/protein recognition. Finally, such results observed with the simplest models are applied to a more elaborate one to demonstrate their generality.

Effect of long-range repulsive Coulomb interactions on packing structure of adhesive particles.

PubMed

Chen, Sheng; Li, Shuiqing; Liu, Wenwei; Makse, Hernán A

2016-02-14

The packing of charged micron-sized particles is investigated using discrete element simulations based on adhesive contact dynamic model. The formation process and the final obtained structures of ballistic packings are studied to show the effect of interparticle Coulomb force. It is found that increasing the charge on particles causes a remarkable decrease of the packing volume fraction ϕ and the average coordination number 〈Z〉, indicating a looser and chainlike structure. Force-scaling analysis shows that the long-range Coulomb interaction changes packing structures through its influence on particle inertia before they are bonded into the force networks. Once contact networks are formed, the expansion effect caused by repulsive Coulomb forces are dominated by short-range adhesion. Based on abundant results from simulations, a dimensionless adhesion parameter Ad*, which combines the effects of the particle inertia, the short-range adhesion and the long-range Coulomb interaction, is proposed and successfully scales the packing results for micron-sized particles within the latest derived adhesive loose packing (ALP) regime. The structural properties of our packings follow well the recent theoretical prediction which is described by an ensemble approach based on a coarse-grained volume function, indicating some kind of universality in the low packing density regime of the phase diagram regardless of adhesion or particle charge. Based on the comprehensive consideration of the complicated inter-particle interactions, our findings provide insight into the roles of short-range adhesion and repulsive Coulomb force during packing formation and should be useful for further design of packings.

Steric and electrostatic surface forces on sulfonated PEG graft surfaces with selective albumin adsorption.

PubMed

Bremmell, Kristen E; Britcher, Leanne; Griesser, Hans J

2013-06-01

Addition of ionized terminal groups to PEG graft layers may cause additional interfacial forces to modulate the net interfacial interactions between PEG graft layers and proteins. In this study we investigated the effect of terminal sulfonate groups, characterizing PEG-aldehyde (PEG-CHO) and sulfonated PEG (PEG-SO3) graft layers by XPS and colloid probe AFM interaction force measurements as a function of ionic strength, in order to determine surface forces relevant to protein resistance and models of bio-interfacial interaction of such graft coatings. On the PEG-CHO surface the measured interaction force does not alter with ionic strength, typical of a repulsive steric barrier coating. An analogous repulsive interaction force of steric origin was also observed on the PEG-SO3 graft coating; however, the net interaction force changed with ionic strength. Interaction forces were modelled by steric and electrical double layer interaction theories, with fitting to a scaling theory model enabling determination of the spacing and stretching of the grafted chains. Albumin, fibrinogen, and lysozyme did not adsorb on the PEG-CHO coating, whereas the PEG graft with terminal sulfonate groups showed substantial adsorption of albumin but not fibrinogen or lysozyme from 0.15 M salt solutions. Under lower ionic strength conditions albumin adsorption was again minimized as a result of the increased electrical double-layer interaction observed with the PEG-SO3 modified surface. This unique and unexpected adsorption behaviour of albumin provides an alternative explanation to the "negative cilia" model used by others to rationalize observed thromboresistance on PEG-sulfonate coatings. Copyright © 2013 Elsevier B.V. All rights reserved.

Dark matter repulsion could thwart direct detection

DOE PAGES

Davoudiasl, Hooman

2017-11-20

We consider a feeble repulsive interaction between ordinary matter and dark matter, with a range similar to or larger than the size of the Earth. Dark matter can thus be repelled from the Earth, leading to null results in direct detection experiments, regardless of the strength of the short-distance interactions of dark matter with atoms. Generically, such a repulsive force would not allow trapping of dark matter inside astronomical bodies. In this scenario, accelerator-based experiments may furnish the only robust signals of asymmetric dark matter models, which typically lack indirect signals from self-annihilation. Finally, some of the variants of ourmore » hypothesis are also briefly discussed.« less

Dark matter repulsion could thwart direct detection

DOE Office of Scientific and Technical Information (OSTI.GOV)

Davoudiasl, Hooman

We consider a feeble repulsive interaction between ordinary matter and dark matter, with a range similar to or larger than the size of the Earth. Dark matter can thus be repelled from the Earth, leading to null results in direct detection experiments, regardless of the strength of the short-distance interactions of dark matter with atoms. Generically, such a repulsive force would not allow trapping of dark matter inside astronomical bodies. In this scenario, accelerator-based experiments may furnish the only robust signals of asymmetric dark matter models, which typically lack indirect signals from self-annihilation. Finally, some of the variants of ourmore » hypothesis are also briefly discussed.« less

Repulsion of polarized particles from two-dimensional materials

NASA Astrophysics Data System (ADS)

Rodríguez-Fortuño, Francisco J.; Picardi, Michela F.; Zayats, Anatoly V.

2018-05-01

Repulsion of nanoparticles, molecules, and atoms from surfaces can have important applications in nanomechanical devices, microfluidics, optical manipulation, and atom optics. Here, through the solution of a classical scattering problem, we show that a dipole source oscillating at a frequency ω can experience a robust and strong repulsive force when its near-field interacts with a two-dimensional material. As an example, the case of graphene is considered, showing that a broad bandwidth of repulsion can be obtained at frequencies for which propagation of plasmon modes is allowed 0 <ℏ ω <(5 /3 ) μc , where μc is the chemical potential tunable electrically or by chemical doping.

Grain-grain interaction in stationary dusty plasma

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lampe, Martin; Joyce, Glenn

We present a particle-in-cell simulation study of the steady-state interaction between two stationary dust grains in uniform stationary plasma. Both the electrostatic force and the shadowing force on the grains are calculated explicitly. The electrostatic force is always repulsive. For two grains of the same size, the electrostatic force is very nearly equal to the shielded electric field due to a single isolated grain, acting on the charge of the other grain. For two grains of unequal size, the electrostatic force on the smaller grain is smaller than the isolated-grain field, and the force on the larger grain is largermore » than the isolated-grain field. In all cases, the attractive shadowing force exceeds the repulsive electrostatic force when the grain separation d is greater than an equilibrium separation d{sub 0}. d{sub 0} is found to be between 6λ{sub D} and 9λ{sub D} in all cases. The binding energy is estimated to be between 19 eV and 900 eV for various cases.« less

Entropic Repulsion Between Fluctuating Surfaces

NASA Astrophysics Data System (ADS)

Janke, W.

The statistical mechanics of fluctuating surfaces plays an important role in a variety of physical systems, ranging from biological membranes to world sheets of strings in theories of fundamental interactions. In many applications it is a good approximation to assume that the surfaces possess no tension. Their statistical properties are then governed by curvature energies only, which allow for gigantic out-of-plane undulations. These fluctuations are the “entropic” origin of long-range repulsive forces in layered surface systems. Theoretical estimates of these forces for simple model surfaces are surveyed and compared with recent Monte Carlo simulations.

Effect of the Magnus force on skyrmion relaxation dynamics

NASA Astrophysics Data System (ADS)

Brown, Barton L.; Täuber, Uwe C.; Pleimling, Michel

2018-01-01

We perform systematic Langevin molecular dynamics simulations of interacting skyrmions in thin films. The interplay between the Magnus force, the repulsive skyrmion-skyrmion interaction, and the thermal noise yields different regimes during nonequilibrium relaxation. In the noise-dominated regime, the Magnus force enhances the disordering effects of the thermal noise. In the Magnus-force-dominated regime, the Magnus force cooperates with the skyrmion-skyrmion interaction to yield a dynamic regime with slow decaying correlations. These two regimes are characterized by different values of the aging exponent. In general, the Magnus force accelerates the approach to the steady state.

Visualizing the orientational dependence of an intermolecular potential

NASA Astrophysics Data System (ADS)

Sweetman, Adam; Rashid, Mohammad A.; Jarvis, Samuel P.; Dunn, Janette L.; Rahe, Philipp; Moriarty, Philip

2016-02-01

Scanning probe microscopy can now be used to map the properties of single molecules with intramolecular precision by functionalization of the apex of the scanning probe tip with a single atom or molecule. Here we report on the mapping of the three-dimensional potential between fullerene (C60) molecules in different relative orientations, with sub-Angstrom resolution, using dynamic force microscopy (DFM). We introduce a visualization method which is capable of directly imaging the variation in equilibrium binding energy of different molecular orientations. We model the interaction using both a simple approach based around analytical Lennard-Jones potentials, and with dispersion-force-corrected density functional theory (DFT), and show that the positional variation in the binding energy between the molecules is dominated by the onset of repulsive interactions. Our modelling suggests that variations in the dispersion interaction are masked by repulsive interactions even at displacements significantly larger than the equilibrium intermolecular separation.

van der Waals torque and force between anisotropic topological insulator slabs

NASA Astrophysics Data System (ADS)

Lu, Bing-Sui

2018-01-01

We investigate the character of the van der Waals (vdW) torque and force between two coplanar and dielectrically anisotropic topological insulator (TI) slabs separated by a vacuum gap in the nonretardation regime, where the optic axes of the slabs are each perpendicular to the normal direction to the slab-gap interface and also generally differently oriented from each other. We find that in addition to the magnetoelectric coupling strength, the anisotropy can also influence the sign of the vdW force, viz., a repulsive vdW force can become attractive if the anisotropy is increased sufficiently. In addition, the vdW force oscillates as a function of the angular difference between the optic axes of the TI slabs, being most repulsive/least attractive (least repulsive/most attractive) for angular differences that are integer (half-integer) multiples of π . Our third finding is that the vdW torque for TI slabs is generally weaker than that for ordinary dielectric slabs. Our work provides an instance in which the vector potential appears in a calculation of the vdW interaction for which the limit is nonretarded or static.

Crossing Over from Attractive to Repulsive Interactions in a Tunneling Bosonic Josephson Junction.

PubMed

Spagnolli, G; Semeghini, G; Masi, L; Ferioli, G; Trenkwalder, A; Coop, S; Landini, M; Pezzè, L; Modugno, G; Inguscio, M; Smerzi, A; Fattori, M

2017-06-09

We explore the interplay between tunneling and interatomic interactions in the dynamics of a bosonic Josephson junction. We tune the scattering length of an atomic ^{39}K Bose-Einstein condensate confined in a double-well trap to investigate regimes inaccessible to other superconducting or superfluid systems. In the limit of small-amplitude oscillations, we study the transition from Rabi to plasma oscillations by crossing over from attractive to repulsive interatomic interactions. We observe a critical slowing down in the oscillation frequency by increasing the strength of an attractive interaction up to the point of a quantum phase transition. With sufficiently large initial oscillation amplitude and repulsive interactions, the system enters the macroscopic quantum self-trapping regime, where we observe coherent undamped oscillations with a self-sustained average imbalance of the relative well population. The exquisite agreement between theory and experiments enables the observation of a broad range of many body coherent dynamical regimes driven by tunable tunneling energy, interactions and external forces, with applications spanning from atomtronics to quantum metrology.

Inferring coarse-grain histone-DNA interaction potentials from high-resolution structures of the nucleosome

NASA Astrophysics Data System (ADS)

Meyer, Sam; Everaers, Ralf

2015-02-01

The histone-DNA interaction in the nucleosome is a fundamental mechanism of genomic compaction and regulation, which remains largely unknown despite increasing structural knowledge of the complex. In this paper, we propose a framework for the extraction of a nanoscale histone-DNA force-field from a collection of high-resolution structures, which may be adapted to a larger class of protein-DNA complexes. We applied the procedure to a large crystallographic database extended by snapshots from molecular dynamics simulations. The comparison of the structural models first shows that, at histone-DNA contact sites, the DNA base-pairs are shifted outwards locally, consistent with locally repulsive forces exerted by the histones. The second step shows that the various force profiles of the structures under analysis derive locally from a unique, sequence-independent, quadratic repulsive force-field, while the sequence preferences are entirely due to internal DNA mechanics. We have thus obtained the first knowledge-derived nanoscale interaction potential for histone-DNA in the nucleosome. The conformations obtained by relaxation of nucleosomal DNA with high-affinity sequences in this potential accurately reproduce the experimental values of binding preferences. Finally we address the more generic binding mechanisms relevant to the 80% genomic sequences incorporated in nucleosomes, by computing the conformation of nucleosomal DNA with sequence-averaged properties. This conformation differs from those found in crystals, and the analysis suggests that repulsive histone forces are related to local stretch tension in nucleosomal DNA, mostly between adjacent contact points. This tension could play a role in the stability of the complex.

A molecular dynamics study on the role of attractive and repulsive forces in internal energy, internal pressure and structure of dense fluids

NASA Astrophysics Data System (ADS)

Goharshadi, Elaheh K.; Morsali, Ali; Mansoori, G. Ali

2007-01-01

Isotherms of experimental data of internal pressure of dense fluids versus molar volume, Vm are shown to have each a maximum point at a Vmax below the critical molar volume. In this study, we investigated the role of attractive and repulsive intermolecular energies on this behavior using a molecular dynamics simulation technique. In the simulation, we choose the Lennard-Jones (LJ) intermolecular potential energy function. The LJ potential is known to be an effective potential representing a statistical average of the true pair and many-body interactions in simple molecular systems. The LJ potential function is divided into attractive and repulsive parts. MD calculations have produced internal energy, potential energy, transitional kinetic energy, and radial distribution function (RDF) for argon at 180 K and 450 K using LJ potential, LJ repulsive, and LJ attractive parts. It is shown that the LJ potential function is well capable of predicting the inflection point in the internal energy-molar volume curve as well as maximum point in the internal pressure-molar volume curve. It is also shown that at molar volumes higher than Vmax, the attractive forces have strong influence on determination of internal energy and internal pressure. At volumes lower than Vmax, neither repulsive nor attractive forces are dominating. Also, the coincidence between RDFs resulting from LJ potential and repulsive parts of LJ potential improves as molar volume approaches Vmax from high molar volumes. The coincidence becomes complete at Vmax ⩾ V.

Density-Dependent Formulation of Dispersion-Repulsion Interactions in Hybrid Multiscale Quantum/Molecular Mechanics (QM/MM) Models.

PubMed

Curutchet, Carles; Cupellini, Lorenzo; Kongsted, Jacob; Corni, Stefano; Frediani, Luca; Steindal, Arnfinn Hykkerud; Guido, Ciro A; Scalmani, Giovanni; Mennucci, Benedetta

2018-03-13

Mixed multiscale quantum/molecular mechanics (QM/MM) models are widely used to explore the structure, reactivity, and electronic properties of complex chemical systems. Whereas such models typically include electrostatics and potentially polarization in so-called electrostatic and polarizable embedding approaches, respectively, nonelectrostatic dispersion and repulsion interactions are instead commonly described through classical potentials despite their quantum mechanical origin. Here we present an extension of the Tkatchenko-Scheffler semiempirical van der Waals (vdW TS ) scheme aimed at describing dispersion and repulsion interactions between quantum and classical regions within a QM/MM polarizable embedding framework. Starting from the vdW TS expression, we define a dispersion and a repulsion term, both of them density-dependent and consistently based on a Lennard-Jones-like potential. We explore transferable atom type-based parametrization strategies for the MM parameters, based on either vdW TS calculations performed on isolated fragments or on a direct estimation of the parameters from atomic polarizabilities taken from a polarizable force field. We investigate the performance of the implementation by computing self-consistent interaction energies for the S22 benchmark set, designed to represent typical noncovalent interactions in biological systems, in both equilibrium and out-of-equilibrium geometries. Overall, our results suggest that the present implementation is a promising strategy to include dispersion and repulsion in multiscale QM/MM models incorporating their explicit dependence on the electronic density.

Interaction forces between DPPC bilayers on glass

PubMed Central

Orozco-Alcaraz, Raquel; Kuhl, Tonya L.

2013-01-01

The Surface Force Apparatus (SFA) was utilized to obtain force-distance profiles between silica supported membranes formed by Langmuir-Blodgett deposition of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). In the absence of a membrane, a long range electrostatic and short range steric repulsion is measured due to deprotonation of silica in water and roughness of the silica film. The electrostatic repulsion is partially screened by the lipid membrane and a van der Waals adhesion comparable to that measured with well packed DPPC membranes on mica is measured. This finding suggest that electrostatic interactions due to the underlying negatively charged silica are likely present in other systems of glass supported membranes. In contrast, the charge of an underlying mica substrate is almost completely screened when a lipid membrane is deposited on the mica. The difference in the two systems is attributed to stronger physisorption of zwitterionic lipids to molecularly smooth mica compared to rougher silica. PMID:23199333

FORCES DICTATING COLLOIDAL INTERACTIONS BETWEEN VIRUSES AND SOIL

EPA Science Inventory

The fate and transport of viruses in soil and aquatic environments were studied with respect to the different forces involved in the process of sorption of these viruses on soil particles. In accordance with the classical DLVO theory, we have calculated the repulsive electrostat...

Soft repulsive mixtures under gravity: Brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium shaking

NASA Astrophysics Data System (ADS)

Kruppa, Tobias; Neuhaus, Tim; Messina, René; Löwen, Hartmut

2012-04-01

A binary mixture of particles interacting via long-ranged repulsive forces is studied in gravity by computer simulation and theory. The more repulsive A-particles create a depletion zone of less repulsive B-particles around them reminiscent to a bubble. Applying Archimedes' principle effectively to this bubble, an A-particle can be lifted in a fluid background of B-particles. This "depletion bubble" mechanism explains and predicts a brazil-nut effect where the heavier A-particles float on top of the lighter B-particles. It also implies an effective attraction of an A-particle towards a hard container bottom wall which leads to boundary layering of A-particles. Additionally, we have studied a periodic inversion of gravity causing perpetuous mutual penetration of the mixture in a slit geometry. In this nonequilibrium case of time-dependent gravity, the boundary layering persists. Our results are based on computer simulations and density functional theory of a two-dimensional binary mixture of colloidal repulsive dipoles. The predicted effects also occur for other long-ranged repulsive interactions and in three spatial dimensions. They are therefore verifiable in settling experiments on dipolar or charged colloidal mixtures as well as in charged granulates and dusty plasmas.

Soft repulsive mixtures under gravity: brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium shaking.

PubMed

Kruppa, Tobias; Neuhaus, Tim; Messina, René; Löwen, Hartmut

2012-04-07

A binary mixture of particles interacting via long-ranged repulsive forces is studied in gravity by computer simulation and theory. The more repulsive A-particles create a depletion zone of less repulsive B-particles around them reminiscent to a bubble. Applying Archimedes' principle effectively to this bubble, an A-particle can be lifted in a fluid background of B-particles. This "depletion bubble" mechanism explains and predicts a brazil-nut effect where the heavier A-particles float on top of the lighter B-particles. It also implies an effective attraction of an A-particle towards a hard container bottom wall which leads to boundary layering of A-particles. Additionally, we have studied a periodic inversion of gravity causing perpetuous mutual penetration of the mixture in a slit geometry. In this nonequilibrium case of time-dependent gravity, the boundary layering persists. Our results are based on computer simulations and density functional theory of a two-dimensional binary mixture of colloidal repulsive dipoles. The predicted effects also occur for other long-ranged repulsive interactions and in three spatial dimensions. They are therefore verifiable in settling experiments on dipolar or charged colloidal mixtures as well as in charged granulates and dusty plasmas.

Self-bound droplets of a dilute magnetic quantum liquid

NASA Astrophysics Data System (ADS)

Schmitt, Matthias; Wenzel, Matthias; Böttcher, Fabian; Ferrier-Barbut, Igor; Pfau, Tilman

2016-11-01

Self-bound many-body systems are formed through a balance of attractive and repulsive forces and occur in many physical scenarios. Liquid droplets are an example of a self-bound system, formed by a balance of the mutual attractive and repulsive forces that derive from different components of the inter-particle potential. It has been suggested that self-bound ensembles of ultracold atoms should exist for atom number densities that are 108 times lower than in a helium droplet, which is formed from a dense quantum liquid. However, such ensembles have been elusive up to now because they require forces other than the usual zero-range contact interaction, which is either attractive or repulsive but never both. On the basis of the recent finding that an unstable bosonic dipolar gas can be stabilized by a repulsive many-body term, it was predicted that three-dimensional self-bound quantum droplets of magnetic atoms should exist. Here we report the observation of such droplets in a trap-free levitation field. We find that this dilute magnetic quantum liquid requires a minimum, critical number of atoms, below which the liquid evaporates into an expanding gas as a result of the quantum pressure of the individual constituents. Consequently, around this critical atom number we observe an interaction-driven phase transition between a gas and a self-bound liquid in the quantum degenerate regime with ultracold atoms. These droplets are the dilute counterpart of strongly correlated self-bound systems such as atomic nuclei and helium droplets.

Self-bound droplets of a dilute magnetic quantum liquid.

PubMed

Schmitt, Matthias; Wenzel, Matthias; Böttcher, Fabian; Ferrier-Barbut, Igor; Pfau, Tilman

2016-11-10

Self-bound many-body systems are formed through a balance of attractive and repulsive forces and occur in many physical scenarios. Liquid droplets are an example of a self-bound system, formed by a balance of the mutual attractive and repulsive forces that derive from different components of the inter-particle potential. It has been suggested that self-bound ensembles of ultracold atoms should exist for atom number densities that are 10 8 times lower than in a helium droplet, which is formed from a dense quantum liquid. However, such ensembles have been elusive up to now because they require forces other than the usual zero-range contact interaction, which is either attractive or repulsive but never both. On the basis of the recent finding that an unstable bosonic dipolar gas can be stabilized by a repulsive many-body term, it was predicted that three-dimensional self-bound quantum droplets of magnetic atoms should exist. Here we report the observation of such droplets in a trap-free levitation field. We find that this dilute magnetic quantum liquid requires a minimum, critical number of atoms, below which the liquid evaporates into an expanding gas as a result of the quantum pressure of the individual constituents. Consequently, around this critical atom number we observe an interaction-driven phase transition between a gas and a self-bound liquid in the quantum degenerate regime with ultracold atoms. These droplets are the dilute counterpart of strongly correlated self-bound systems such as atomic nuclei and helium droplets.

Refined potentials for rare gas atom adsorption on rare gas and alkali-halide surfaces

NASA Technical Reports Server (NTRS)

Wilson, J. W.; Heinbockel, J. H.; Outlaw, R. A.

1985-01-01

The utilization of models of interatomic potential for physical interaction to estimate the long range attractive potential for rare gases and ions is discussed. The long range attractive force is calculated in terms of the atomic dispersion properties. A data base of atomic dispersion parameters for rare gas atoms, alkali ion, and halogen ions is applied to the study of the repulsive core; the procedure for evaluating the repulsive core of ion interactions is described. The interaction of rare gas atoms on ideal rare gas solid and alkali-halide surfaces is analyzed; zero coverage absorption potentials are derived.

Three-body effects in Casimir-Polder repulsion

NASA Astrophysics Data System (ADS)

Milton, Kimball A.; Abalo, E. K.; Parashar, Prachi; Pourtolami, Nima; Brevik, Iver; Ellingsen, Simen Å.; Buhmann, Stefan Yoshi; Scheel, Stefan

2015-04-01

In this paper we study an archetypical scenario in which repulsive Casimir-Polder forces between an atom or molecule and two macroscopic bodies can be achieved. This is an extension of previous studies of the interaction between a polarizable atom and a wedge, in which repulsion occurs if the atom is sufficiently anisotropic and close enough to the symmetry plane of the wedge. A similar repulsion occurs if such an atom passes a thin cylinder or a wire. An obvious extension is to compute the interaction between such an atom and two facing wedges, which includes as a special case the interaction of an atom with a conducting screen possessing a slit, or between two parallel wires. To this end we further extend the electromagnetic multiple-scattering formalism for three-body interactions. To test this machinery we reinvestigate the interaction of a polarizable atom between two parallel conducting plates. In that case, three-body effects are shown to be small and are dominated by three- and four-scattering terms. The atom-wedge calculation is illustrated by an analogous scalar situation, described in the Appendix. The wedge-wedge-atom geometry is difficult to analyze because this is a scale-free problem. However, it is not so hard to investigate the three-body corrections to the interaction between an anisotropic atom or nanoparticle and a pair of parallel conducting cylinders and show that the three-body effects are very small and do not affect the Casimir-Polder repulsion at large distances between the cylinders. Finally, we consider whether such highly anisotropic atoms needed for repulsion are practically realizable. Since this appears rather difficult to accomplish, it may be more feasible to observe such effects with highly anisotropic nanoparticles.

Potential of mean force between like-charged nanoparticles: Many-body effect

NASA Astrophysics Data System (ADS)

Zhang, Xi; Zhang, Jin-Si; Shi, Ya-Zhou; Zhu, Xiao-Long; Tan, Zhi-Jie

2016-03-01

Ion-mediated interaction is important for the properties of polyelectrolytes such as colloids and nucleic acids. The effective pair interactions between two polyelectrolytes have been investigated extensively, but the many-body effect for multiple polyelectrolytes still remains elusive. In this work, the many-body effect in potential of mean force (PMF) between like-charged nanoparticles in various salt solutions has been comprehensively examined by Monte Carlo simulation and the nonlinear Poisson-Boltzmann theory. Our calculations show that, at high 1:1 salt, the PMF is weakly repulsive and appears additive, while at low 1:1 salt, the additive assumption overestimates the repulsive many-body PMF. At low 2:2 salt, the pair PMF appears weakly repulsive while the many-body PMF can become attractive. In contrast, at high 2:2 salt, the pair PMF is apparently attractive while the many-body effect can cause a weaker attractive PMF than that from the additive assumption. Our microscopic analyses suggest that the elusive many-body effect is attributed to ion-binding which is sensitive to ion concentration, ion valence, number of nanoparticles and charges on nanoparticles.

Loss of electrostatic cell-surface repulsion mediates myelin membrane adhesion and compaction in the central nervous system.

PubMed

Bakhti, Mostafa; Snaidero, Nicolas; Schneider, David; Aggarwal, Shweta; Möbius, Wiebke; Janshoff, Andreas; Eckhardt, Matthias; Nave, Klaus-Armin; Simons, Mikael

2013-02-19

During the development of the central nervous system (CNS), oligodendrocytes wrap their plasma membrane around axons to form a multilayered stack of tightly attached membranes. Although intracellular myelin compaction and the role of myelin basic protein has been investigated, the forces that mediate the close interaction of myelin membranes at their external surfaces are poorly understood. Such extensive bilayer-bilayer interactions are usually prevented by repulsive forces generated by the glycocalyx, a dense and confluent layer of large and negatively charged oligosaccharides. Here we investigate the molecular mechanisms underlying myelin adhesion and compaction in the CNS. We revisit the role of the proteolipid protein and analyze the contribution of oligosaccharides using cellular assays, biophysical tools, and transgenic mice. We observe that differentiation of oligodendrocytes is accompanied by a striking down-regulation of components of their glycocalyx. Both in vitro and in vivo experiments indicate that the adhesive properties of the proteolipid protein, along with the reduction of sialic acid residues from the cell surface, orchestrate myelin membrane adhesion and compaction in the CNS. We suggest that loss of electrostatic cell-surface repulsion uncovers weak and unspecific attractive forces in the bilayer that bring the extracellular surfaces of a membrane into close contact over long distances.

Path planning for mobile robot using the novel repulsive force algorithm

NASA Astrophysics Data System (ADS)

Sun, Siyue; Yin, Guoqiang; Li, Xueping

2018-01-01

A new type of repulsive force algorithm is proposed to solve the problem of local minimum and the target unreachable of the classic Artificial Potential Field (APF) method in this paper. The Gaussian function that is related to the distance between the robot and the target is added to the traditional repulsive force, solving the problem of the goal unreachable with the obstacle nearby; variable coefficient is added to the repulsive force component to resize the repulsive force, which can solve the local minimum problem when the robot, the obstacle and the target point are in the same line. The effectiveness of the algorithm is verified by simulation based on MATLAB and actual mobile robot platform.

Analytical theory for the dark-soliton interaction in nonlocal nonlinear materials with an arbitrary degree of nonlocality

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kong Qian; Department of Physics, Shanghai University, Shanghai 200444; Wang, Q.

2010-07-15

We investigate theoretically the interaction of dark solitons in materials with a spatially nonlocal nonlinearity. In particular we do this analytically and for arbitrary degree of nonlocality. We employ the variational technique to show that nonlocality induces an attractive force in the otherwise repulsive soliton interaction.

Lift on side by side intruders of various geometries within a granular flow

NASA Astrophysics Data System (ADS)

Acevedo-Escalante, M. F.; Caballero-Robledo, G. A.

2017-06-01

Obstacles within fluids have been widely used in engineering and in physics to study hydrodynamic interactions. In granular matter, objects within a granular flow have helped to understand fundamental features of drag and lift forces. In our group, we have studied numerically the flow mediated interaction between two static disks within a vertical granular flow in a two-dimensional container where the flow velocity and the distance between obstacles were varied. Attractive and repulsive forces were found depending on flow velocity and separation between intruders. The simulations evidenced a relationship between the average flow velocity in a specific section ahead of the obstacles and the attractive-repulsive lift. On the other hand, it was showed that the lift force on an object dragged within a granular medium depends on the shape of the intruder. Here we present experimental results of the interaction between two side-by-side intruders of different shapes within a vertical granular flow. We built a quasi-two-dimensional container in which we placed the intruders and using load cells we measured lift and drag forces during the discharge process for different flow velocities.

Impact of self-assembled surfactant structures on rheology of concentrated nanoparticle dispersions.

PubMed

Zaman, A A; Singh, P; Moudgil, B M

2002-07-15

Rheological behavior of surfactant-stabilized colloidal dispersions of silica particles under extreme conditions (low pH, high ionic strength) has been investigated in relation to interparticle forces and stability of the dispersion. The surfactant used as the dispersing agent was C(12)TAB, a cationic surfactant. Stability analysis through turbidity measurements indicated that there is a sharp increase in the stability of the dispersion when the surfactant concentration is in the range of 8 to 10 mM in the system. The state of the dispersion changes from an unstable regime to a stable regime above a critical concentration of C(12)TAB in the system. In the case of interaction forces measured between the silica substrate and AFM tip, no repulsive force was observed up to a surfactant concentration of 8 mM and a transition from no repulsive forces to steric repulsive forces occurred between 8 and 10 mM. Rheological measurements as a function of C(12)TAB concentration indicated a significant decrease in the viscosity and linear viscoelastic functions of the dispersion over the same range of surfactant concentration (8 to 10 mM C(12)TAB), showing a strong correlation between the viscosity behavior, interparticle forces, and structure development in the dispersion.

Dynamics of Two Interactive Bubbles in An Acoustic Field - Part II: Experiments

NASA Astrophysics Data System (ADS)

Ashgriz, Nasser; Barbat, Tiberiu; Liu, Ching-Shi

1996-11-01

The motion of two air bubbles levitated in water, in the presence of a high-frequency acoustic field is experimentally studied. The interaction force between them is named "secondary Bjerknes force" and may be significant in microgravity environments; in our experiments the buoyancy effect is compensated through the action of the "primary Bjerknes forces" - interaction between each bubble oscillation and external sound field. The stationary sound field is produced by a piezoceramic tranducer, in the range of 22-24 kHz. The experiments succesfully demonstrate the existence of three patterns of interaction between bubbles of various sizes: attraction, repulsion and oscillation. Bubbles attraction is quantitatively studied using a high speed video, for "large" bubbles (in the range 0.5-2 mm radius); bubbles repulsion and oscillations are only observed with a regular video, for "small" bubbles (around the resonance size at these frequencies, 0.12 mm). Velocities and accelerations of each bubble are computed from the time history of the motion. The theoretical equations of motion are completed with a drag force formula for single bubbles and solved numerically. Experimental results, for the case of two attracting bubbles, are in good agreement with the numerical model, especially for values of the mutual distance greater than 3 large bubble radii.

Exploiting Repulsive and Attractive Optical Forces in Advanced Nanophotonic Systems

DTIC Science & Technology

2015-10-26

in the same device. Such all-optical interaction is achieved without involving any optoelectronic interaction or nonlinear optical effect and thus...other cavity and tilt the see-saw, causing detuning of both cavities but in opposite directions. Furthermore, the see- saw oscillation can “shuttle

Beyond Born-Mayer: Improved models for short-range repulsion in ab initio force fields

DOE PAGES

Van Vleet, Mary J.; Misquitta, Alston J.; Stone, Anthony J.; ...

2016-06-23

Short-range repulsion within inter-molecular force fields is conventionally described by either Lennard-Jones or Born-Mayer forms. Despite their widespread use, these simple functional forms are often unable to describe the interaction energy accurately over a broad range of inter-molecular distances, thus creating challenges in the development of ab initio force fields and potentially leading to decreased accuracy and transferability. Herein, we derive a novel short-range functional form based on a simple Slater-like model of overlapping atomic densities and an iterated stockholder atom (ISA) partitioning of the molecular electron density. We demonstrate that this Slater-ISA methodology yields a more accurate, transferable, andmore » robust description of the short-range interactions at minimal additional computational cost compared to standard Lennard-Jones or Born-Mayer approaches. Lastly, we show how this methodology can be adapted to yield the standard Born-Mayer functional form while still retaining many of the advantages of the Slater-ISA approach.« less

From hydration repulsion to dry adhesion between asymmetric hydrophilic and hydrophobic surfaces

PubMed Central

Kanduč, Matej; Netz, Roland R.

2015-01-01

Using all-atom molecular dynamics (MD) simulations at constant water chemical potential in combination with basic theoretical arguments, we study hydration-induced interactions between two overall charge-neutral yet polar planar surfaces with different wetting properties. Whether the water film between the two surfaces becomes unstable below a threshold separation and cavitation gives rise to long-range attraction, depends on the sum of the two individual surface contact angles. Consequently, cavitation-induced attraction also occurs for a mildly hydrophilic surface interacting with a very hydrophobic surface. If both surfaces are very hydrophilic, hydration repulsion dominates at small separations and direct attractive force contribution can—if strong enough—give rise to wet adhesion in this case. In between the regimes of cavitation-induced attraction and hydration repulsion we find a narrow range of contact angle combinations where the surfaces adhere at contact in the absence of cavitation. This dry adhesion regime is driven by direct surface–surface interactions. We derive simple laws for the cavitation transition as well as for the transition between hydration repulsion and dry adhesion, which favorably compare with simulation results in a generic adhesion state diagram as a function of the two surface contact angles. PMID:26392526

Nanobubbles do not sit alone at the solid-liquid interface.

PubMed

Peng, Hong; Hampton, Marc A; Nguyen, Anh V

2013-05-21

The unexpected stability and anomalous contact angle of gaseous nanobubbles at the hydrophobic solid-liquid interface has been an issue of debate for almost two decades. In this work silicon-nitride tipped AFM cantilevers are used to probe the highly ordered pyrolytic graphite (HOPG)-water interface with and without solvent-exchange (a common nanobubble production method). Without solvent-exchange the force obtained by the single force and force mapping techniques is consistent over the HOPG atomic layers and described by DLVO theory (strong EDL repulsion). With solvent-exchange the force is non-DLVO (no EDL repulsion) and the range of the attractive jump-in (>10 nm) over the surface is grouped into circular areas of longer range, consistent with nanobubbles, and the area of shorter range. The non-DLVO nature of the area between nanobubbles suggests that the interaction is no longer between a silicon-nitride tip and HOPG. Interfacial gas enrichment (IGE) covering the entire area between nanobubbles is suggested to be responsible for the non-DLVO forces. The absence of EDL repulsion suggests that both IGE and nanobubbles are not charged. The coexistence of nanobubbles and IGE provides further evidence of nanobubble stability by dynamic equilibrium. The IGE cannot be removed by contact mode scanning of a cantilever tip in pure water, but in a surfactant (SDS) solution the mechanical action of the tip and the chemical action of the surfactant molecules can successfully remove the enrichment. Strong EDL repulsion between the tip and nanobubbles/IGE in surfactant solutions is due to the polar heads of the adsorbed surfactant molecules.

Effects of a hyperonic many-body force on BΛ values of hypernuclei

NASA Astrophysics Data System (ADS)

Isaka, M.; Yamamoto, Y.; Rijken, Th. A.

2017-04-01

The stiff equation of state (EoS) giving the neutron-star mass of 2 M⊙ suggests the existence of strongly repulsive many-body effects (MBE) not only in nucleon channels but also in hyperonic ones. As a specific model for MBE, the repulsive multi-Pomeron exchange potential (MPP) is added to the two-body interaction together with the phenomenological three-body attraction. For various versions of the Nijmegen interaction models, the MBE parts are determined so as to reproduce the observed data of BΛ. The mass dependence of BΛ values is shown to be reproduced well by adding MBE to the strong MPP repulsion, assuring the stiff EoS of hyperon-mixed neutron-star matter, in which P -state components of the adopted interaction model lead to almost vanishing contributions. The nuclear matter Λ N G -matrix interactions are derived and used in Λ hypernuclei on the basis of the averaged-density approximation (ADA). The BΛ values of hypernuclei with 9 ≤A ≤59 are analyzed in the framework of antisymmetrized molecular dynamics with use of the two types of Λ N G -matrix interactions including strong and weak MPP repulsions. The calculated values of BΛ reproduce the experimental data well within a few hundred keV. The values of BΛ in p states also can be reproduced well, when the ADA is modified to be suitable also for weakly bound Λ states.

Assessing the attractive/repulsive force balance in axial cyclohexane C-Hax ···Yax contacts: A combined computational analysis in monosubstituted cyclohexanes.

PubMed

Silva Lopez, Carlos; Nieto Faza, Olalla; De Proft, Frank; Kolocouris, Antonios

2016-11-15

The interactions of axial substituents in monosubstituted cyclohexane rings are studied in this work using an array of different computational techniques. Additionally, the anomalous axial preference for some bulky substituents is related to stabilizing dispersion interactions. We find that the C-H ax ···Y ax contacts for various substituents with distances ranging from 2 to ∼5 Å may include attractive dispersion forces that can affect the conformational equilibrium; these forces co-exist with Pauli repulsive forces effected by Y ax group due to van der Waals sphere penetration. At distances between 2 and 3 Å stabilizing electron transfer interactions were calculated and the combination of natural bond orbital and QTAIM analysis showed that, in certain cases, Y ax  =  t Bu, C ax -O or C ax  = O or S ax  = O or C ax  = S this interaction can be characterized as an improper H-bond. DFT-D3 and non-covalent interactions calculations (NCIs) in cyclohexane derivatives with Y ax  = SiOR 3 including H Yax ···H cy surfaces at distances ranging between 4 and 6 Å suggest that dispersion has a clear effect on the experimentally observed stabilization of the axial conformer. NCIs computed from the reduced density gradient help to visually identify and analyze these interactions. © 2016 Wiley Periodicals, Inc. © 2016 Wiley Periodicals, Inc.

Cheerios Effect Controlled by Electrowetting.

PubMed

Yuan, Junqi; Feng, Jian; Cho, Sung Kwon

2015-08-04

The Cheerios effect is a common phenomenon in which small floating objects are either attracted or repelled by the sidewall due to capillary interaction. This attractive or repulsive behavior is highly dependent on the slope angles (angles of the interface on the wall or floating object with respect to a horizontal line) that can be mainly controlled by the wettability of the wall and floating object and the density of the object. In this paper, electrowetting on dielectric (EWOD) is implemented to the wall or floating object in order to actively control the wettability and thus capillary interaction. As such, the capillary force on buoyant and dense floating objects can be easily switched between repulsion and attraction by simply applying an electrical input. In addition, the theoretical prediction for the capillary force is verified experimentally by measuring the motion of floating particle and the critical contact angle on the wall at which the capillary force changes from attraction to repulsion. This successive verification is enabled by the merit of EWOD that allows for continuous change in the contact angle. Finally, the control method is extended to continuously move a floating object along a linear path and to continuously rotate a dumbbell-like floating object in centimeter scales using arrays of EWOD electrodes. A continuous linear motion is also accomplished in a smaller scale where the channel width (3 mm) is comparable to the capillary length.

Domain wall structure and interactions in 50 nm wide Cobalt nanowires

NASA Astrophysics Data System (ADS)

Tu, Kun-Hua; Ojha, Shuchi; Ross, Caroline A.

2018-05-01

Arrays of cobalt nanowires with widths of 50 nm, thickness of 5 and 20 nm and periodicity of 70 nm were fabricated by pattern transfer from a self-assembled block copolymer film. Transverse domain walls (DWs) were imaged by magnetic force microscopy, indicating repulsive interactions between DWs of the same sign in the 20 nm thick wires. Micromagnetic simulations were used to identify the interactions in the six distinct cases of a pair of transverse DWs in adjacent wires, considering all the possible combinations of head-to-head and tail-to-tail DWs and the orientation of the core magnetization. The boundary between repulsive and attractive DW interactions is mapped out for wires as a function of thickness, width and interwire spacing.

Proton Distribution in Heavy Nuclei

DOE R&D Accomplishments Database

Johnson, M. H; Teller, E.

1953-11-13

It is reasoned that, from considerations connected with beta-decay stability and Coulomb repulsion forces, a neutron excess is developed on the surface of heavy nuclei. Several consequences of this qualitative analysis in nucleon interactions are briefly noted. (K.S.)

Electrokinetic mechanism of wettability alternation at oil-water-rock interface

NASA Astrophysics Data System (ADS)

Tian, Huanhuan; Wang, Moran

2017-12-01

Design of ions for injection water may change the wettability of oil-brine-rock (OBR) system, which has very important applications in enhanced oil recovery. Though ion-tuned wettability has been verified by various experiments, the mechanism is still not clear. In this review paper, we first present a comprehensive summarization of possible wettability alteration mechanisms, including fines migration or dissolution, multicomponent ion-exchange (MIE), electrical double layer (EDL) interaction between rock and oil, and repulsive hydration force. To clarify the key mechanism, we introduce a complete frame of theories to calculate attribution of EDL repulsion to wettability alteration by assuming constant binding forces (no MIE) and rigid smooth surface (no fines migration or dissolution). The frame consists of three parts: the classical Gouy-Chapman model coupled with interface charging mechanisms to describe EDL in oil-brine-rock systems, three methods with different boundary assumptions to evaluate EDL interaction energy, and the modified Young-Dupré equation to link EDL interaction energy with contact angle. The quantitative analysis for two typical oil-brine-rock systems provides two physical maps that show how the EDL interaction influences contact angle at different ionic composition. The result indicates that the contribution of EDL interaction to ion-tuned wettability for the studied system is not quite significant. The classical and advanced experimental work using microfabrication is reviewed briefly on the contribution of EDL repulsion to wettability alteration and compared with the theoretical results. It is indicated that the roughness of real rock surface may enhance EDL interaction. Finally we discuss some pending questions, perspectives and promising applications based on the mechanism.

Molecular Theory and the Effects of Solute Attractive Forces on Hydrophobic Interactions.

PubMed

Chaudhari, Mangesh I; Rempe, Susan B; Asthagiri, D; Tan, L; Pratt, L R

2016-03-03

The role of solute attractive forces on hydrophobic interactions is studied by coordinated development of theory and simulation results for Ar atoms in water. We present a concise derivation of the local molecular field (LMF) theory for the effects of solute attractive forces on hydrophobic interactions, a derivation that clarifies the close relation of LMF theory to the EXP approximation applied to this problem long ago. The simulation results show that change from purely repulsive atomic solute interactions to include realistic attractive interactions diminishes the strength of hydrophobic bonds. For the Ar-Ar rdfs considered pointwise, the numerical results for the effects of solute attractive forces on hydrophobic interactions are opposite in sign and larger in magnitude than predicted by LMF theory. That comparison is discussed from the point of view of quasichemical theory, and it is suggested that the first reason for this difference is the incomplete evaluation within LMF theory of the hydration energy of the Ar pair. With a recent suggestion for the system-size extrapolation of the required correlation function integrals, the Ar-Ar rdfs permit evaluation of osmotic second virial coefficients B2. Those B2's also show that incorporation of attractive interactions leads to more positive (repulsive) values. With attractive interactions in play, B2 can change from positive to negative values with increasing temperatures. This is consistent with the puzzling suggestions of decades ago that B2 ≈ 0 for intermediate cases of temperature or solute size. In all cases here, B2 becomes more attractive with increasing temperature.

Molecular theory and the effects of solute attractive forces on hydrophobic interactions

DOE PAGES

Chaudhari, Mangesh I.; Rempe, Susan B.; Asthagiri, D.; ...

2015-12-22

The role of solute attractive forces on hydrophobic interactions is studied by coordinated development of theory and simulation results for Ar atoms in water. In this paper, we present a concise derivation of the local molecular field (LMF) theory for the effects of solute attractive forces on hydrophobic interactions, a derivation that clarifies the close relation of LMF theory to the EXP approximation applied to this problem long ago. The simulation results show that change from purely repulsive atomic solute interactions to include realistic attractive interactions diminishes the strength of hydrophobic bonds. For the Ar–Ar rdfs considered pointwise, the numericalmore » results for the effects of solute attractive forces on hydrophobic interactions are opposite in sign and larger in magnitude than predicted by LMF theory. That comparison is discussed from the point of view of quasichemical theory, and it is suggested that the first reason for this difference is the incomplete evaluation within LMF theory of the hydration energy of the Ar pair. With a recent suggestion for the system-size extrapolation of the required correlation function integrals, the Ar–Ar rdfs permit evaluation of osmotic second virial coefficients B 2. Those B 2’s also show that incorporation of attractive interactions leads to more positive (repulsive) values. With attractive interactions in play, B 2 can change from positive to negative values with increasing temperatures. Furthermore, this is consistent with the puzzling suggestions of decades ago that B 2 ≈ 0 for intermediate cases of temperature or solute size. In all cases here, B 2 becomes more attractive with increasing temperature.« less

Calculation of noncontact forces between silica nanospheres.

PubMed

Sun, Weifu; Zeng, Qinghua; Yu, Aibing

2013-02-19

Quantification of the interactions between nanoparticles is important in understanding their dynamic behaviors and many related phenomena. In this study, molecular dynamics simulation is used to calculate the interaction potentials (i.e., van der Waals attraction, Born repulsion, and electrostatic interaction) between two silica nanospheres of equal radius in the range of 0.975 to 5.137 nm. The results are compared with those obtained from the conventional Hamaker approach, leading to the development of modified formulas to calculate the van der Waals attraction and Born repulsion between nanospheres, respectively. Moreover, Coulomb's law is found to be valid for calculating the electrostatic potential between nanospheres. The developed formulas should be useful in the study of the dynamic behaviors of nanoparticle systems under different conditions.

Investigation on the individual contributions of N-H...O=C and C-H...O=C interactions to the binding energies of beta-sheet models.

PubMed

Wang, Chang-Sheng; Sun, Chang-Liang

2010-04-15

In this article, the binding energies of 16 antiparallel and parallel beta-sheet models are estimated using the analytic potential energy function we proposed recently and the results are compared with those obtained from MP2, AMBER99, OPLSAA/L, and CHARMM27 calculations. The comparisons indicate that the analytic potential energy function can produce reasonable binding energies for beta-sheet models. Further comparisons suggest that the binding energy of the beta-sheet models might come mainly from dipole-dipole attractive and repulsive interactions and VDW interactions between the two strands. The dipole-dipole attractive and repulsive interactions are further obtained in this article. The total of N-H...H-N and C=O...O=C dipole-dipole repulsive interaction (the secondary electrostatic repulsive interaction) in the small ring of the antiparallel beta-sheet models is estimated to be about 6.0 kcal/mol. The individual N-H...O=C dipole-dipole attractive interaction is predicted to be -6.2 +/- 0.2 kcal/mol in the antiparallel beta-sheet models and -5.2 +/- 0.6 kcal/mol in the parallel beta-sheet models. The individual C(alpha)-H...O=C attractive interaction is -1.2 +/- 0.2 kcal/mol in the antiparallel beta-sheet models and -1.5 +/- 0.2 kcal/mol in the parallel beta-sheet models. These values are important in understanding the interactions at protein-protein interfaces and developing a more accurate force field for peptides and proteins. 2009 Wiley Periodicals, Inc.

Interactions regulating the head-to-tail directed assembly of biological Janus rods

DOE Office of Scientific and Technical Information (OSTI.GOV)

Greene, A. C.; Bachand, M.; Gomez, A.

We can generalize the directed, head-to-tail self-assembly of microtubule filaments in the context of Janus colloidal rods. Specifically, their assembly at the tens of micron-length scale involves a careful balance between long-range electrostatic repulsion and short-range attractive forces. We show that the addition of counterion salts increases the rate of directed assembly by screening the electrostatic forces and enhancing the effectiveness of short-range interactions at the microtubule ends.

Interactions regulating the head-to-tail directed assembly of biological Janus rods

DOE PAGES

Greene, A. C.; Bachand, M.; Gomez, A.; ...

2017-03-31

We can generalize the directed, head-to-tail self-assembly of microtubule filaments in the context of Janus colloidal rods. Specifically, their assembly at the tens of micron-length scale involves a careful balance between long-range electrostatic repulsion and short-range attractive forces. We show that the addition of counterion salts increases the rate of directed assembly by screening the electrostatic forces and enhancing the effectiveness of short-range interactions at the microtubule ends.

The role of electrostatics in protein-protein interactions of a monoclonal antibody.

PubMed

Roberts, D; Keeling, R; Tracka, M; van der Walle, C F; Uddin, S; Warwicker, J; Curtis, R

2014-07-07

Understanding how protein-protein interactions depend on the choice of buffer, salt, ionic strength, and pH is needed to have better control over protein solution behavior. Here, we have characterized the pH and ionic strength dependence of protein-protein interactions in terms of an interaction parameter kD obtained from dynamic light scattering and the osmotic second virial coefficient B22 measured by static light scattering. A simplified protein-protein interaction model based on a Baxter adhesive potential and an electric double layer force is used to separate out the contributions of longer-ranged electrostatic interactions from short-ranged attractive forces. The ionic strength dependence of protein-protein interactions for solutions at pH 6.5 and below can be accurately captured using a Deryaguin-Landau-Verwey-Overbeek (DLVO) potential to describe the double layer forces. In solutions at pH 9, attractive electrostatics occur over the ionic strength range of 5-275 mM. At intermediate pH values (7.25 to 8.5), there is a crossover effect characterized by a nonmonotonic ionic strength dependence of protein-protein interactions, which can be rationalized by the competing effects of long-ranged repulsive double layer forces at low ionic strength and a shorter ranged electrostatic attraction, which dominates above a critical ionic strength. The change of interactions from repulsive to attractive indicates a concomitant change in the angular dependence of protein-protein interaction from isotropic to anisotropic. In the second part of the paper, we show how the Baxter adhesive potential can be used to predict values of kD from fitting to B22 measurements, thus providing a molecular basis for the linear correlation between the two protein-protein interaction parameters.

Mechanism underlying bioinertness of self-assembled monolayers of oligo(ethyleneglycol)-terminated alkanethiols on gold: protein adsorption, platelet adhesion, and surface forces.

PubMed

Hayashi, Tomohiro; Tanaka, Yusaku; Koide, Yuki; Tanaka, Masaru; Hara, Masahiko

2012-08-07

The mechanism underlying the bioinertness of the self-assembled monolayers of oligo(ethylene glycol)-terminated alkanethiol (OEG-SAM) was investigated with protein adsorption experiments, platelet adhesion tests, and surface force measurements with an atomic force microscope (AFM). In this work, we performed systematic analysis with SAMs having various terminal groups (-OEG, -OH, -COOH, -NH(2), and -CH(3)). The results of the protein adsorption experiment by the quartz crystal microbalance (QCM) method suggested that having one EG unit and the neutrality of total charges of the terminal groups are essential for protein-resistance. In particular, QCM with energy dissipation analyses indicated that proteins absorb onto the OEG-SAM via a very weak interaction compared with other SAMs. Contrary to the protein resistance, at least three EG units as well as the charge neutrality of the SAM are found to be required for anti-platelet adhesion. When the identical SAMs were formed on both AFM probe and substrate, our force measurements revealed that only the OEG-SAMs possessing more than two EG units showed strong repulsion in the range of 4 to 6 nm. In addition, we found that the SAMs with other terminal groups did not exhibit such repulsion. The repulsion between OEG-SAMs was always observed independent of solution conditions [NaCl concentration (between 0 and 1 M) and pH (between 3 and 11)] and was not observed in solution mixed with ethanol, which disrupts the three-dimensional network of the water molecules. We therefore concluded that the repulsion originated from structured interfacial water molecules. Considering the correlation between the above results, we propose that the layer of the structured interfacial water with a thickness of 2 to 3 nm (half of the range of the repulsion observed in the surface force measurements) plays an important role in deterring proteins and platelets from adsorption or adhesion.

Correlational Effects of the Molecular-Tilt Configuration and the Intermolecular van der Waals Interaction on the Charge Transport in the Molecular Junction.

PubMed

Shin, Jaeho; Gu, Kyungyeol; Yang, Seunghoon; Lee, Chul-Ho; Lee, Takhee; Jang, Yun Hee; Wang, Gunuk

2018-06-25

Molecular conformation, intermolecular interaction, and electrode-molecule contacts greatly affect charge transport in molecular junctions and interfacial properties of organic devices by controlling the molecular orbital alignment. Here, we statistically investigated the charge transport in molecular junctions containing self-assembled oligophenylene molecules sandwiched between an Au probe tip and graphene according to various tip-loading forces ( F L ) that can control the molecular-tilt configuration and the van der Waals (vdW) interactions. In particular, the molecular junctions exhibited two distinct transport regimes according to the F L dependence (i.e., F L -dependent and F L -independent tunneling regimes). In addition, the charge-injection tunneling barriers at the junction interfaces are differently changed when the F L ≤ 20 nN. These features are associated to the correlation effects between the asymmetry-coupling factor (η), the molecular-tilt angle (θ), and the repulsive intermolecular vdW force ( F vdW ) on the molecular-tunneling barriers. A more-comprehensive understanding of these charge transport properties was thoroughly developed based on the density functional theory calculations in consideration of the molecular-tilt configuration and the repulsive vdW force between molecules.

Modeling the Interaction between AFM Tips and Pinned Surface Nanobubbles.

PubMed

Guo, Zhenjiang; Liu, Yawei; Xiao, Qianxiang; Schönherr, Holger; Zhang, Xianren

2016-01-26

Although the morphology of surface nanobubbles has been studied widely with different AFM modes, AFM images may not reflect the real shapes of the nanobubbles due to AFM tip-nanobubble interactions. In addition, the interplay between surface nanobubble deformation and induced capillary force has not been well understood in this context. In our work we used constraint lattice density functional theory to investigate the interaction between AFM tips and pinned surface nanobubbles systematically, especially concentrating on the effects of tip hydrophilicity and shape. For a hydrophilic tip contacting a nanobubble, its hydrophilic nature facilitates its departure from the bubble surface, displaying a weak and intermediate-range attraction. However, when the tip squeezes the nanobubble during the approach process, the nanobubble shows an elastic effect that prevents the tip from penetrating the bubble, leading to a strong nanobubble deformation and repulsive interactions. On the contrary, a hydrophobic tip can easily pierce the vapor-liquid interface of the nanobubble during the approach process, leading to the disappearance of the repulsive force. In the retraction process, however, the adhesion between the tip and the nanobubble leads to a much stronger lengthening effect on nanobubble deformation and a strong long-range attractive force. The trends of force evolution from our simulations agree qualitatively well with recent experimental AFM observations. This favorable agreement demonstrates that our model catches the main intergradient of tip-nanobubble interactions for pinned surface nanobubbles and may therefore provide important insight into how to design minimally invasive AFM experiments.

Selfbound quantum droplets

NASA Astrophysics Data System (ADS)

Langen, Tim; Wenzel, Matthias; Schmitt, Matthias; Boettcher, Fabian; Buehner, Carl; Ferrier-Barbut, Igor; Pfau, Tilman

2017-04-01

Self-bound many-body systems are formed through a balance of attractive and repulsive forces and occur in many physical scenarios. Liquid droplets are an example of a self-bound system, formed by a balance of the mutual attractive and repulsive forces that derive from different components of the inter-particle potential. On the basis of the recent finding that an unstable bosonic dipolar gas can be stabilized by a repulsive many-body term, it was predicted that three-dimensional self-bound quantum droplets of magnetic atoms should exist. Here we report on the observation of such droplets using dysprosium atoms, with densities 108 times lower than a helium droplet, in a trap-free levitation field. We find that this dilute magnetic quantum liquid requires a minimum, critical number of atoms, below which the liquid evaporates into an expanding gas as a result of the quantum pressure of the individual constituents. Consequently, around this critical atom number we observe an interaction-driven phase transition between a gas and a self-bound liquid in the quantum degenerate regime with ultracold atoms.

Like-charge attraction and opposite-charge decomplexation between polymers and DNA molecules

NASA Astrophysics Data System (ADS)

Buyukdagli, Sahin

2017-02-01

We scrutinize the effect of polyvalent ions on polymer-DNA interactions. We extend a recently developed test-charge theory [S. Buyukdagli et al., Phys. Rev. E 94, 042502 (2016), 10.1103/PhysRevE.94.042502] to the case of a stiff polymer interacting with a DNA molecule in an electrolyte mixture. The theory accounts for one-loop level electrostatic correlation effects such as the ionic cloud deformation around the strongly charged DNA molecule as well as image-charge forces induced by the low DNA permittivity. Our model can reproduce and explain various characteristics of the experimental phase diagrams for polymer solutions. First, the addition of polyvalent cations to the electrolyte solution results in the attraction of the negatively charged polymer by the DNA molecule. The glue of the like-charge attraction is the enhanced shielding of the polymer charges by the dense counterion layer at the DNA surface. Second, through the shielding of the DNA-induced electrostatic potential, mono- and polyvalent cations of large concentration both suppress the like-charge attraction. Within the same formalism, we also predict a new opposite-charge repulsion effect between the DNA molecule and a positively charged polymer. In the presence of polyvalent anions such as sulfate or phosphate, their repulsion by the DNA charges leads to the charge screening deficiency of the region around the DNA molecule. This translates into a repulsive force that results in the decomplexation of the polymer from DNA. This opposite-charge repulsion phenomenon can be verified by current experiments and the underlying mechanism can be beneficial to gene therapeutic applications where the control over polymer-DNA interactions is the key factor.

Evaluation of synthetic linear motor-molecule actuation energetics

PubMed Central

Brough, Branden; Northrop, Brian H.; Schmidt, Jacob J.; Tseng, Hsian-Rong; Houk, Kendall N.; Stoddart, J. Fraser; Ho, Chih-Ming

2006-01-01

By applying atomic force microscope (AFM)-based force spectroscopy together with computational modeling in the form of molecular force-field simulations, we have determined quantitatively the actuation energetics of a synthetic motor-molecule. This multidisciplinary approach was performed on specifically designed, bistable, redox-controllable [2]rotaxanes to probe the steric and electrostatic interactions that dictate their mechanical switching at the single-molecule level. The fusion of experimental force spectroscopy and theoretical computational modeling has revealed that the repulsive electrostatic interaction, which is responsible for the molecular actuation, is as high as 65 kcal·mol−1, a result that is supported by ab initio calculations. PMID:16735470

Interactions and aggregation of apoferritin molecules in solution: effects of added electrolytes.

PubMed Central

Petsev, D N; Thomas, B R; Yau, S; Vekilov, P G

2000-01-01

We have studied the structure of the protein species and the protein-protein interactions in solutions containing two apoferritin molecular forms, monomers and dimers, in the presence of Na(+) and Cd(2+) ions. We used chromatographic, and static and dynamic light scattering techniques, and atomic force microscopy (AFM). Size-exclusion chromatography was used to isolate these two protein fractions. The sizes and shapes of the monomers and dimers were determined by dynamic light scattering and AFM. Although the monomer is an apparent sphere with a diameter corresponding to previous x-ray crystallography determinations, the dimer shape corresponds to two, bound monomer spheres. Static light scattering was applied to characterize the interactions between solute molecules of monomers and dimers in terms of the second osmotic virial coefficients. The results for the monomers indicate that Na(+) ions cause strong intermolecular repulsion even at concentrations higher than 0.15 M, contrary to the predictions of the commonly applied Derjaguin-Landau-Verwey-Overbeek theory. We argue that the reason for such behavior is hydration force due to the formation of a water shell around the protein molecules with the help of the sodium ions. The addition of even small amounts of Cd(2+) changes the repulsive interactions to attractive but does not lead to oligomer formation, at least at the protein concentrations used. Thus, the two ions provide examples of strong specificity of their interactions with the protein molecules. In solutions of the apoferritin dimer, the molecules attract even in the presence of Na(+) only, indicating a change in the surface of the apoferritin molecule. In view of the strong repulsion between the monomers, this indicates that the dimers and higher oligomers form only after partial denaturation of some of the apoferritin monomers. These observations suggest that aggregation and self-assembly of protein molecules or molecular subunits may be driven by forces other than those responsible for crystallization and other phase transitions in the protein solution. PMID:10733984

Experimental comparison of forces resisting viral DNA packaging and driving DNA ejection

NASA Astrophysics Data System (ADS)

Keller, Nicholas; Berndsen, Zachary T.; Jardine, Paul J.; Smith, Douglas E.

2017-05-01

We compare forces resisting DNA packaging and forces driving DNA ejection in bacteriophage phi29 with theoretical predictions. Ejection of DNA from prohead-motor complexes is triggered by heating complexes after in vitro packaging and force is inferred from the suppression of ejection by applied osmotic pressure. Ejection force from 0 % to 80 % filling is found to be in quantitative agreement with predictions of a continuum mechanics model that assumes a repulsive DNA-DNA interaction potential based on DNA condensation studies and predicts an inverse-spool conformation. Force resisting DNA packaging from ˜80 % to 100 % filling inferred from optical tweezers studies is also consistent with the predictions of this model. The striking agreement with these two different measurements suggests that the overall energetics of DNA packaging is well described by the model. However, since electron microscopy studies of phi29 do not reveal a spool conformation, our findings suggest that the spool model overestimates the role of bending rigidity and underestimates the role of intrastrand repulsion. Below ˜80 % filling the inferred forces resisting packaging are unexpectedly lower than the inferred ejection forces, suggesting that in this filling range the forces are less accurately determined or strongly temperature dependent.

Experimental comparison of forces resisting viral DNA packaging and driving DNA ejection.

PubMed

Keller, Nicholas; Berndsen, Zachary T; Jardine, Paul J; Smith, Douglas E

2017-05-01

We compare forces resisting DNA packaging and forces driving DNA ejection in bacteriophage phi29 with theoretical predictions. Ejection of DNA from prohead-motor complexes is triggered by heating complexes after in vitro packaging and force is inferred from the suppression of ejection by applied osmotic pressure. Ejection force from 0% to 80% filling is found to be in quantitative agreement with predictions of a continuum mechanics model that assumes a repulsive DNA-DNA interaction potential based on DNA condensation studies and predicts an inverse-spool conformation. Force resisting DNA packaging from ∼80% to 100% filling inferred from optical tweezers studies is also consistent with the predictions of this model. The striking agreement with these two different measurements suggests that the overall energetics of DNA packaging is well described by the model. However, since electron microscopy studies of phi29 do not reveal a spool conformation, our findings suggest that the spool model overestimates the role of bending rigidity and underestimates the role of intrastrand repulsion. Below ∼80% filling the inferred forces resisting packaging are unexpectedly lower than the inferred ejection forces, suggesting that in this filling range the forces are less accurately determined or strongly temperature dependent.

Diffusing colloidal probes of protein-carbohydrate interactions.

PubMed

Eichmann, Shannon L; Meric, Gulsum; Swavola, Julia C; Bevan, Michael A

2013-02-19

We present diffusing colloidal probe measurements of weak, multivalent, specific protein-polysaccharide interactions mediated by a competing monosaccharide. Specifically, we used integrated evanescent wave and video microscopy methods to monitor the three-dimensional Brownian excursions of conconavilin A (ConA) decorated colloids interacting with dextran-functionalized surfaces in the presence of glucose. Particle trajectories were interpreted as binding lifetime histograms, binding isotherms, and potentials of mean force. Binding lifetimes and isotherms showed clear trends of decreasing ConA-dextran-specific binding with increasing glucose concentration, consistent with expectations. Net potentials were accurately captured by superposition of a short-range, glucose-independent ConA-dextran repulsion and a longer-range, glucose-dependent dextran bridging attraction modeled as a harmonic potential. For glucose concentrations greater than 100 mM, the net ConA-dextran potential was found to have only a nonspecific repulsion, similar to that of bovine serum albumin (BSA) decorated colloids over dextran determined in control experiments. Our results demonstrate the first use of optical microscopy methods to quantify the connections between potentials of mean force and the binding behavior of ConA-decorated colloids on dextran-functionalized surfaces.

Electrostatic interaction between stereocilia: I. Its role in supporting the structure of the hair bundle.

PubMed

Dolgobrodov, S G; Lukashkin, A N; Russell, I J

2000-12-01

This paper provides theoretical estimates for the forces of electrostatic interaction between adjacent stereocilia in auditory and vestibular hair cells. Estimates are given for parameters within the measured physiological range using constraints appropriate for the known geometry of the hair bundle. Stereocilia are assumed to possess an extended, negatively charged surface coat, the glycocalyx. Different charge distribution profiles within the glycocalyx are analysed. It is shown that charged glycocalices on the apical surface of the hair cells can support spatial separation between adjacent stereocilia in the hair bundles through electrostatic repulsion between stereocilia. The charge density profile within the glycocalyx is a crucial parameter. In fact, attraction instead of repulsion between adjacent stereocilia will be observed if the charge of the glycocalyx is concentrated near the membrane of the stereocilia, thereby making this type of charge distribution unlikely. The forces of electrostatic interaction between stereocilia may influence the mechanical properties of the hair bundle and, being strongly non-linear, contribute to the non-linear phenomena that have been recorded from the periphery of the auditory and vestibular systems.

Landau instability and mobility edges of the interacting one-dimensional Bose gas in weak random potentials

NASA Astrophysics Data System (ADS)

Cherny, Alexander Yu; Caux, Jean-Sébastien; Brand, Joachim

2018-01-01

We study the frictional force exerted on the trapped, interacting 1D Bose gas under the influence of a moving random potential. Specifically we consider weak potentials generated by optical speckle patterns with finite correlation length. We show that repulsive interactions between bosons lead to a superfluid response and suppression of frictional force, which can inhibit the onset of Anderson localisation. We perform a quantitative analysis of the Landau instability based on the dynamic structure factor of the integrable Lieb-Liniger model and demonstrate the existence of effective mobility edges.

Electron Bubbles in Superfluid (3) 3 He-A: Exploring the Quasiparticle-Ion Interaction

NASA Astrophysics Data System (ADS)

Shevtsov, Oleksii; Sauls, J. A.

2017-06-01

When an electron is forced into liquid ^3He, it forms an "electron bubble", a heavy ion with radius, R˜eq 1.5 nm, and mass, M˜eq 100 m_3, where m_3 is the mass of a ^3He atom. These negative ions have proven to be powerful local probes of the physical properties of the host quantum fluid, especially the excitation spectra of the superfluid phases. We recently developed a theory for Bogoliubov quasiparticles scattering off electron bubbles embedded in a chiral superfluid that provides a detailed understanding of the spectrum of Weyl Fermions bound to the negative ion, as well as a theory for the forces on moving electron bubbles in superfluid ^3He-A (Shevtsov and Sauls in Phys Rev B 94:064511, 2016). This theory is shown to provide quantitative agreement with measurements reported by the RIKEN group (Ikegami et al. in Science 341(6141):59, 2013) for the drag force and anomalous Hall effect of moving electron bubbles in superfluid ^3He-A. In this report, we discuss the sensitivity of the forces on the moving ion to the effective interaction between normal-state quasiparticles and the ion. We consider models for the quasiparticle-ion (QP-ion) interaction, including the hard-sphere potential, constrained random-phase-shifts, and interactions with short-range repulsion and intermediate-range attraction. Our results show that the transverse force responsible for the anomalous Hall effect is particularly sensitive to the structure of the QP-ion potential and that strong short-range repulsion, captured by the hard-sphere potential, provides an accurate model for computing the forces acting on the moving electron bubble in superfluid 3He-A.

Electromagnetic theory of the nuclear interaction. Application to the deuteron {sup 2}H

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schaeffer, Bernard

2012-06-20

Bieler of the Rutherford laboratory imagined in 1924 a magnetic attraction equilibrating an electrostatic repulsion between the protons. Since the discovery of the neutron and the magnetic moments of the nucleons proving that the neutron contains electric charges, nobody, as far as I know, has tried to apply electromagnetism to the nuclear interaction. The electrostatic and magnetic interactions are completely neglected except for a mean Coulomb repulsion. As it is well known, there is an attraction between an electric charge and a neutral conductor. In the neutron, the positive charges are repelled and the negative charges attracted by a nearbymore » proton. There is a net attraction explaining quantitatively the so-called strong force as it is shown in this paper. In the deuteron, the magnetic repulsion equilibrates the electrostatically induced neutron-proton attraction. The experimental value (- 2.2 MeV) is surrounded by - 1.6 MeV and - 2.5 MeV, depending on the calculation method. No arbitrary fitting parameter is used, only physical constants: it is a true ab initio calculation. The theoretical ratio between nuclear and chemical energies has been found to be (m{sub p}/m{sub e}{alpha}), proving that the usual assumption that the electromagnetic interaction is too feeble to predict the nuclear interaction is incorrect.« less

Dark matter and the equivalence principle

NASA Technical Reports Server (NTRS)

Frieman, Joshua A.; Gradwohl, Ben-Ami

1991-01-01

If the dark matter in galaxies and clusters is nonbaryonic, it can interact with additional long-range fields that are invisible to experimental tests of the equivalence principle. The astrophysical and cosmological implications of a long-range force coupled only to the dark matter are discussed and rather tight constraints on its strength are found. If the force is repulsive (attractive), the masses of galaxy groups and clusters (and the mean density of the universe inferred from them) have been systematically underestimated (overestimated). Such an interaction also has unusual implications for the growth of large-scale structure.

Computer Simulation of the Forces Acting on a Submerged Polystyrene Probe as it Approaches the Succinonitrile Melt-Solid Interface

NASA Technical Reports Server (NTRS)

Bune, Andris V.; Kaukler, William; Whitaker, Ann (Technical Monitor)

2001-01-01

A Modeling approach to simulate both mesoscale and microscopic forces acting in a typical AFM experiment is presented. A mesoscale level interaction between the cantilever tip and the sample surface is primarily described by the balance of attractive Van der Waals and repulsive forces. Ultimately, the goal is to measure the forces between a particle and the crystal-melt interface. Two modes of AFM operation are considered in this paper - a stationary and a "tapping" one. The continuous mechanics approach to model tip-surface interaction is presented. At microscopic levels, tip contamination and details of tip-surface interaction are modeled using a molecular dynamics approach for the case of polystyrene - succinonitrile contact. Integration of the mesoscale model with a molecular dynamic model is discussed.

Social dynamics in emergency evacuations: Disentangling crowd's attraction and repulsion effects

NASA Astrophysics Data System (ADS)

Haghani, Milad; Sarvi, Majid

2017-06-01

The social dynamics of crowds in emergency escape scenarios have been conventionally modelled as the net effect of virtual forces exerted by the crowd on each individual (as self-driven particles), with the magnitude of the influence formulated as decreasing functions of inter-individual distances and the direction of effect assumed to be transitioning from repulsion to attraction by distance. Here, we revisit this conventional assumption using laboratory experimental data. We show based on robust econometric hypothesis-testing methods that individuals' perception of other escapees differs based on whether those individuals are jamming around exit destinations or are on the move towards the destinations. Also, for moving crowds, it differs based on whether the escape destination chosen by the moving flow is visible or invisible to the individual. The presence of crowd jams around a destination, also the movement of crowd flows towards visible destinations are both perceived on average as repulsion (or disutility) effects (with the former showing significantly larger magnitude than the latter). The movement of crowd flows towards an invisible destination, however, is on average perceived as attraction (or utility) effect. Yet, further hypothesis testing showed that neither of those effects in isolation determines adequately whether an individual would merge with or diverge from the crowd. Rather, the social interaction factors act (at significant levels) in conjunction with the physical factors of the environments (including spatial distances to exit destinations and destinations' visibility). In brief, our finding disentangles the conditions under which individuals are more likely to show mass behaviour from the situations where they are more likely to break from the herd. It identifies two factors that moderate the perception of social interactions, ;crowds' jam/movement status; and ;environmental setup;. Our results particularly challenge the taxonomy of attraction-repulsion social interaction forces defined purely based on the distance of the individual to the surrounding crowd, by showing that crowds could be in far distance and yet be perceived as repulsion effect, or they could be in close distance and yet act as attraction effect.

Computer Simulation of the Forces Acting on the Polystyrene Probe Submerged into the Succinonitrile Near Phase Transition

NASA Technical Reports Server (NTRS)

Bune, Andris V.; Kaukler, William F.; Whitaker, Ann F. (Technical Monitor)

2001-01-01

Modeling approach to simulate both mesoscale and microscopic forces acting in a typical AFM experiment is presented. At mesoscale level interaction between the cantilever tip and the sample surface is primarily described by the balance of attractive Van der Waals and repulsive forces. The model of cantilever oscillations is applicable to both non-contact and "tapping" AFM. This model can be farther enhanced to describe nanoparticle manipulation by cantilever. At microscopic level tip contamination and details of tip-surface interaction can be simulated using molecular dynamics approach. Integration of mesoscale model with molecular dynamic model is discussed.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chaudhari, Mangesh I.; Rempe, Susan B.; Asthagiri, D.

The role of solute attractive forces on hydrophobic interactions is studied by coordinated development of theory and simulation results for Ar atoms in water. In this paper, we present a concise derivation of the local molecular field (LMF) theory for the effects of solute attractive forces on hydrophobic interactions, a derivation that clarifies the close relation of LMF theory to the EXP approximation applied to this problem long ago. The simulation results show that change from purely repulsive atomic solute interactions to include realistic attractive interactions diminishes the strength of hydrophobic bonds. For the Ar–Ar rdfs considered pointwise, the numericalmore » results for the effects of solute attractive forces on hydrophobic interactions are opposite in sign and larger in magnitude than predicted by LMF theory. That comparison is discussed from the point of view of quasichemical theory, and it is suggested that the first reason for this difference is the incomplete evaluation within LMF theory of the hydration energy of the Ar pair. With a recent suggestion for the system-size extrapolation of the required correlation function integrals, the Ar–Ar rdfs permit evaluation of osmotic second virial coefficients B 2. Those B 2’s also show that incorporation of attractive interactions leads to more positive (repulsive) values. With attractive interactions in play, B 2 can change from positive to negative values with increasing temperatures. Furthermore, this is consistent with the puzzling suggestions of decades ago that B 2 ≈ 0 for intermediate cases of temperature or solute size. In all cases here, B 2 becomes more attractive with increasing temperature.« less

Casimir force phase transitions in the graphene family

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rodriguez-Lopez, Pablo; Kort-Kamp, Wilton J. M.; Dalvit, Diego A. R.

The Casimir force is a universal interaction induced by electromagnetic quantum fluctuations between any types of objects. We found that the expansion of the graphene family by adding silicene, germanene and stanene (2D allotropes of Si, Ge, and Sn), lends itself as a platform to probe Dirac-like physics in honeycomb staggered systems in such a ubiquitous interaction. Here, we discover Casimir force phase transitions between these staggered 2D materials induced by the complex interplay between Dirac physics, spin-orbit coupling and externally applied fields. Particularly, we find that the interaction energy experiences different power law distance decays, magnitudes and dependences onmore » characteristic physical constants. Furthermore, due to the topological properties of these materials, repulsive and quantized Casimir interactions become possible.« less

Casimir force phase transitions in the graphene family

DOE PAGES

Rodriguez-Lopez, Pablo; Kort-Kamp, Wilton J. M.; Dalvit, Diego A. R.; ...

2017-03-15

The Casimir force is a universal interaction induced by electromagnetic quantum fluctuations between any types of objects. We found that the expansion of the graphene family by adding silicene, germanene and stanene (2D allotropes of Si, Ge, and Sn), lends itself as a platform to probe Dirac-like physics in honeycomb staggered systems in such a ubiquitous interaction. Here, we discover Casimir force phase transitions between these staggered 2D materials induced by the complex interplay between Dirac physics, spin-orbit coupling and externally applied fields. Particularly, we find that the interaction energy experiences different power law distance decays, magnitudes and dependences onmore » characteristic physical constants. Furthermore, due to the topological properties of these materials, repulsive and quantized Casimir interactions become possible.« less

Repulsive Casimir force in Bose–Einstein Condensate

NASA Astrophysics Data System (ADS)

Mehedi Faruk, Mir; Biswas, Shovon

2018-04-01

We study the Casimir effect for a three dimensional system of ideal free massive Bose gas in a slab geometry with Zaremba and anti-periodic boundary conditions. It is found that for these type of boundary conditions the resulting Casimir force is repulsive in nature, in contrast with usual periodic, Dirichlet or Neumann boundary condition where the Casimir force is attractive (Martin and Zagrebnov 2006 Europhys. Lett. 73 15). Casimir forces in these boundary conditions also maintain a power law decay function below condensation temperature and exponential decay function above the condensation temperature albeit with a positive sign, identifying the repulsive nature of the force.

Thermal fluctuations and stability of a particle levitated by a repulsive Casimir force in a liquid.

PubMed

Inui, Norio; Goto, Kosuke

2013-11-01

We study the vertical Brownian motion of a gold particle levitated by a repulsive Casimir force to a silica plate immersed in bromobenzene. The time evolution of the particle distribution starting from an equilibrium position, where the Casimir force and gravitational force are balanced, is considered by solving the Langevin equation using the Monte Carlo method. When the gold particle is very close to the silica plate, the Casimir force changes from repulsive to attractive, and the particle eventually sticks to the surface. The escape rate from a metastable position is calculated by solving the Fokker-Plank equation; it agrees with the value obtained by Kramers' escape theory. The duration of levitation increases as the particle radius increases up to around 2.3 μm. As an example, we show that a 1-μm-diameter gold particle can be levitated for a significantly long time by the repulsive Casimir force at room temperature.

Gauche effect in 1,2-difluoroethane. Hyperconjugation, bent bonds, steric repulsion.

PubMed

Goodman, Lionel; Gu, Hongbing; Pophristic, Vojislava

2005-02-17

Natural bond orbital deletion calculations show that whereas the gauche preference arises from vicinal hyperconjugative interaction between anti C-H bonds and C-F* antibonds, the cis C-H/C-F* interactions are substantial (approximately 25% of the anti interaction). The established significantly >60 degrees FCCF dihedral angle for the equilibrium conformer can then be rationalized in terms of the hyperconjugation model alone by taking into account both anti interactions that maximize near 60 degrees and the smaller cis interactions that maximize at a much larger dihedral angle. This explanation does not invoke repulsive forces to rationalize the 72 degrees equilibrium conformer angle. The relative minimum energy for the trans conformer is the consequence of a balance between decreasing hyperconjugative stabilization and decreasing steric destabilization as the FCCF torsional angle approaches 180 degrees . The torsional coordinate is predicted to be strongly contaminated by CCF bending, with the result that approximately half of the trans --> gauche stabilization energy stems from mode coupling.

Self diffusion of interacting membrane proteins.

PubMed Central

Abney, J R; Scalettar, B A; Owicki, J C

1989-01-01

A two-dimensional version of the generalized Smoluchowski equation is used to analyze the time (or distance) dependent self diffusion of interacting membrane proteins in concentrated membrane systems. This equation provides a well established starting point for descriptions of the diffusion of particles that interact through both direct and hydrodynamic forces; in this initial work only the effects of direct interactions are explicitly considered. Data describing diffusion in the presence of hard-core repulsions, soft repulsions, and soft repulsions with weak attractions are presented. The effect that interactions have on the self-diffusion coefficient of a real protein molecule from mouse liver gap junctions is also calculated. The results indicate that self diffusion is always inhibited by direct interactions; this observation is interpreted in terms of the caging that will exist at finite protein concentration. It is also noted that, over small distance scales, the diffusion coefficient is determined entirely by the very strong Brownian forces; therefore, as a function of displacement the self-diffusion coefficient decays (rapidly) from its value at infinite dilution to its steady-state interaction-averaged value. The steady-state self-diffusion coefficient describes motion over distance scales that range from approximately 10 nm to cellular dimensions and is the quantity measured in fluorescence recovery after photobleaching experiments. The short-ranged behavior of the diffusion coefficient is important on the interparticle-distance scale and may therefore influence the rate at which nearest-neighbor collisional processes take place. The hard-disk theoretical results presented here are in excellent agreement with lattice Monte-Carlo results obtained by other workers. The concentration dependence of experimentally measured diffusion coefficients of antibody-hapten complexes bound to the membrane surface is consistent with that predicted by the theory. The variation in experimental diffusion coefficients of integral membrane proteins is greater than that predicted by the theory, and may also reflect protein-induced perturbations in membrane viscosity. PMID:2720077

Micro-mechanics of electrostatically stabilized suspensions of cellulose nanofibrils under steady state shear flow.

PubMed

Martoïa, F; Dumont, P J J; Orgéas, L; Belgacem, M N; Putaux, J-L

2016-02-14

In this study, we characterized and modeled the rheology of TEMPO-oxidized cellulose nanofibril (NFC) aqueous suspensions with electrostatically stabilized and unflocculated nanofibrous structures. These colloidal suspensions of slender and wavy nanofibers exhibited a yield stress and a shear thinning behavior at low and high shear rates, respectively. Both the shear yield stress and the consistency of these suspensions were power-law functions of the NFC volume fraction. We developed an original multiscale model for the prediction of the rheology of these suspensions. At the nanoscale, the suspensions were described as concentrated systems where NFCs interacted with the Newtonian suspending fluid through Brownian motion and long range fluid-NFC hydrodynamic interactions, as well as with each other through short range hydrodynamic and repulsive colloidal interaction forces. These forces were estimated using both the experimental results and 3D networks of NFCs that were numerically generated to mimic the nanostructures of NFC suspensions under shear flow. They were in good agreement with theoretical and measured forces for model colloidal systems. The model showed the primary role played by short range hydrodynamic and colloidal interactions on the rheology of NFC suspensions. At low shear rates, the origin of the yield stress of NFC suspensions was attributed to the combined contribution of repulsive colloidal interactions and the topology of the entangled NFC networks in the suspensions. At high shear rates, both concurrent colloidal and short (in some cases long) range hydrodynamic interactions could be at the origin of the shear thinning behavior of NFC suspensions.

Phantom energy mediates a long-range repulsive force.

PubMed

Amendola, Luca

2004-10-29

Scalar field models with nonstandard kinetic terms have been proposed in the context of k inflation, of Born-Infeld Lagrangians, of phantom energy and, more in general, of low-energy string theory. In general, scalar fields are expected to couple to matter inducing a new interaction. In this Letter I derive the cosmological perturbation equations and the Yukawa correction to gravity for such general models. I find three interesting results: first, when the field behaves as phantom energy (equation of state less than -1), then the coupling strength is negative, inducing a long-range repulsive force; second, the dark-energy field might cluster on astrophysical scales; third, applying the formalism to a Brans-Dicke theory with a general kinetic term it is shown that its Newtonian effects depend on a single parameter that generalizes the Brans-Dicke constant.

Optomechanical and photothermal interactions in suspended photonic crystal membranes.

PubMed

Woolf, David; Hui, Pui-Chuen; Iwase, Eiji; Khan, Mughees; Rodriguez, Alejandro W; Deotare, Parag; Bulu, Irfan; Johnson, Steven G; Capasso, Federico; Loncar, Marko

2013-03-25

We present here an optomechanical system fabricated with novel stress management techniques that allow us to suspend an ultrathin defect-free silicon photonic-crystal membrane above a Silicon-on-Insulator (SOI) substrate with a gap that is tunable to below 200 nm. Our devices are able to generate strong attractive and repulsive optical forces over a large surface area with simple in- and out- coupling and feature the strongest repulsive optomechanical coupling in any geometry to date (gOM/2π ≈65 GHz/nm). The interplay between the optomechanical and photo-thermal-mechanical dynamics is explored, and the latter is used to achieve cooling and amplification of the mechanical mode, demonstrating that our platform is well-suited for potential applications in low-power mass, force, and refractive-index sensing as well as optomechanical accelerometry.

Unbinding transition from fluid membranes with associated polymers.

PubMed

Benhamou, M; Kaidi, H

2013-10-01

We consider two neighboring fluid membranes that are associated with long flexible polymers (proteins or other macromolecules). We are interested in two physical systems consisting of i) two adjacent membranes with end-grafted (or adsorbed) polymers (system I), or ii) two membranes confining a polymer solution (system II). In addition to the pure interactions between membranes, the presence of polymers gives rise to new induced mediated interactions, which are repulsive, for system I, and attractive, for system II. In fact, repulsive induced interactions are caused by the excluded-volume forces between grafted polymers, while attractive ones, by entropy loss, due to free motion of polymers between membranes. The main goal is a quantitative study of the unbinding transition thermodynamics that is drastically affected by the associated polymers. For system I, the repulsive polymer-mediated force delays this transition that can happen at low temperature. To investigate the unbinding phenomenon, we first present an exact mathematical analysis of the total potential that is the sum of the primitive and induced potentials. This mathematical study enables us to classify the total interaction potentials, in terms of all parameters of the problem. Second, use is made of the standard variational method to calculate the first moments of the membrane separation. Special attention is paid to the determination of the unbinding temperature. In particular, we discuss its dependence on the extra parameters related to the associated polymers, which are the surface coverage and the polymer layer thickness on each membrane (for system I) or the polymer density and the gyration radius of coils (for system II). Third, we compute the disjoining pressure upon membrane separation. Finally, we emphasize that the presence of polymers may be a mechanism to delay or to accentuate the appearance of the unbinding transition between fluid membranes.

Quantum liquids get thin

NASA Astrophysics Data System (ADS)

Ferrier-Barbut, Igor; Pfau, Tilman

2018-01-01

A liquid exists when interactions that attract its constituent particles to each other are counterbalanced by a repulsion acting at higher densities. Other characteristics of liquids are short-range correlations and the existence of surface tension (1). Ultracold atom experiments provide a privileged platform with which to observe exotic states of matter, but the densities are far too low to obtain a conventional liquid because the atoms are too far apart to create repulsive forces arising from the Pauli exclusion principle of the atoms' internal electrons. The observation of quantum liquid droplets in an ultracold mixture of two quantum fluids is now reported on page 301 of this issue by Cabrera et al. (2) and a recent preprint by Semeghini et al. (3). Unlike conventional liquids, these liquids arise from a weak attraction and repulsive many-body correlations in the mixtures.

Molecular simulation of the effect of cholesterol on lipid-mediated protein-protein interactions.

PubMed

de Meyer, Frédérick J-M; Rodgers, Jocelyn M; Willems, Thomas F; Smit, Berend

2010-12-01

Experiments and molecular simulations have shown that the hydrophobic mismatch between proteins and membranes contributes significantly to lipid-mediated protein-protein interactions. In this article, we discuss the effect of cholesterol on lipid-mediated protein-protein interactions as function of hydrophobic mismatch, protein diameter and protein cluster size, lipid tail length, and temperature. To do so, we study a mesoscopic model of a hydrated bilayer containing lipids and cholesterol in which proteins are embedded, with a hybrid dissipative particle dynamics-Monte Carlo method. We propose a mechanism by which cholesterol affects protein interactions: protein-induced, cholesterol-enriched, or cholesterol-depleted lipid shells surrounding the proteins affect the lipid-mediated protein-protein interactions. Our calculations of the potential of mean force between proteins and protein clusters show that the addition of cholesterol dramatically reduces repulsive lipid-mediated interactions between proteins (protein clusters) with positive mismatch, but does not affect attractive interactions between proteins with negative mismatch. Cholesterol has only a modest effect on the repulsive interactions between proteins with different mismatch. Copyright © 2010 Biophysical Society. Published by Elsevier Inc. All rights reserved.

Self-assembly of skyrmion-dressed chiral nematic colloids with tangential anchoring.

PubMed

Pandey, M B; Porenta, T; Brewer, J; Burkart, A; Copar, S; Zumer, S; Smalyukh, Ivan I

2014-06-01

We describe dipolar nematic colloids comprising mutually bound solid microspheres, three-dimensional skyrmions, and point defects in a molecular alignment field of chiral nematic liquid crystals. Nonlinear optical imaging and numerical modeling based on minimization of Landau-de Gennes free energy reveal that the particle-induced skyrmions resemble torons and hopfions, while matching surface boundary conditions at the interfaces of liquid crystal and colloidal spheres. Laser tweezers and videomicroscopy reveal that the skyrmion-colloidal hybrids exhibit purely repulsive elastic pair interactions in the case of parallel dipoles and an unexpected reversal of interaction forces from repulsive to attractive as the center-to-center distance decreases for antiparallel dipoles. The ensuing elastic self-assembly gives rise to colloidal chains of antiparallel dipoles with particles entangled by skyrmions.

Glass transition temperature of polymer nano-composites with polymer and filler interactions

NASA Astrophysics Data System (ADS)

Hagita, Katsumi; Takano, Hiroshi; Doi, Masao; Morita, Hiroshi

2012-02-01

We systematically studied versatile coarse-grained model (bead spring model) to describe filled polymer nano-composites for coarse-grained (Kremer-Grest model) molecular dynamics simulations. This model consists of long polymers, crosslink, and fillers. We used the hollow structure as the filler to describe rigid spherical fillers with small computing costs. Our filler model consists of surface particles of icosahedra fullerene structure C320 and a repulsive force from the center of the filler is applied to the surface particles in order to make a sphere and rigid. The filler's diameter is 12 times of beads of the polymers. As the first test of our model, we study temperature dependence of volumes of periodic boundary conditions under constant pressures through NPT constant Andersen algorithm. It is found that Glass transition temperature (Tg) decrease with increasing filler's volume fraction for the case of repulsive interaction between polymer and fillers and Tg weakly increase for attractive interaction.

Chemistry of the calcite/water interface: Influence of sulfate ions and consequences in terms of cohesion forces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pourchet, Sylvie, E-mail: sylvie.pourchet@u-bourgogne.fr; Pochard, Isabelle; Brunel, Fabrice

2013-10-15

Calcite suspensions are used to mimic the behavior of more complex cementitious systems. Therefore the characterization of calcite–water interface in strong alkaline conditions, through ionic adsorption, electrokinetic measurements, static rheology and atomic force microscopy is a prerequisite. Calcium, a potential determining ion for calcite, adsorbs specifically onto the weakly positively charged calcite surface in water. This leads to an increase of the repulsive electric double layer force and thus weakens the particle cohesion. Sulfate adsorption, made at constant calcium concentration and ionic strength, significantly increases the attractive interactions between the calcite particles despite its very low adsorption. This is attributedmore » to a lowering of the electrostatic repulsion in connection with the evolution of the zeta potential. The linear relationship found between the yield stress and ζ{sup 2} proves that the classical DLVO theory applies for these systems, contrary to what was previously observed with C–S–H particles under the same conditions.« less

Understanding and modulating opalescence and viscosity in a monoclonal antibody formulation

PubMed Central

Salinas, Branden A; Sathish, Hasige A; Bishop, Steven M; Harn, Nick; Carpenter, John F; Randolph, Theodore W

2014-01-01

Opalescence and high viscosities can pose challenges for high concentration formulation of antibodies. Both phenomena result from protein-protein intermolecular interactions that can be modulated with solution ionic strength. We studied a therapeutic monoclonal antibody that exhibits high viscosity in solutions at low ionic strength (~20 centipoise (cP) at 90 mg/mL and 23°C) and significant opalescence at isotonic ionic strength (approximately 100 nephelometric turbidity units at 90 mg/mL and 23°C). The intermolecular interactions responsible for these effects were characterized using membrane osmometry, static light scattering and zeta potential measurements. The net protein-protein interactions were repulsive at low ionic strength (~4 mM) and attractive at isotonic ionic strengths. The high viscosities are attributed to electroviscous forces at low ionic strength and the significant opalescence at isotonic ionic strength is correlated with attractive antibody interactions. Furthermore there appears to be a connection to critical phenomena and it is suggested that the extent of opalescence is dependent on the proximity to the critical point. We demonstrate that by balancing the repulsive and attractive forces via intermediate ionic strengths and by increasing the mAb concentration above the apparent critical concentration both opalescence and viscosity can be simultaneously minimized. PMID:19475558

Repulsive DNA-DNA interactions accelerate viral DNA packaging in phage Phi29.

PubMed

Keller, Nicholas; delToro, Damian; Grimes, Shelley; Jardine, Paul J; Smith, Douglas E

2014-06-20

We use optical tweezers to study the effect of attractive versus repulsive DNA-DNA interactions on motor-driven viral packaging. Screening of repulsive interactions accelerates packaging, but induction of attractive interactions by spermidine(3+) causes heterogeneous dynamics. Acceleration is observed in a fraction of complexes, but most exhibit slowing and stalling, suggesting that attractive interactions promote nonequilibrium DNA conformations that impede the motor. Thus, repulsive interactions facilitate packaging despite increasing the energy of the theoretical optimum spooled DNA conformation.

Free cooling phase-diagram of hard-spheres with short- and long-range interactions

NASA Astrophysics Data System (ADS)

Gonzalez, S.; Thornton, A. R.; Luding, S.

2014-10-01

We study the stability, the clustering and the phase-diagram of free cooling granular gases. The systems consist of mono-disperse particles with additional non-contact (long-range) interactions, and are simulated here by the event-driven molecular dynamics algorithm with discrete (short-range shoulders or wells) potentials (in both 2D and 3D). Astonishingly good agreement is found with a mean field theory, where only the energy dissipation term is modified to account for both repulsive or attractive non-contact interactions. Attractive potentials enhance cooling and structure formation (clustering), whereas repulsive potentials reduce it, as intuition suggests. The system evolution is controlled by a single parameter: the non-contact potential strength scaled by the fluctuation kinetic energy (granular temperature). When this is small, as expected, the classical homogeneous cooling state is found. However, if the effective dissipation is strong enough, structure formation proceeds, before (in the repulsive case) non-contact forces get strong enough to undo the clustering (due to the ongoing dissipation of granular temperature). For both repulsive and attractive potentials, in the homogeneous regime, the cooling shows a universal behaviour when the (inverse) control parameter is used as evolution variable instead of time. The transition to a non-homogeneous regime, as predicted by stability analysis, is affected by both dissipation and potential strength. This can be cast into a phase diagram where the system changes with time, which leaves open many challenges for future research.

Stability of a thin elastic film close to a rigid plate

NASA Astrophysics Data System (ADS)

Chen, Yi-chao; Fried, Eliot; Tortorelli, Daniel A.

2012-05-01

We introduce and study a variational model for the formation of patterns induced by bringing the surface of a rigid plate into contact proximity with the surface of a polymeric film strongly bonded to a substrate. We treat the film as a homogeneous, isotropic, hyperelastic solid and account for both attractive and repulsive van der Waals interactions between the film surface and the proximate contractor. Aside from confirming the intuitive expectation that the presence of a repulsive contribution to the van der Waals potential should stabilize patterns that form on the film surface, we elucidate the role of repulsive interactions at the onset of instability. For a recently proposed van der Waals potential involving two parameters, the Hamaker constant A and the equilibrium spacing de, our results include estimates for the critical gap dc at which undulations appear on the film surface, the corresponding wavenumber kc of the undulations, and a lower bound fm for the attractive force needed to induce the undulations. To leading order, dc˜(Ah/μ), kc˜1/h, and fm˜(μ3A/h3), where h and μ denote the thickness and infinitesimal shear modulus of the film. Correction terms due to repulsive interactions indicate that, while kc may be influenced by μ and A, dc may also be influenced by de. Granted knowledge of μ and A, our results also suggest a simple experimental protocol for determining de.

Polymer translocation under time-dependent driving forces: resonant activation induced by attractive polymer-pore interactions.

PubMed

Ikonen, Timo; Shin, Jaeoh; Sung, Wokyung; Ala-Nissila, Tapio

2012-05-28

We study the driven translocation of polymers under time-dependent driving forces using N-particle Langevin dynamics simulations. We consider the force to be either sinusoidally oscillating in time or dichotomic noise with exponential correlation time, to mimic both plausible experimental setups and naturally occurring biological conditions. In addition, we consider both the case of purely repulsive polymer-pore interactions and the case with additional attractive polymer-pore interactions, typically occurring inside biological pores. We find that the nature of the interaction fundamentally affects the translocation dynamics. For the non-attractive pore, the translocation time crosses over to a fast translocation regime as the frequency of the driving force decreases. In the attractive pore case, because of a free energy well induced inside the pore, the translocation time can be a minimum at the optimal frequency of the force, the so-called resonant activation. In the latter case, we examine the effect of various physical parameters on the resonant activation, and explain our observations using simple theoretical arguments.

Nanophotonic force microscopy: characterizing particle-surface interactions using near-field photonics.

PubMed

Schein, Perry; Kang, Pilgyu; O'Dell, Dakota; Erickson, David

2015-02-11

Direct measurements of particle-surface interactions are important for characterizing the stability and behavior of colloidal and nanoparticle suspensions. Current techniques are limited in their ability to measure pico-Newton scale interaction forces on submicrometer particles due to signal detection limits and thermal noise. Here we present a new technique for making measurements in this regime, which we refer to as nanophotonic force microscopy. Using a photonic crystal resonator, we generate a strongly localized region of exponentially decaying, near-field light that allows us to confine small particles close to a surface. From the statistical distribution of the light intensity scattered by the particle we are able to map out the potential well of the trap and directly quantify the repulsive force between the nanoparticle and the surface. As shown in this Letter, our technique is not limited by thermal noise, and therefore, we are able to resolve interaction forces smaller than 1 pN on dielectric particles as small as 100 nm in diameter.

Stretching of a polymer chain anchored to a surface: the massive field theory approach

NASA Astrophysics Data System (ADS)

Usatenko, Zoryana

2014-09-01

Taking into account the well-known correspondence between the field theoretical φ4 O(n)-vector model in the limit n → 0 and the behaviour of long-flexible polymer chains, the investigation of stretching of an ideal and a real polymer chain with excluded volume interactions in a good solvent anchored to repulsive and inert surfaces is performed. The calculations of the average stretching force which arises when the free end of a polymer chain moves away from a repulsive or inert surface are performed up to one-loop order of the massive field theory approach in fixed space dimensions d = 3. The analysis of the obtained results indicates that the average stretching force for a real polymer chain anchored to a repulsive surface demonstrates different behaviour for the cases \\tilde{z}\\ll1 and \\tilde{z}\\gg1 , where \\tilde{z}=z^\\prime/Rz . Besides, the results obtained in the framework of the massive field theory approach are in good agreement with previous theoretical results for an ideal polymer chain and results of a density functional theory approach for the region of small applied forces when deformation of a polymer chain in the direction of the applied force is not bigger than the linear extension of a polymer chain in this direction. The better agreement between these two methods is observed in the case where the number of monomers increases and the polymer chain becomes longer.

Polymer Brushes under High Load

PubMed Central

Balko, Suzanne M.; Kreer, Torsten; Costanzo, Philip J.; Patten, Tim E.; Johner, Albert; Kuhl, Tonya L.; Marques, Carlos M.

2013-01-01

Polymer coatings are frequently used to provide repulsive forces between surfaces in solution. After 25 years of design and study, a quantitative model to explain and predict repulsion under strong compression is still lacking. Here, we combine experiments, simulations, and theory to study polymer coatings under high loads and demonstrate a validated model for the repulsive forces, proposing that this universal behavior can be predicted from the polymer solution properties. PMID:23516470

Electrostatic forces in planetary rings

NASA Technical Reports Server (NTRS)

Goertz, C. K.; Shan, Linhua; Havnes, O.

1988-01-01

The average charge on a particle in a particle-plasma cloud, the plasma potential inside the cloud, and the Coulomb force acting on the particle are calculated. The net repulsive electrostatic force on a particle depends on the plasma density, temperature, density of particles, particle size, and the gradient of the particle density. In a uniformly dense ring the electrostatic repulsion is zero. It is also shown that the electrostatic force acts like a pressure force, that even a collisionless ring can be stable against gravitational collapse, and that a finite ring thickness does not necessarily imply a finite velocity dispersion. A simple criterion for the importance of electrostatic forces in planetary rings is derived which involves the calculation of the vertical ring thickness which would result if only electrostatic repulsion were responsible for the finite ring thickness. Electrostatic forces are entirely negligible in the main rings of Saturn and the E and G rings. They may also be negligible in the F ring. However, the Uranian rings and Jupiter's ring seem to be very much influenced by electrostatic repulsion. In fact, electrostatic forces could support a Jovian ring which is an order of magnitude more dense than observed.

Hadron resonance gas with repulsive interactions and fluctuations of conserved charges

DOE PAGES

Huovinen, Pasi; Petreczky, Peter

2017-12-11

We discuss the role of repulsive baryon-baryon interactions in a hadron gas using relativistic virial expansion and repulsive mean field approaches. The fluctuations of the baryon number as well as strangeness-baryon correlations are calculated in the hadron resonance gas with repulsive interactions and compared with the recent lattice QCD results. In particular, we calculate the difference between the second and fourth order fluctuations and correlations of baryon number and strangeness, that have been proposed as probes of deconfinement. We show that for not too high temperatures these differences could be understood in terms of repulsive interactions.

Long-range repulsion of colloids driven by ion exchange and diffusiophoresis

PubMed Central

Florea, Daniel; Musa, Sami; Huyghe, Jacques M. R.; Wyss, Hans M.

2014-01-01

Interactions between surfaces and particles in aqueous suspension are usually limited to distances smaller than 1 μm. However, in a range of studies from different disciplines, repulsion of particles has been observed over distances of up to hundreds of micrometers, in the absence of any additional external fields. Although a range of hypotheses have been suggested to account for such behavior, the physical mechanisms responsible for the phenomenon still remain unclear. To identify and isolate these mechanisms, we perform detailed experiments on a well-defined experimental system, using a setup that minimizes the effects of gravity and convection. Our experiments clearly indicate that the observed long-range repulsion is driven by a combination of ion exchange, ion diffusion, and diffusiophoresis. We develop a simple model that accounts for our data; this description is expected to be directly applicable to a wide range of systems exhibiting similar long-range forces. PMID:24748113

Investigation of electric charge on inertial particle dynamics in turbulence

NASA Astrophysics Data System (ADS)

Lu, Jiang; Shaw, Raymond

2014-11-01

The behavior of electrically charged, inertial particles in homogeneous, isotropic turbulence is investigated. Both like-charged and oppositely-charged particle interactions are considered. Direct numerical simulations (DNS) of turbulence in a periodic box using the pseudospectral numerical method are performed, with Lagrangian tracking of the particles. We study effects of mutual electrostatic repulsion and attraction on the particle dynamics, as quantified by the radial distribution function (RDF) and the radial relative velocity. For the like-charged particle case, the Coulomb force leads to a short range repulsion behavior and an RDF reminiscent of that for a dilute gas. For the oppositely-charged particle case, the Coulomb force increases the RDF beyond that already occurring for neutral inertial particles. For both cases, the relative velocities are calculated as a function of particle separation distance and show distinct deviations from the expected scaling within the dissipation range. This research was supported by NASA Grant NNX113AF90G.

Energy component analysis of π interactions.

PubMed

Sherrill, C David

2013-04-16

Fundamental features of biomolecules, such as their structure, solvation, and crystal packing and even the docking of drugs, rely on noncovalent interactions. Theory can help elucidate the nature of these interactions, and energy component analysis reveals the contributions from the various intermolecular forces: electrostatics, London dispersion terms, induction (polarization), and short-range exchange-repulsion. Symmetry-adapted perturbation theory (SAPT) provides one method for this type of analysis. In this Account, we show several examples of how SAPT provides insight into the nature of noncovalent π-interactions. In cation-π interactions, the cation strongly polarizes electrons in π-orbitals, leading to substantially attractive induction terms. This polarization is so important that a cation and a benzene attract each other when placed in the same plane, even though a consideration of the electrostatic interactions alone would suggest otherwise. SAPT analysis can also support an understanding of substituent effects in π-π interactions. Trends in face-to-face sandwich benzene dimers cannot be understood solely in terms of electrostatic effects, especially for multiply substituted dimers, but SAPT analysis demonstrates the importance of London dispersion forces. Moreover, detailed SAPT studies also reveal the critical importance of charge penetration effects in π-stacking interactions. These effects arise in cases with substantial orbital overlap, such as in π-stacking in DNA or in crystal structures of π-conjugated materials. These charge penetration effects lead to attractive electrostatic terms where a simpler analysis based on atom-centered charges, electrostatic potential plots, or even distributed multipole analysis would incorrectly predict repulsive electrostatics. SAPT analysis of sandwich benzene, benzene-pyridine, and pyridine dimers indicates that dipole/induced-dipole terms present in benzene-pyridine but not in benzene dimer are relatively unimportant. In general, a nitrogen heteroatom contracts the electron density, reducing the magnitude of both the London dispersion and the exchange-repulsion terms, but with an overall net increase in attraction. Finally, using recent advances in SAPT algorithms, researchers can now perform SAPT computations on systems with 200 atoms or more. We discuss a recent study of the intercalation complex of proflavine with a trinucleotide duplex of DNA. Here, London dispersion forces are the strongest contributors to binding, as is typical for π-π interactions. However, the electrostatic terms are larger than usual on a fractional basis, which likely results from the positive charge on the intercalator and its location between two electron-rich base pairs. These cation-π interactions also increase the induction term beyond those of typical noncovalent π-interactions.

Direct Measurements of Long-Range Repulsive Interactions in the L_α phase of Polymer-Coated Highly Flexible Membranes

NASA Astrophysics Data System (ADS)

Warriner, Heidi E.; Safinya, Cyrus R.

1997-03-01

Using two complimentary techniques, we have measured repulsive interactions in the L_α phase of very flexible membranes composed of the surfactant C12E5 and small amounts of polymer-lipids derived from polyethylene glycol (PEG-DMPE 5000, PEG-DMPE 2000 and PEG-DMPE 550). In the first method, the lamellar repeat distance of samples in equilibrium with a dextran solution of known osmotic pressure is determined, yielding a direct measurement of pressure versus distance. These data immediately differentiate the repulsive interaction between flexible polymer-decorated membranes from polymer-brush forces found in rigid lamellar systems. In the second method, fits to high-resolution x-ray data yield the η parameter, proportional to (κB)-1\\over2, where B is the layer compressional modulus and κ is the bending rigidity of a single membrane. Combining the two types of data to eliminate B, one can quantitatively determine the κ of a decorated membrane as a function of polymer-lipid concentration. For the bare C12E5 membrane, where κ is known , a direct comparison of the compressibility modulus values derived via the two methods is also possible. This work supported by NSF-DMR-9624091; PRF-31352-AC7 CULAR-STB/UC:96-118.

Origins of microstructural transformations in charged vesicle suspensions: the crowding hypothesis.

PubMed

Seth, Mansi; Ramachandran, Arun; Murch, Bruce P; Leal, L Gary

2014-09-02

It is observed that charged unilamellar vesicles in a suspension can spontaneously deflate and subsequently transition to form bilamellar vesicles, even in the absence of externally applied triggers such as salt or temperature gradients. We provide strong evidence that the driving force for this deflation-induced transition is the repulsive electrostatic pressure between charged vesicles in concentrated suspensions, above a critical effective volume fraction. We use volume fraction measurements and cryogenic transmission electron microscopy imaging to quantitatively follow both the macroscopic and microstructural time-evolution of cationic diC18:1 DEEDMAC vesicle suspensions at different surfactant and salt concentrations. A simple model is developed to estimate the extent of deflation of unilamellar vesicles caused by electrostatic interactions with neighboring vesicles. It is determined that when the effective volume fraction of the suspension exceeds a critical value, charged vesicles in a suspension can experience "crowding" due to overlap of their electrical double layers, which can result in deflation and subsequent microstructural transformations to reduce the effective volume fraction of the suspension. Ordinarily in polydisperse colloidal suspensions, particles interacting via a repulsive potential transform into a glassy state above a critical volume fraction. The behavior of charged vesicle suspensions reported in this paper thus represents a new mechanism for the relaxation of repulsive interactions in crowded situations.

Swarming and pattern formation due to selective attraction and repulsion.

PubMed

Romanczuk, Pawel; Schimansky-Geier, Lutz

2012-12-06

We discuss the collective dynamics of self-propelled particles with selective attraction and repulsion interactions. Each particle, or individual, may respond differently to its neighbours depending on the sign of their relative velocity. Thus, it is able to distinguish approaching (coming closer) and retreating (moving away) individuals. This differentiation of the social response is motivated by the response to looming visual stimuli and may be seen as a generalization of the previously proposed escape and pursuit interactions motivated by empirical evidence for cannibalism as a driving force of collective migration in locusts and Mormon crickets. The model can account for different types of behaviour such as pure attraction, pure repulsion or escape and pursuit, depending on the values (signs) of the different response strengths. It provides, in the light of recent experimental results, an interesting alternative to previously proposed models of collective motion with an explicit velocity-alignment interaction. We discuss the derivation of a coarse-grained description of the system dynamics, which allows us to derive analytically the necessary condition for emergence of collective motion. Furthermore, we analyse systematically the onset of collective motion and clustering in numerical simulations of the model for varying interaction strengths. We show that collective motion arises only in a subregion of the parameter space, which is consistent with the analytical prediction and corresponds to an effective escape and/or pursuit response.

Measuring the interaction force between a high temperature superconductor and a permanent magnet

NASA Astrophysics Data System (ADS)

Valenzuela, S. O.; Jorge, G. A.; Rodríguez, E.

1999-11-01

Repulsive and attractive forces are both possible between a superconducting sample and a permanent magnet, and they can give rise to magnetic levitation or free-suspension phenomena, respectively. We show experiments to quantify this magnetic interaction, which represents a promising field with regard to short-term technological applications of high temperature superconductors. The measuring technique employs an electronic balance and a rare-earth magnet that induces a magnetic moment in a melt-textured YBa2Cu3O7 superconductor immersed in liquid nitrogen. The simple design of the experiments allows a fast and easy implementation in the advanced physics laboratory with a minimum cost. Actual levitation and suspension demonstrations can be done simultaneously as a help to interpret magnetic force measurements.

New Angles on Standard Force Fields: Toward a General Approach for Treating Atomic-Level Anisotropy

DOE PAGES

Van Vleet, Mary J.; Misquitta, Alston J.; Schmidt, J. R.

2017-12-21

Nearly all standard force fields employ the “sum-of-spheres” approximation, which models intermolecular interactions purely in terms of interatomic distances. Nonetheless, atoms in molecules can have significantly nonspherical shapes, leading to interatomic interaction energies with strong orientation dependencies. Neglecting this “atomic-level anisotropy” can lead to significant errors in predicting interaction energies. Herein, we propose a simple, transferable, and computationally efficient model (MASTIFF) whereby atomic-level orientation dependence can be incorporated into ab initio intermolecular force fields. MASTIFF includes anisotropic exchange-repulsion, charge penetration, and dispersion effects, in conjunction with a standard treatment of anisotropic long-range (multipolar) electrostatics. To validate our approach, we benchmarkmore » MASTIFF against various sum-of-spheres models over a large library of intermolecular interactions between small organic molecules. MASTIFF achieves quantitative accuracy, with respect to both high-level electronic structure theory and experiment, thus showing promise as a basis for “next-generation” force field development.« less

New Angles on Standard Force Fields: Toward a General Approach for Treating Atomic-Level Anisotropy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Van Vleet, Mary J.; Misquitta, Alston J.; Schmidt, J. R.

Nearly all standard force fields employ the “sum-of-spheres” approximation, which models intermolecular interactions purely in terms of interatomic distances. Nonetheless, atoms in molecules can have significantly nonspherical shapes, leading to interatomic interaction energies with strong orientation dependencies. Neglecting this “atomic-level anisotropy” can lead to significant errors in predicting interaction energies. Herein, we propose a simple, transferable, and computationally efficient model (MASTIFF) whereby atomic-level orientation dependence can be incorporated into ab initio intermolecular force fields. MASTIFF includes anisotropic exchange-repulsion, charge penetration, and dispersion effects, in conjunction with a standard treatment of anisotropic long-range (multipolar) electrostatics. To validate our approach, we benchmarkmore » MASTIFF against various sum-of-spheres models over a large library of intermolecular interactions between small organic molecules. MASTIFF achieves quantitative accuracy, with respect to both high-level electronic structure theory and experiment, thus showing promise as a basis for “next-generation” force field development.« less

Inter-layer potential for hexagonal boron nitride

NASA Astrophysics Data System (ADS)

Leven, Itai; Azuri, Ido; Kronik, Leeor; Hod, Oded

2014-03-01

A new interlayer force-field for layered hexagonal boron nitride (h-BN) based structures is presented. The force-field contains three terms representing the interlayer attraction due to dispersive interactions, repulsion due to anisotropic overlaps of electron clouds, and monopolar electrostatic interactions. With appropriate parameterization, the potential is able to simultaneously capture well the binding and lateral sliding energies of planar h-BN based dimer systems as well as the interlayer telescoping and rotation of double walled boron-nitride nanotubes of different crystallographic orientations. The new potential thus allows for the accurate and efficient modeling and simulation of large-scale h-BN based layered structures.

Ultracold fermions in a one-dimensional bipartite optical lattice: Metal-insulator transitions driven by shaking

NASA Astrophysics Data System (ADS)

Di Liberto, M.; Malpetti, D.; Japaridze, G. I.; Morais Smith, C.

2014-08-01

We theoretically investigate the behavior of a system of fermionic atoms loaded in a bipartite one-dimensional optical lattice that is under the action of an external time-periodic driving force. By using Floquet theory, an effective model is derived. The bare hopping coefficients are renormalized by zeroth-order Bessel functions of the first kind with different arguments for the nearest-neighbor and next-nearest-neighbor hopping. The insulating behavior characterizing the system at half filling in the absence of driving is dynamically suppressed, and for particular values of the driving parameter the system becomes either a standard metal or an unconventional metal with four Fermi points. The existence of the four-Fermi-point metal relies on the fact that, as a consequence of the shaking procedure, the next-nearest-neighbor hopping coefficients become significant compared to the nearest-neighbor ones. We use the bosonization technique to investigate the effect of on-site Hubbard interactions on the four-Fermi-point metal-insulator phase transition. Attractive interactions are expected to enlarge the regime of parameters where the unconventional metallic phase arises, whereas repulsive interactions reduce it. This metallic phase is known to be a Luther-Emery liquid (spin-gapped metal) for both repulsive and attractive interactions, contrary to the usual Hubbard model, which exhibits a Mott-insulator phase for repulsive interactions. Ultracold fermions in driven one-dimensional bipartite optical lattices provide an interesting platform for the realization of this long-studied four-Fermi-point unconventional metal.

Role of DNA-DNA Interactions on the Structure and Thermodynamics of Bacteriophages Lambda and P4

PubMed Central

Petrov, Anton S.; Harvey, Stephen C.

2010-01-01

Electrostatic interactions play an important role in both packaging of DNA inside bacteriophages and its release into bacterial cells. While at physiological conditions DNA strands repel each other, the presence of polyvalent cations such as spermine and spermidine in solutions leads to the formation of DNA condensates. In this study, we discuss packaging of DNA into bacteriophages P4 and Lambda under repulsive and attractive conditions using a coarse-grained model of DNA and capsids. Packaging under repulsive conditions leads to the appearance of the coaxial spooling conformations; DNA occupies all available space inside the capsid. Under the attractive potential both packed systems reveal toroidal conformations, leaving the central part of the capsids empty. We also present a detailed thermodynamic analysis of packaging and show that the forces required to pack the genomes in the presence of polyamines are significantly lower than those observed under repulsive conditions. The analysis reveals that in both the repulsive and attractive regimes the entropic penalty of DNA confinement has a significant non-negligible contribution into the total energy of packaging. Additionally we report the results of simulations of DNA condensation inside partially packed Lambda. We found that at low densities DNA behaves as free unconfined polymer and condenses into the toroidal structures; at higher densities rearrangement of the genome into toroids becomes hindered, and condensation results in the formation of non-equilibrium structures. In all cases packaging in a specific conformation occurs as a result of interplay between bending stresses experienced by the confined polymer and interactions between the strands. PMID:21074621

A smoothed particle hydrodynamics framework for modelling multiphase interactions at meso-scale

NASA Astrophysics Data System (ADS)

Li, Ling; Shen, Luming; Nguyen, Giang D.; El-Zein, Abbas; Maggi, Federico

2018-01-01

A smoothed particle hydrodynamics (SPH) framework is developed for modelling multiphase interactions at meso-scale, including the liquid-solid interaction induced deformation of the solid phase. With an inter-particle force formulation that mimics the inter-atomic force in molecular dynamics, the proposed framework includes the long-range attractions between particles, and more importantly, the short-range repulsive forces to avoid particle clustering and instability problems. Three-dimensional numerical studies have been conducted to demonstrate the capabilities of the proposed framework to quantitatively replicate the surface tension of water, to model the interactions between immiscible liquids and solid, and more importantly, to simultaneously model the deformation of solid and liquid induced by the multiphase interaction. By varying inter-particle potential magnitude, the proposed SPH framework has successfully simulated various wetting properties ranging from hydrophobic to hydrophilic surfaces. The simulation results demonstrate the potential of the proposed framework to genuinely study complex multiphase interactions in wet granular media.

Probing surface charge potentials of clay basal planes and edges by direct force measurements.

PubMed

Zhao, Hongying; Bhattacharjee, Subir; Chow, Ross; Wallace, Dean; Masliyah, Jacob H; Xu, Zhenghe

2008-11-18

The dispersion and gelation of clay suspensions have major impact on a number of industries, such as ceramic and composite materials processing, paper making, cement production, and consumer product formulation. To fundamentally understand controlling mechanisms of clay dispersion and gelation, it is necessary to study anisotropic surface charge properties and colloidal interactions of clay particles. In this study, a colloidal probe technique was employed to study the interaction forces between a silica probe and clay basal plane/edge surfaces. A muscovite mica was used as a representative of 2:1 phyllosilicate clay minerals. The muscovite basal plane was prepared by cleavage, while the edge surface was obtained by a microtome cutting technique. Direct force measurements demonstrated the anisotropic surface charge properties of the basal plane and edge surface. For the basal plane, the long-range forces were monotonically repulsive within pH 6-10 and the measured forces were pH-independent, thereby confirming that clay basal planes have permanent surface charge from isomorphic substitution of lattice elements. The measured interaction forces were fitted well with the classical DLVO theory. The surface potentials of muscovite basal plane derived from the measured force profiles were in good agreement with those reported in the literature. In the case of edge surfaces, the measured forces were monotonically repulsive at pH 10, decreasing with pH, and changed to be attractive at pH 5.6, strongly suggesting that the charge on the clay edge surfaces is pH-dependent. The measured force profiles could not be reasonably fitted with the classical DLVO theory, even with very small surface potential values, unless the surface roughness was considered. The surface element integration (SEI) method was used to calculate the DLVO forces to account for the surface roughness. The surface potentials of the muscovite edges were derived by fitting the measured force profiles with the surface element integrated DLVO model. The point of zero charge of the muscovite edge surface was estimated to be pH 7-8.

Modeling polymer-induced interactions between two grafted surfaces: comparison between interfacial statistical associating fluid theory and self-consistent field theory.

PubMed

Jain, Shekhar; Ginzburg, Valeriy V; Jog, Prasanna; Weinhold, Jeffrey; Srivastava, Rakesh; Chapman, Walter G

2009-07-28

The interaction between two polymer grafted surfaces is important in many applications, such as nanocomposites, colloid stabilization, and polymer alloys. In our previous work [Jain et al., J. Chem. Phys. 128, 154910 (2008)], we showed that interfacial statistical associating fluid density theory (iSAFT) successfully calculates the structure of grafted polymer chains in the absence/presence of a free polymer. In the current work, we have applied this density functional theory to calculate the force of interaction between two such grafted monolayers in implicit good solvent conditions. In particular, we have considered the case where the segment sizes of the free (sigma(f)) and grafted (sigma(g)) polymers are different. The interactions between the two monolayers in the absence of the free polymer are always repulsive. However, in the presence of the free polymer, the force either can be purely repulsive or can have an attractive minimum depending upon the relative chain lengths of the free (N(f)) and grafted polymers (N(g)). The attractive minimum is observed only when the ratio alpha = N(f)/N(g) is greater than a critical value. We find that these critical values of alpha satisfy the following scaling relation: rho(g) square root(N(g)) beta(3) proportional to alpha(-lambda), where beta = sigma(f)/sigma(g) and lambda is the scaling exponent. For beta = 1 or the same segment sizes of the free and grafted polymers, this scaling relation is in agreement with those from previous theoretical studies using self-consistent field theory (SCFT). Detailed comparisons between iSAFT and SCFT are made for the structures of the monolayers and their forces of interaction. These comparisons lead to interesting implications for the modeling of nanocomposite thermodynamics.

Current quantization and fractal hierarchy in a driven repulsive lattice gas.

PubMed

Rotondo, Pietro; Sellerio, Alessandro Luigi; Glorioso, Pietro; Caracciolo, Sergio; Cosentino Lagomarsino, Marco; Gherardi, Marco

2017-11-01

Driven lattice gases are widely regarded as the paradigm of collective phenomena out of equilibrium. While such models are usually studied with nearest-neighbor interactions, many empirical driven systems are dominated by slowly decaying interactions such as dipole-dipole and Van der Waals forces. Motivated by this gap, we study the nonequilibrium stationary state of a driven lattice gas with slow-decayed repulsive interactions at zero temperature. By numerical and analytical calculations of the particle current as a function of the density and of the driving field, we identify (i) an abrupt breakdown transition between insulating and conducting states, (ii) current quantization into discrete phases where a finite current flows with infinite differential resistivity, and (iii) a fractal hierarchy of excitations, related to the Farey sequences of number theory. We argue that the origin of these effects is the competition between scales, which also causes the counterintuitive phenomenon that crystalline states can melt by increasing the density.

Current quantization and fractal hierarchy in a driven repulsive lattice gas

NASA Astrophysics Data System (ADS)

Rotondo, Pietro; Sellerio, Alessandro Luigi; Glorioso, Pietro; Caracciolo, Sergio; Cosentino Lagomarsino, Marco; Gherardi, Marco

2017-11-01

Driven lattice gases are widely regarded as the paradigm of collective phenomena out of equilibrium. While such models are usually studied with nearest-neighbor interactions, many empirical driven systems are dominated by slowly decaying interactions such as dipole-dipole and Van der Waals forces. Motivated by this gap, we study the nonequilibrium stationary state of a driven lattice gas with slow-decayed repulsive interactions at zero temperature. By numerical and analytical calculations of the particle current as a function of the density and of the driving field, we identify (i) an abrupt breakdown transition between insulating and conducting states, (ii) current quantization into discrete phases where a finite current flows with infinite differential resistivity, and (iii) a fractal hierarchy of excitations, related to the Farey sequences of number theory. We argue that the origin of these effects is the competition between scales, which also causes the counterintuitive phenomenon that crystalline states can melt by increasing the density.

Topological interactions in spacetimes with thick line defects

NASA Astrophysics Data System (ADS)

Moraes, Fernando; Carvalho, A. M.; Costa, Ismael V.; Oliveira, F. A.; Furtado, Claudio

2003-08-01

In this work we study the topologically induced electric self-energy and self-force on a long, straight, wire in two distinct, but similar, spacetimes: (i) the Gott-Hiscock thick cosmic string spacetime, and (ii) the spacetime of a continuous distribution of infinitely thin cosmic strings over a disk of finite radius. In each case we obtain the electric self-energy and self-force both in the internal and external regions of the defect distribution. The self-force is always repulsive, independently of the sign of the charge, and is maximum on the string’s surface, in both cases.

Forces dictating colloidal interactions between viruses and soil

USGS Publications Warehouse

Chattopadhyay, Sandip; Puls, Robert W.

2000-01-01

The fate and transport of viruses in soil and aquatic environments were studied with respect to the different forces involved in the process of sorption of these viruses on soil particles. In accordance with the classical DLVO theory, we have calculated the repulsive electrostatic forces and the attractive van der Waals forces. Bacteriophages have been used as model sorbates, while different clays have been used as model sorbents. The equations used for the determination of the change in free energy for the process (ΔG) takes into consideration the roughness of the sorbent surfaces. Results indicate that attractive van der Waals forces predominate the process of sorption of the selected bacteriophages on clays.

Anomalous Protein-Protein Interactions in Multivalent Salt Solution.

PubMed

Pasquier, Coralie; Vazdar, Mario; Forsman, Jan; Jungwirth, Pavel; Lund, Mikael

2017-04-13

The stability of aqueous protein solutions is strongly affected by multivalent ions, which induce ion-ion correlations beyond the scope of classical mean-field theory. Using all-atom molecular dynamics (MD) and coarse grained Monte Carlo (MC) simulations, we investigate the interaction between a pair of protein molecules in 3:1 electrolyte solution. In agreement with available experimental findings of "reentrant protein condensation", we observe an anomalous trend in the protein-protein potential of mean force with increasing electrolyte concentration in the order: (i) double-layer repulsion, (ii) ion-ion correlation attraction, (iii) overcharge repulsion, and in excess of 1:1 salt, (iv) non Coulombic attraction. To efficiently sample configurational space we explore hybrid continuum solvent models, applicable to many-protein systems, where weakly coupled ions are treated implicitly, while strongly coupled ones are treated explicitly. Good agreement is found with the primitive model of electrolytes, as well as with atomic models of protein and solvent.

Electrostatics of polymer translocation events in electrolyte solutions.

PubMed

Buyukdagli, Sahin; Ala-Nissila, T

2016-07-07

We develop an analytical theory that accounts for the image and surface charge interactions between a charged dielectric membrane and a DNA molecule translocating through the membrane. Translocation events through neutral carbon-based membranes are driven by a competition between the repulsive DNA-image-charge interactions and the attractive coupling between the DNA segments on the trans and the cis sides of the membrane. The latter effect is induced by the reduction of the coupling by the dielectric membrane. In strong salt solutions where the repulsive image-charge effects dominate the attractive trans-cis coupling, the DNA molecule encounters a translocation barrier of ≈10 kBT. In dilute electrolytes, the trans-cis coupling takes over image-charge forces and the membrane becomes a metastable attraction point that can trap translocating polymers over long time intervals. This mechanism can be used in translocation experiments in order to control DNA motion by tuning the salt concentration of the solution.

Nanophotonic force microscopy: Characterizing particle–surface interactions using near-field photonics

DOE PAGES

Schein, Perry; Kang, Pilgyu; O’Dell, Dakota; ...

2015-01-27

Direct measurements of particle–surface interactions are important for characterizing the stability and behavior of colloidal and nanoparticle suspensions. Current techniques are limited in their ability to measure pico-Newton scale interaction forces on submicrometer particles due to signal detection limits and thermal noise. In this paper, we present a new technique for making measurements in this regime, which we refer to as nanophotonic force microscopy. Using a photonic crystal resonator, we generate a strongly localized region of exponentially decaying, near-field light that allows us to confine small particles close to a surface. From the statistical distribution of the light intensity scatteredmore » by the particle we are able to map out the potential well of the trap and directly quantify the repulsive force between the nanoparticle and the surface. Finally, as shown in this Letter, our technique is not limited by thermal noise, and therefore, we are able to resolve interaction forces smaller than 1 pN on dielectric particles as small as 100 nm in diameter.« less

The Layered Structure of The Universe

NASA Astrophysics Data System (ADS)

Kursunoglu, Behram N.

2003-06-01

It has now become a habit for the cosmologists to introduce attraction or repulsion generating substances to describe the observed cosmological behavior of matter. Examples are dark energy to provide repulsive force to cause increasing acceleration accompanying the expansion of the universe, quintessence providing repulsive force. In this paper we believe that what is needed in the final analysis is attraction and repulsion. We show here that universe can be conceived to consist of attractive and repulsive layers of matter expanding with increasing acceleration. The generalized theory of gravitation as developed originally by Einstein and Schrödinger as a non-symmetric theory was modified by this author using Bianchi-Einstein Identities yielding coupling between the field and electric charge as well as between the field and magnetic charge, and there appears a fundamental length parameter ro where quintessence constitute magnetic repulsive layers while dark energy and all other kinds of names invented by cosmologists refer to attractive electric layers. This layered structure of the universe resembles the layered structure of the elementary particle predicted by this theory decades ago (1, 3, and 6). This implies a layer Doughnut structure of the universe. We have therefore, obtained a unification of the structure of the universe and the structure of elementary particles. Overall the forces consist of long range attractive, long range repulsive, short-range attractive, and short-range repulsive variety. We further discovered the existence of space oscillations whose roles in the expansion of the universe with increasing acceleration and further the impact in the propagation of the gravitational waves can be expected to play a role in their observation.

Nuclear Stability and Nucleon-Nucleon Interactions in Introductory and General Chemistry Textbooks

ERIC Educational Resources Information Center

Millevolte, Anthony

2010-01-01

The nucleus is a highly dense and highly charged substructure of atoms. In the nuclei of all atoms beyond hydrogen, multiple protons are in close proximity to each other in spite of strong electrostatic repulsions between them. The attractive internucleon strong force is described and its origin explained by using a simple quark model for the…

Pipette-surface interaction: current enhancement and intrinsic force.

PubMed

Clarke, Richard W; Zhukov, Alexander; Richards, Owen; Johnson, Nicholas; Ostanin, Victor; Klenerman, David

2013-01-09

There is an intrinsic repulsion between glass and cell surfaces that allows noninvasive scanning ion conductance microscopy (SICM) of cells and which must be overcome in order to form the gigaseals used for patch clamping investigations of ion channels. However, the interactions of surfaces in physiological solutions of electrolytes, including the presence of this repulsion, for example, do not obviously agree with the standard Derjaguin-Landau-Verwey-Overbeek (DLVO) colloid theory accurate at much lower salt concentrations. In this paper we investigate the interactions of glass nanopipettes in this high-salt regime with a variety of surfaces and propose a way to resolve DLVO theory with the results. We demonstrate the utility of this understanding to SICM by topographically mapping a live cell's cytoskeleton. We also report an interesting effect whereby the ion current though a nanopipette can increase under certain conditions upon approaching an insulating surface, rather than decreasing as would be expected. We propose that this is due to electroosmotic flow separation, a high-salt electrokinetic effect. Overall these experiments yield key insights into the fundamental interactions that take place between surfaces in strong solutions of electrolytes.

Effect of attractive interactions between polymers on the effective force acting between colloids immersed in a polymer system: Analytic liquid-state theory.

PubMed

Chervanyov, A I

2016-12-28

By making use of the polymer reference interaction site model, we analytically study the effect of attractive interactions between polymers on the effective forces acting between colloids immersed in a polymer system. The performed theoretical analysis has no restrictions with respect to the polymer density and relative sizes of the colloids and polymers. The polymer mediated (PM) potential acting between colloids is shown to significantly depend on the strength and range of the polymer-polymer interactions. In the nano-particle limit, where the colloid radius is much smaller than the polymer gyration radius, the presence of attractive polymer-polymer interactions causes only quantitative changes to the PM potential. In the opposite limit of relatively large colloids, the polymer-polymer interactions revert the sign of the total effective force acting between colloids so that this force becomes attractive at sufficiently large polymer densities. With the objective to study an intricate interplay between the attractive PM forces and steric repulsion in different polymer density regimes, we calculate the second virial coefficient B of the total effective potential acting between colloids. The dependence of B on the polymer density is discussed in detail, revealing several novel features of the PM interactions caused by the presence of attractive polymer-polymer interactions.

True non-contact atomic force microscopy imaging of heterogeneous biological samples in liquids: topography and material contrast.

PubMed

Almonte, Lisa; Colchero, Jaime

2017-02-23

The present work analyses how the tip-sample interaction signals critically determine the operation of an Atomic Force Microscope (AFM) set-up immersed in liquid. On heterogeneous samples, the conservative tip-sample interaction may vary significantly from point to point - in particular from attractive to repulsive - rendering correct feedback very challenging. Lipid membranes prepared on a mica substrate are analyzed as reference samples which are locally heterogeneous (material contrast). The AFM set-up is operated dynamically at low oscillation amplitude and all available experimental data signals - the normal force, as well as the amplitude and frequency - are recorded simultaneously. From the analysis of how the dissipation (oscillation amplitude) and the conservative interaction (normal force and resonance frequency) vary with the tip-sample distance we conclude that dissipation is the only appropriate feedback source for stable and correct topographic imaging. The normal force and phase then carry information about the sample composition ("chemical contrast"). Dynamic AFM allows imaging in a non-contact regime where essentially no forces are applied, rendering dynamic AFM a truly non-invasive technique.

Manipulation and Investigation of Uniformly-Spaced Nanowire Array on a Substrate via Dielectrophoresis and Electrostatic Interaction.

PubMed

Choi, U Hyeok; Park, Ji Hun; Kim, Jaekyun

2018-06-21

Directed-assembly of nanowires on the dielectrics-covered parallel electrode structure is capable of producing uniformly-spaced nanowire array at the electrode gap due to dielectrophoretic nanowire attraction and electrostatic nanowire repulsion. Beyond uniformly-spaced nanowire array formation, the control of spacing in the array is beneficial in that it should be the experimental basis of the precise positioning of functional nanowires on a circuit. Here, we investigate the material parameters and bias conditions to modulate the nanowire spacing in the ordered array, where the nanowire array formation is readily attained due to the electrostatic nanowire interaction. A theoretical model for the force calculation and the simulation of the induced charge in the assembled nanowire verifies that the longer nanowires on thicker dielectric layer tend to be assembled with a larger pitch due to the stronger nanowire-nanowire electrostatic repulsion, which is consistent with the experimental results. It was claimed that the stronger dielectrophoretic force is likely to attract more nanowires that are suspended in solution at the electrode gap, causing them to be less-spaced. Thus, we propose a generic mechanism, competition of dielectrophoretic and electrostatic force, to determine the nanowire pitch in an ordered array. Furthermore, this spacing-controlled nanowire array offers a way to fabricate the high-density nanodevice array without nanowire registration.

Repulsive hydration forces between calcite surfaces and their effect on the brittle strength of calcite-bearing rocks

NASA Astrophysics Data System (ADS)

Røyne, Anja; Dalby, Kim N.; Hassenkam, Tue

2015-06-01

The long-term mechanical strength of calcite-bearing rocks is highly dependent on the presence and nature of pore fluids, and it has been suggested that the observed effects are due to changes in nanometer-scale surface forces near fracture tips and grain contacts. In this letter, we present measurements of forces between two calcite surfaces in air and water-glycol mixtures using the atomic force microscope. We show a time- and load-dependent adhesion at low water concentrations and a strong repulsion in the presence of water, which is most likely due to hydration of the strongly hydrophilic calcite surfaces. We argue that this hydration repulsion can explain the commonly observed water-induced decrease in strength in calcitic rocks and single calcite crystals. Furthermore, this relatively simple experimental setup may serve as a useful tool for analyzing surface forces in other mineral-fluid combinations.

The observation of negative permittivity in stripe and bubble phases

NASA Astrophysics Data System (ADS)

Smet, Jurgen

The physics of itinerant two-dimensional electrons is by and large governed by repulsive Coulomb forces. However, cases exist where the interplay of attractive and repulsive interaction components may instigate spontaneous symmetry lowering and clustering of charges in geometric patterns such as bubbles and stripes, provided these interactions act on different length scales. The existence of these phases in higher Landau levels has so far been concluded from transport behavior. Here, we report surface acoustic wave experiments. They probe the permittivity at small wave vector. This technique offers true directionality, whereas in transport the current distribution is complex and strongly affected by the inhomogeneous density pattern. Outside the charge density wave regime, the measured permittivity is always positive. However, negative permittivity is observed in the bubble phase irrespective of the propagation direction. For the stripe phase the permittivity takes on both positive as well as negative values depending on the propagation direction. This confirms the stripe phase to be a strongly anisotropic medium. The observation of negative permittivity is considered an immediate consequence of the exchange related attractive interaction. It makes charge clustering favorable in higher Landau levels where the repulsive direct Coulomb interaction acts on a longer length scale and is responsible for a negative compressibility of the electronic system. This work has been carried out with B. Friess, K. von Klitzing (MPI-FKF), Y. Peng, F. von Oppen (FU Berlin), B. Rosenow (Uni Leipzig) and V. Umansky (Weizmann Institute of Science).

Do the repulsive and attractive pair forces play separate roles for the physics of liquids?

PubMed

Bøhling, Lasse; Veldhorst, Arno A; Ingebrigtsen, Trond S; Bailey, Nicholas P; Hansen, Jesper S; Toxvaerd, Søren; Schrøder, Thomas B; Dyre, Jeppe C

2013-01-23

According to standard liquid-state theory repulsive and attractive pair forces play distinct roles for the physics of liquids. This paradigm is put into perspective here by demonstrating a continuous series of pair potentials that have virtually the same structure and dynamics, although only some of them have attractive forces of significance. Our findings reflect the fact that the motion of a given particle is determined by the total force on it, whereas the quantity usually discussed in liquid-state theory is the individual pair force.

Bi-stable frequency up-conversion piezoelectric energy harvester driven by non-contact magnetic repulsion

NASA Astrophysics Data System (ADS)

Tang, Q. C.; Yang, Y. L.; Li, Xinxin

2011-12-01

This paper presents miniaturized energy harvesters, where the frequency up-conversion technique is used to improve the bandwidth of vibration energy harvesters. The proposed and developed miniature piezoelectric energy harvester utilizes magnetic repulsion forces to achieve non-contact frequency up-conversion, thereby avoiding mechanical collision and wear for long-term working durability. A pair of piezoelectric resonant cantilevers is micro-fabricated to generate electric power. A simplified model involving linear oscillators and magnetic interaction is deployed to demonstrate the feasibility of the device design. A bench-top harvester has been fabricated and characterized, resulting in average power generation of over 10 µW within a broad frequency range of 10-22 Hz under 1g acceleration.

Matrix theory for baryons: an overview of holographic QCD for nuclear physics.

PubMed

Aoki, Sinya; Hashimoto, Koji; Iizuka, Norihiro

2013-10-01

We provide, for non-experts, a brief overview of holographic QCD (quantum chromodynamics) and a review of the recent proposal (Hashimoto et al 2010 (arXiv:1003.4988[hep-th])) of a matrix-like description of multi-baryon systems in holographic QCD. Based on the matrix model, we derive the baryon interaction at short distances in multi-flavor holographic QCD. We show that there is a very universal repulsive core of inter-baryon forces for a generic number of flavors. This is consistent with a recent lattice QCD analysis for Nf = 2, 3 where the repulsive core looks universal. We also provide a comparison of our results with the lattice QCD and the operator product expansion analysis.

Simulation of shear thickening in attractive colloidal suspensions.

PubMed

Pednekar, Sidhant; Chun, Jaehun; Morris, Jeffrey F

2017-03-01

The influence of attractive forces between particles under conditions of large particle volume fraction, ϕ, is addressed using numerical simulations which account for hydrodynamic, Brownian, conservative and frictional contact forces. The focus is on conditions for which a significant increase in the apparent viscosity at small shear rates, and possibly the development of a yield stress, is observed. The high shear rate behavior for Brownian suspensions has been shown in recent work [R. Mari, R. Seto, J. F. Morris and M. M. Denn PNAS, 2015, 112, 15326-15330] to be captured by the inclusion of pairwise forces of two forms, one a contact frictional interaction and the second a repulsive force often found in stabilized colloidal dispersions. Under such conditions, shear thickening is observed when shear stress is comparable to the sum of the Brownian stress, kT/a 3 , and a characteristic stress based on the combination of interparticle force, i.e. σ ∼ F 0 /a 2 with kT the thermal energy, F 0 the repulsive force scale and a the particle radius. At sufficiently large ϕ, this shear thickening can be very abrupt. Here it is shown that when attractive interactions are present with the noted forces, the shear thickening is obscured, as the viscosity shear thins with increasing shear rate, eventually descending from an infinite value (yield stress conditions) to a plateau at large stress; this plateau is at the same level as the large-shear rate viscosity found in the shear thickened state without attractive forces. It is shown that this behavior is consistent with prior observations in shear thickening suspensions modified to be attractive through depletion flocculation [V. Gopalakrishnan and C. F. Zukoski J. Rheol., 2004, 48, 1321-1344]. The contributions of the contact, attractive, and hydrodynamics forces to the bulk stress are presented, as are the contact networks found at different attractive strengths.

Repulsive nature of optical potentials for high-energy heavy-ion scattering

DOE Office of Scientific and Technical Information (OSTI.GOV)

Furumoto, T.; Sakuragi, Y.; Yamamoto, Y.

2010-10-15

The recent works by the present authors predicted that the real part of heavy-ion optical potentials changes its character from attraction to repulsion around the incident energy per nucleon E/A=200-300 MeV on the basis of the complex G-matrix interaction and the double-folding model (DFM) and revealed that the three-body force plays an important role there. In the present paper, we have precisely analyzed the energy dependence of the calculated DFM potentials and its relation to the elastic-scattering angular distributions in detail in the case of the {sup 12}C+{sup 12}C system in the energy range of E/A=100-400 MeV. The tensor forcemore » contributes substantially to the energy dependence of the real part of the DFM potentials and plays an important role to lower the attractive-to-repulsive transition energy. The nearside and farside (N/F) decompositions of the elastic-scattering amplitudes clarify the close relation between the attractive-to-repulsive transition of the potentials and the characteristic evolution of the calculated angular distributions with the increase of the incident energy. Based on the present analysis, we propose experimental measurements for the predicted strong diffraction phenomena of the elastic-scattering angular distribution caused by the N/F interference around the attractive-to-repulsive transition energy together with the reduced diffractions below and above the transition energy.« less

Drag and Lift Forces Between a Rotating Conductive Sphere and a Cylindrical Magnet

NASA Technical Reports Server (NTRS)

Nurge, Mark A.; Youngquist, Robert C.

2017-01-01

Modeling the interaction between a non-uniform magnetic field and a rotating conductive object allows study of the drag force which is used in applications such as eddy current braking and linear induction motors as well as the transition to a repulsive force that is the basis for magnetic levitation systems. Here, we study the interaction between a non-uniform field generated by a cylindrical magnet and a rotating conductive sphere. Each eddy current in the sphere generates a magnetic field which in turn generates another eddy current, eventually feeding back on itself. A two step mathematics process is developed to find a closed form solution in terms of only two eddy currents. However, the complete solution requires decomposition of the magnetic field into a summation of spherical harmonics, making it more suitable for a graduate level electromagnetism lecture or lab. Finally, the forces associated with these currents are calculated and then verified experimentally.

Drag and lift forces between a rotating conductive sphere and a cylindrical magnet

NASA Astrophysics Data System (ADS)

Nurge, Mark A.; Youngquist, Robert C.; Starr, Stanley O.

2018-06-01

Modeling the interaction between a non-uniform magnetic field and a rotating conductive object provides insight into the drag force, which is used in applications such as eddy current braking and linear induction motors, as well as the transition to a repulsive force, which is the basis for magnetic levitation systems. Here, we study the interaction between a non-uniform field generated by a cylindrical magnet and a rotating conductive sphere. Each eddy current in the sphere generates a magnetic field which in turn generates another eddy current, eventually feeding back on itself. A two-step mathematical process is developed to find a closed-form solution in terms of only three eddy currents. However, the complete solution requires decomposition of the magnetic field into a summation of spherical harmonics, making it more suitable for a graduate-level electromagnetism lecture or lab. Finally, the forces associated with these currents are calculated and then verified experimentally.

Ion Trap Quantum Computing

DTIC Science & Technology

2011-12-01

quantum computer architecture schemes, but there are several problems that will be discussed later. 15 IV. ION TRAPS Wolfgang Paul was the first...famous physics experiment [62]. Wolfgang Paul demonstrated a similar apparatus during his Nobel Prize speech [63]. This device is hyperbolic-parabolic...Although it does not apply to linear traps, it is useful to understand the interaction between the Coulomb force and the repulsive quantum-mechanical Pauli

Membrane-Mediated Cooperativity of Proteins

NASA Astrophysics Data System (ADS)

Weikl, Thomas R.

2018-04-01

Besides direct protein-protein interactions, indirect interactions mediated by membranes play an important role for the assembly and cooperative function of proteins in membrane shaping and adhesion. The intricate shapes of biological membranes are generated by proteins that locally induce membrane curvature. Indirect curvature-mediated interactions between these proteins arise because the proteins jointly affect the bending energy of the membranes. These curvature-mediated interactions are attractive for crescent-shaped proteins and are a driving force in the assembly of the proteins during membrane tubulation. Membrane adhesion results from the binding of receptor and ligand proteins that are anchored in the apposing membranes. The binding of these proteins strongly depends on nanoscale shape fluctuations of the membranes, leading to a fluctuation-mediated binding cooperativity. A length mismatch between receptor-ligand complexes in membrane adhesion zones causes repulsive curvature-mediated interactions that are a driving force for the length-based segregation of proteins during membrane adhesion.

Elasticity-induced force reversal between active spinning particles in dense passive media

PubMed Central

Aragones, J. L.; Steimel, J. P.; Alexander-Katz, A.

2016-01-01

The self-organization of active particles is governed by their dynamic effective interactions. Such interactions are controlled by the medium in which such active agents reside. Here we study the interactions between active agents in a dense non-active medium. Our system consists of actuated, spinning, active particles embedded in a dense monolayer of passive, or non-active, particles. We demonstrate that the presence of the passive monolayer alters markedly the properties of the system and results in a reversal of the forces between active spinning particles from repulsive to attractive. The origin of such reversal is due to the coupling between the active stresses and elasticity of the system. This discovery provides a mechanism for the interaction between active agents in complex and structured media, opening up opportunities to tune the interaction range and directionality via the mechanical properties of the medium. PMID:27112961

Quantum Monte Carlo calculations of neutron matter with chiral three-body forces

DOE PAGES

Tews, I.; Gandolfi, Stefano; Gezerlis, A.; ...

2016-02-02

Chiral effective field theory (EFT) enables a systematic description of low-energy hadronic interactions with controlled theoretical uncertainties. For strongly interacting systems, quantum Monte Carlo (QMC) methods provide some of the most accurate solutions, but they require as input local potentials. We have recently constructed local chiral nucleon-nucleon (NN) interactions up to next-to-next-to-leading order (N 2LO). Chiral EFT naturally predicts consistent many-body forces. In this paper, we consider the leading chiral three-nucleon (3N) interactions in local form. These are included in auxiliary field diffusion Monte Carlo (AFDMC) simulations. We present results for the equation of state of neutron matter and formore » the energies and radii of neutron drops. Specifically, we study the regulator dependence at the Hartree-Fock level and in AFDMC and find that present local regulators lead to less repulsion from 3N forces compared to the usual nonlocal regulators.« less

The initial single yeast cell adhesion on glass via optical trapping and Derjaguin-Landau-Verwey-Overbeek predictions

NASA Astrophysics Data System (ADS)

Castelain, Mickaël; Pignon, Frédéric; Piau, Jean-Michel; Magnin, Albert

2008-04-01

We used an optical tweezer to investigate the adhesion of yeast Saccharomyces cerevisiae onto a glass substrate at the initial contact. Micromanipulation of free-living objects with single-beam gradient optical trap enabled to highlight mechanisms involved in this initial contact. As a function of the ionic strength and with a displacement parallel to the glass surface, the yeast adheres following different successive ways: (i) Slipping and rolling at 1.5mM NaCl, (ii) slipping, rolling, and sticking at 15mM NaCl, and (iii) only sticking at 150mM. These observations were numerous and reproducible. A kinetic evolution of these adhesion phenomena during yeast movement was clearly established. The nature, range, and relative intensity of forces involved in these different adhesion mechanisms have been worked out as a quantitative analysis from Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended DLVO theories. Calculations show that the adhesion mechanisms observed and their affinity with ionic strength were mainly governed by the Lifshitz-van der Waals interaction forces and the electrical double-layer repulsion to which are added specific contact forces linked to "sticky" glycoprotein secretion, considered to be the main forces capable of overcoming the short-range Lewis acid-base repulsions.

High-Q enhancement of attractive and repulsive optical forces between coupled whispering-gallery- mode resonators.

PubMed

Povinelli, Michelle; Johnson, Steven; Lonèar, Marko; Ibanescu, Mihai; Smythe, Elizabeth; Capasso, Federico; Joannopoulos, J

2005-10-03

We have calculated the optically-induced force between coupled high-Q whispering gallery modes of microsphere resonators. Attractive and repulsive forces are found, depending whether the bi-sphere mode is symmetric or antisymmetric. The magnitude of the force is linearly proportional to the total power in the spheres and consequently linearly enhanced by Q. Forces on the order of 100 nN are found for Q=108, large enough to cause displacements in the range of 1mum when the sphere is attached to a fiber stem with spring constant 0.004 N/m.

Sample positioning in microgravity

NASA Technical Reports Server (NTRS)

Sridharan, Govind (Inventor)

1991-01-01

Repulsion forces arising from laser beams are provided to produce mild positioning forces on a sample in microgravity vacuum environments. The system of the preferred embodiment positions samples using a plurality of pulsed lasers providing opposing repulsion forces. The lasers are positioned around the periphery of a confinement area and expanded to create a confinement zone. The grouped laser configuration, in coordination with position sensing devices, creates a feedback servo whereby stable position control of a sample within microgravity environment can be achieved.

Sample positioning in microgravity

NASA Technical Reports Server (NTRS)

Sridharan, Govind (Inventor)

1993-01-01

Repulsion forces arising from laser beams are provided to produce mild positioning forces on a sample in microgravity vacuum environments. The system of the preferred embodiment positions samples using a plurality of pulsed lasers providing opposing repulsion forces. The lasers are positioned around the periphery of a confinement area and expanded to create a confinement zone. The grouped laser configuration, in coordination with position sensing devices, creates a feedback servo whereby stable position control of a sample within microgravity environment can be achieved.

Spatial variation in deposition rate coefficients of an adhesion-deficient bacterial strain in quartz sand.

PubMed

Tong, Meiping; Camesano, Terri A; Johnson, William P

2005-05-15

The transport of bacterial strain DA001 was examined in packed quartz sand under a variety of environmentally relevant ionic strength and flow conditions. Under all conditions, the retained bacterial concentrations decreased with distance from the column inlet at a rate that was faster than loglinear, indicating that the deposition rate coefficient decreased with increasing transport distance. The hyperexponential retained profile contrasted againstthe nonmonotonic retained profiles that had been previously observed for this same bacterial strain in glass bead porous media, demonstrating that the form of deviation from log-linear behavior is highly sensitive to system conditions. The deposition rate constants in quartz sand were orders of magnitude below those expected from filtration theory, even in the absence of electrostatic energy barriers. The degree of hyperexponential deviation of the retained profiles from loglinear behavior did not decrease with increasing ionic strength in quartz sand. These observations demonstrate thatthe observed low adhesion and deviation from log-linear behavior was not driven by electrostatic repulsion. Measurements of the interaction forces between DA001 cells and the silicon nitride tip of an atomic force microscope (AFM) showed that the bacterium possesses surface polymers with an average equilibrium length of 59.8 nm. AFM adhesion force measurements revealed low adhesion affinities between silicon nitride and DA001 polymers with approximately 95% of adhesion forces having magnitudes < 0.8 nN. Steric repulsion due to surface polymers was apparently responsible for the low adhesion to silicon nitride, indicating that steric interactions from extracellular polymers controlled DA001 adhesion deficiency and deviation from log-linear behavior on quartz sand.

Influence of the shell thickness and charge distribution on the effective interaction between two like-charged hollow spheres.

PubMed

Angelescu, Daniel G; Caragheorgheopol, Dan

2015-10-14

The mean-force and the potential of the mean force between two like-charged spherical shells were investigated in the salt-free limit using the primitive model and Monte Carlo simulations. Apart from an angular homogeneous distribution, a discrete charge distribution where point charges localized on the shell outer surface followed an icosahedral arrangement was considered. The electrostatic coupling of the model system was altered by the presence of mono-, trivalent counterions or small dendrimers, each one bearing a net charge of 9 e. We analyzed in detail how the shell thickness and the radial and angular distribution of the shell charges influenced the effective interaction between the shells. We found a sequence of the potential of the mean force similar to the like-charged filled spheres, ranging from long-range purely repulsive to short-range purely attractive as the electrostatic coupling increased. Both types of potentials were attenuated and an attractive-to-repulsive transition occurred in the presence of trivalent counterions as a result of (i) thinning the shell or (ii) shifting the shell charge from the outer towards the inner surface. The potential of the mean force became more attractive with the icosahedrally symmetric charge model, and additionally, at least one shell tended to line up with 5-fold symmetry axis along the longest axis of the simulation box at the maximum attraction. The results provided a basic framework of understanding the non-specific electrostatic origin of the agglomeration and long-range assembly of the viral nanoparticles.

Coupled electrostatic and material surface stresses yield anomalous particle interactions and deformation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kemp, B. A., E-mail: bkemp@astate.edu; Nikolayev, I.; Sheppard, C. J.

2016-04-14

Like-charges repel, and opposite charges attract. This fundamental tenet is a result of Coulomb's law. However, the electrostatic interactions between dielectric particles remain topical due to observations of like-charged particle attraction and the self-assembly of colloidal systems. Here, we show, using both an approximate description and an exact solution of Maxwell's equations, that nonlinear charged particle forces result even for linear material systems and can be responsible for anomalous electrostatic interactions such as like-charged particle attraction and oppositely charged particle repulsion. Furthermore, these electrostatic interactions and the deformation of such particles have fundamental implications for our understanding of macroscopic electrodynamics.

Design of a 7-DOF slave robot integrated with a magneto-rheological haptic master

NASA Astrophysics Data System (ADS)

Hwang, Yong-Hoon; Cha, Seung-Woo; Kang, Seok-Rae; Choi, Seung-Bok

2017-04-01

In this study, a 7-DOF slave robot integrated with the haptic master is designed and its dynamic motion is controlled. The haptic master is made using a controllable magneto-rheological (MR) clutch and brake and it provides the surgeon with a sense of touch by using both kinetic and kinesthetic information. Due to the size constraint of the slave robot, a wire actuating is adopted to make the desired motion of the end-effector which has 3-DOF instead of a conventional direct-driven motor. Another motions of the link parts that have 4-DOF use direct-driven motor. In total system, for working as a haptic device, the haptic master need to receive the information of repulsive forces applied on the slave robot. Therefore, repulsive forces on the end-effector are sensed by using three uniaxial torque transducer inserted in the wire actuating system and another repulsive forces applied on link part are sensed by using 6-axis transducer that is able to sense forces and torques. Using another 6-axis transducer, verify the reliability of force information on final end of slave robot. Lastly, integrated with a MR haptic master, psycho-physical test is conducted by different operators who can feel the different repulsive force or torque generated from the haptic master which is equivalent to the force or torque occurred on the end-effector to demonstrate the effectiveness of the proposed system.

Quantum mechanics study of the hydroxyethylamines-BACE-1 active site interaction energies

NASA Astrophysics Data System (ADS)

Gueto-Tettay, Carlos; Drosos, Juan Carlos; Vivas-Reyes, Ricardo

2011-06-01

The identification of BACE-1, a key enzyme in the production of Amyloid-β (Aβ) peptides, generated by the proteolytic processing of amyloid precursor protein, was a major advance in the field of Alzheimer's disease as this pathology is characterized by the presence of extracellular senile plaques, mainly comprised of Aβ peptides. Hydroxyethylamines have demonstrated a remarkable potential, like candidate drugs, for this disease using BACE-1 as target. Density Functional Theory calculations were employed to estimate interaction energies for the complexes formed between the hydroxyethylamine derivated inhibitors and 24 residues in the BACE-1 active site. The collected data offered not only a general but a particular quantitative description that gives a deep insight of the interactions in the active site, showing at the same time how ligand structural variations affect them. Polar interactions are the major energetic contributors for complex stabilization and those ones with charged aspartate residues are highlighted, as they contribute over 90% of the total attractive interaction energy. Ligand-ARG296 residue interaction reports the most repulsive value and decreasing of the magnitude of this repulsion can be a key feature for the design of novel and more potent BACE-1 inhibitors. Also it was explained why sultam derivated BACE-1 inhibitors are better ones than lactam based. Hydrophobic interactions concentrated at S1 zone and other relevant repulsions and attractions were also evaluated. The comparison of two different theory levels (X3LYP and M062X) allowed to confirm the relevance of the detected interactions as each theory level has its own strength to depict the forces involved, as is the case of M062X which is better describing the hydrophobic interactions. Those facts were also evaluated and confirmed by comparing the quantitative trend, of selected ligand-residue interactions, with MP2 theory level as reference standard method for electrostatic plus dispersion energies.

Quantum mechanics study of the hydroxyethylamines-BACE-1 active site interaction energies.

PubMed

Gueto-Tettay, Carlos; Drosos, Juan Carlos; Vivas-Reyes, Ricardo

2011-06-01

The identification of BACE-1, a key enzyme in the production of Amyloid-β (Aβ) peptides, generated by the proteolytic processing of amyloid precursor protein, was a major advance in the field of Alzheimer's disease as this pathology is characterized by the presence of extracellular senile plaques, mainly comprised of Aβ peptides. Hydroxyethylamines have demonstrated a remarkable potential, like candidate drugs, for this disease using BACE-1 as target. Density Functional Theory calculations were employed to estimate interaction energies for the complexes formed between the hydroxyethylamine derivated inhibitors and 24 residues in the BACE-1 active site. The collected data offered not only a general but a particular quantitative description that gives a deep insight of the interactions in the active site, showing at the same time how ligand structural variations affect them. Polar interactions are the major energetic contributors for complex stabilization and those ones with charged aspartate residues are highlighted, as they contribute over 90% of the total attractive interaction energy. Ligand-ARG296 residue interaction reports the most repulsive value and decreasing of the magnitude of this repulsion can be a key feature for the design of novel and more potent BACE-1 inhibitors. Also it was explained why sultam derivated BACE-1 inhibitors are better ones than lactam based. Hydrophobic interactions concentrated at S1 zone and other relevant repulsions and attractions were also evaluated. The comparison of two different theory levels (X3LYP and M062X) allowed to confirm the relevance of the detected interactions as each theory level has its own strength to depict the forces involved, as is the case of M062X which is better describing the hydrophobic interactions. Those facts were also evaluated and confirmed by comparing the quantitative trend, of selected ligand-residue interactions, with MP2 theory level as reference standard method for electrostatic plus dispersion energies.

Exploring the Molecular Origins of Bio(in)compatibility: Adhesion Between Proteins and Individual Chains of Poly(ethylene oxide)

NASA Astrophysics Data System (ADS)

Rixman, Monica A.; Ortiz, Christine

2002-03-01

A critical determinant of the biocompatibility of implanted blood-contacting devices is the initial noncovalent adsorption of blood plasma proteins onto the biomaterial surface. Using high-resolution force spectroscopy, we have measured the complex intermolecular interaction forces between individual end-grafted PEO chains and a probe tip covalently bound with human serum albumin, the most abundant blood plasma protein in the human body. On approach, a long-range, nonlinear repulsive force is observed. Upon retraction, however, adhesion between the HSA probe tip and PEO chain occurs, which in many cases is strong enough to allow long-range adhesion and stretching of the individual PEO chains. The known PEO strain-induced conformational transition from the helical (ttg) to the planar (ttt) conformation is clearly observed and seen to shift to lower force values. Statistical analysis of adhesion data, comparison to a variety of control experiments, and theoretical modeling enable us to interpret these experimental results in terms of electrostatic interactions, hydrogen bonding, and steric forces.

Mapping the force field of a hydrogen-bonded assembly

NASA Astrophysics Data System (ADS)

Sweetman, A. M.; Jarvis, S. P.; Sang, Hongqian; Lekkas, I.; Rahe, P.; Wang, Yu; Wang, Jianbo; Champness, N. R.; Kantorovich, L.; Moriarty, P.

2014-05-01

Hydrogen bonding underpins the properties of a vast array of systems spanning a wide variety of scientific fields. From the elegance of base pair interactions in DNA to the symmetry of extended supramolecular assemblies, hydrogen bonds play an essential role in directing intermolecular forces. Yet fundamental aspects of the hydrogen bond continue to be vigorously debated. Here we use dynamic force microscopy (DFM) to quantitatively map the tip-sample force field for naphthalene tetracarboxylic diimide molecules hydrogen-bonded in two-dimensional assemblies. A comparison of experimental images and force spectra with their simulated counterparts shows that intermolecular contrast arises from repulsive tip-sample interactions whose interpretation can be aided via an examination of charge density depletion across the molecular system. Interpreting DFM images of hydrogen-bonded systems therefore necessitates detailed consideration of the coupled tip-molecule system: analyses based on intermolecular charge density in the absence of the tip fail to capture the essential physical chemistry underpinning the imaging mechanism.

Nature and magnitude of aromatic base stacking in DNA and RNA: Quantum chemistry, molecular mechanics, and experiment.

PubMed

Sponer, Jiří; Sponer, Judit E; Mládek, Arnošt; Jurečka, Petr; Banáš, Pavel; Otyepka, Michal

2013-12-01

Base stacking is a major interaction shaping up and stabilizing nucleic acids. During the last decades, base stacking has been extensively studied by experimental and theoretical methods. Advanced quantum-chemical calculations clarified that base stacking is a common interaction, which in the first approximation can be described as combination of the three most basic contributions to molecular interactions, namely, electrostatic interaction, London dispersion attraction and short-range repulsion. There is not any specific π-π energy term associated with the delocalized π electrons of the aromatic rings that cannot be described by the mentioned contributions. The base stacking can be rather reasonably approximated by simple molecular simulation methods based on well-calibrated common force fields although the force fields do not include nonadditivity of stacking, anisotropy of dispersion interactions, and some other effects. However, description of stacking association in condensed phase and understanding of the stacking role in biomolecules remain a difficult problem, as the net base stacking forces always act in a complex and context-specific environment. Moreover, the stacking forces are balanced with many other energy contributions. Differences in definition of stacking in experimental and theoretical studies are explained. Copyright © 2013 Wiley Periodicals, Inc.

Effects of gas interparticle interaction on dissipative wake-mediated forces.

PubMed

Kliushnychenko, O V; Lukyanets, S P

2017-01-01

We examine how the short-range repulsive interaction in a gas of Brownian particles affects behavior of the nonequilibrium depletion forces between obstacles embedded into the gas flow. It is shown that for an ensemble of small and widely separated obstacles the dissipative wake-mediated interaction belongs to the type of induced dipole-dipole interaction governed by an anisotropic screened Coulomb-like potential. For closely located obstacles, formation of a common density perturbation "coat" around them leads to enhancement of dissipative interaction, manifested by characteristic peaks in its dependence on both the bath fraction and the external driving field. Moreover, additional screening of the gas flow due to nonlinear blockade effect gives rise to generation of a pronounced step-like profile of gas density distribution around the obstacles. This can lead to additional enhancement of dissipative interaction between obstacles. The possibility of the dissipative pairing effect and dissipative interaction switching provoked by wake inversion is briefly discussed. All the results are obtained within the classical lattice-gas model.

Condensates of p-wave pairs are exact solutions for rotating two-component Bose gases.

PubMed

Papenbrock, T; Reimann, S M; Kavoulakis, G M

2012-02-17

We derive exact analytical results for the wave functions and energies of harmonically trapped two-component Bose-Einstein condensates with weakly repulsive interactions under rotation. The isospin symmetric wave functions are universal and do not depend on the matrix elements of the two-body interaction. The comparison with the results from numerical diagonalization shows that the ground state and low-lying excitations consist of condensates of p-wave pairs for repulsive contact interactions, Coulomb interactions, and the repulsive interactions between aligned dipoles.

Competing Hydrophobic and Screened-Coulomb Interactions in Hepatitis B Virus Capsid Assembly

PubMed Central

Kegel, Willem K.; Schoot, Paul van der

2004-01-01

Recent experiments show that, in the range from ∼15 to 45°C, an increase in the temperature promotes the spontaneous assembly into capsids of the Escherichia coli-expressed coat proteins of hepatitis B virus. Within that temperature interval, an increase in ionic strength up to five times that of standard physiological conditions also acts to promote capsid assembly. To explain both observations we propose an interaction of mean force between the protein subunits that is the sum of an attractive hydrophobic interaction, driving the self-assembly, and a repulsive electrostatic interaction, opposing the self-assembly. We find that the binding strength of the capsid subunits increases with temperature virtually independently of the ionic strength, and that, at fixed temperature, the binding strength increases with the square root of ionic strength. Both predictions are in quantitative agreement with experiment. We point out the similarities of capsid assembly in general and the micellization of surfactants. Finally we make plausible that electrostatic repulsion between the native core subunits of a large class of virus suppresses the formation in vivo of empty virus capsids, that is, without the presence of the charge-neutralizing nucleic acid. PMID:15189887

Assessing many-body contributions to intermolecular interactions of the AMOEBA force field using energy decomposition analysis of electronic structure calculations.

PubMed

Demerdash, Omar; Mao, Yuezhi; Liu, Tianyi; Head-Gordon, Martin; Head-Gordon, Teresa

2017-10-28

In this work, we evaluate the accuracy of the classical AMOEBA model for representing many-body interactions, such as polarization, charge transfer, and Pauli repulsion and dispersion, through comparison against an energy decomposition method based on absolutely localized molecular orbitals (ALMO-EDA) for the water trimer and a variety of ion-water systems. When the 2- and 3-body contributions according to the many-body expansion are analyzed for the ion-water trimer systems examined here, the 3-body contributions to Pauli repulsion and dispersion are found to be negligible under ALMO-EDA, thereby supporting the validity of the pairwise-additive approximation in AMOEBA's 14-7 van der Waals term. However AMOEBA shows imperfect cancellation of errors for the missing effects of charge transfer and incorrectness in the distance dependence for polarization when compared with the corresponding ALMO-EDA terms. We trace the larger 2-body followed by 3-body polarization errors to the Thole damping scheme used in AMOEBA, and although the width parameter in Thole damping can be changed to improve agreement with the ALMO-EDA polarization for points about equilibrium, the correct profile of polarization as a function of intermolecular distance cannot be reproduced. The results suggest that there is a need for re-examining the damping and polarization model used in the AMOEBA force field and provide further insights into the formulations of polarizable force fields in general.

Assessing many-body contributions to intermolecular interactions of the AMOEBA force field using energy decomposition analysis of electronic structure calculations

NASA Astrophysics Data System (ADS)

Demerdash, Omar; Mao, Yuezhi; Liu, Tianyi; Head-Gordon, Martin; Head-Gordon, Teresa

2017-10-01

In this work, we evaluate the accuracy of the classical AMOEBA model for representing many-body interactions, such as polarization, charge transfer, and Pauli repulsion and dispersion, through comparison against an energy decomposition method based on absolutely localized molecular orbitals (ALMO-EDA) for the water trimer and a variety of ion-water systems. When the 2- and 3-body contributions according to the many-body expansion are analyzed for the ion-water trimer systems examined here, the 3-body contributions to Pauli repulsion and dispersion are found to be negligible under ALMO-EDA, thereby supporting the validity of the pairwise-additive approximation in AMOEBA's 14-7 van der Waals term. However AMOEBA shows imperfect cancellation of errors for the missing effects of charge transfer and incorrectness in the distance dependence for polarization when compared with the corresponding ALMO-EDA terms. We trace the larger 2-body followed by 3-body polarization errors to the Thole damping scheme used in AMOEBA, and although the width parameter in Thole damping can be changed to improve agreement with the ALMO-EDA polarization for points about equilibrium, the correct profile of polarization as a function of intermolecular distance cannot be reproduced. The results suggest that there is a need for re-examining the damping and polarization model used in the AMOEBA force field and provide further insights into the formulations of polarizable force fields in general.

Interaction Mechanisms between Air Bubble and Molybdenite Surface: Impact of Solution Salinity and Polymer Adsorption.

PubMed

Xie, Lei; Wang, Jingyi; Yuan, Duowei; Shi, Chen; Cui, Xin; Zhang, Hao; Liu, Qi; Liu, Qingxia; Zeng, Hongbo

2017-03-07

The surface characteristics of molybdenite (MoS 2 ) such as wettability and surface interactions have attracted much research interest in a wide range of engineering applications, such as froth flotation. In this work, a bubble probe atomic force microscope (AFM) technique was employed to directly measure the interaction forces between an air bubble and molybdenite mineral surface before/after polymer (i.e., guar gum) adsorption treatment. The AFM imaging showed that the polymer coverage on the surface of molybdenite could achieve ∼5.6, ∼44.5, and ∼100% after conditioning in 1, 5, and 10 ppm polymer solution, respectively, which coincided with the polymer coverage results based on contact angle measurements. The electrolyte concentration and surface treatment by polymer adsorption were found to significantly affect bubble-mineral interaction and attachment. The experimental force results on bubble-molybdenite (without polymer treatment) agreed well with the calculations using a theoretical model based on the Reynolds lubrication theory and augmented Young-Laplace equation including the effect of disjoining pressure. The overall surface repulsion was enhanced when the NaCl concentration decreased from 100 to 1 mM, which inhibited the bubble-molybdenite attachment. After conditioning the molybdenite surface in 1 ppm polymer solution, it was more difficult for air bubbles to attach to the molybdenite surface due to the weakened hydrophobic interaction with a shorter decay length. Increasing the polymer concentration to 5 ppm effectively inhibited bubble attachment on mineral surface, which was mainly due to the much reduced hydrophobic interaction as well as the additional steric repulsion between the extended polymer chains and bubble surface. The results provide quantitative information on the interaction mechanism between air bubbles and molybdenite mineral surfaces on the nanoscale, with useful implications for the development of effective polymer depressants and fundamental understanding of bubble-solid interactions in mineral flotation. The methodologies used in this work can be readily extended to studying similar interfacial interactions in many other engineering applications such as froth flotation deinking and bitumen extraction in oil sands industry.

Two-component fluid membranes near repulsive walls: Linearized hydrodynamics of equilibrium and nonequilibrium states.

PubMed

Sankararaman, Sumithra; Menon, Gautam I; Sunil Kumar, P B

2002-09-01

We study the linearized hydrodynamics of a two-component fluid membrane near a repulsive wall, using a model that incorporates curvature-concentration coupling as well as hydrodynamic interactions. This model is a simplified version of a recently proposed one [J.-B. Manneville et al., Phys. Rev. E 64, 021908 (2001)] for nonequilibrium force centers embedded in fluid membranes, such as light-activated bacteriorhodopsin pumps incorporated in phospholipid egg phosphatidyl choline (EPC) bilayers. The pump-membrane system is modeled as an impermeable, two-component bilayer fluid membrane in the presence of an ambient solvent, in which one component, representing active pumps, is described in terms of force dipoles displaced with respect to the bilayer midpoint. We first discuss the case in which such pumps are rendered inactive, computing the mode structure in the bulk as well as the modification of hydrodynamic properties by the presence of a nearby wall. These results should apply, more generally, to equilibrium fluid membranes comprised of two components, in which the effects of curvature-concentration coupling are significant, above the threshold for phase separation. We then discuss the fluctuations and mode structure in the steady state of active two-component membranes near a repulsive wall. We find that proximity to the wall smoothens membrane height fluctuations in the stable regime, resulting in a logarithmic scaling of the roughness even for initially tensionless membranes. This explicitly nonequilibrium result is a consequence of the incorporation of curvature-concentration coupling in our hydrodynamic treatment. This result also indicates that earlier scaling arguments which obtained an increase in the roughness of active membranes near repulsive walls upon neglecting the role played by such couplings may need to be reevaluated.

Size-controlled and redox-responsive supramolecular nanoparticles

PubMed Central

2015-01-01

Summary Control over the assembly and disassembly of nanoparticles is pivotal for their use as drug delivery vehicles. Here, we aim to form supramolecular nanoparticles (SNPs) by combining advantages of the reversible assembly properties of SNPs using host–guest interactions and of a stimulus-responsive moiety. The SNPs are composed of a core of positively charged poly(ethylene imine) grafted with β-cyclodextrin (CD) and a positively charged ferrocene (Fc)-terminated poly(amidoamine) dendrimer, with a monovalent stabilizer at the surface. Fc was chosen for its loss of CD-binding properties when oxidizing it to the ferrocenium cation. The ionic strength was shown to play an important role in controlling the aggregate growth. The attractive supramolecular and repulsive electrostatic interactions constitute a balance of forces in this system at low ionic strengths. At higher ionic strengths, the increased charge screening led to a loss of electrostatic repulsion and therefore to faster aggregate growth. A Job plot showed that a 1:1 stoichiometry of host and guest moieties gave the most efficient aggregate growth. Different stabilizers were used to find the optimal stopper to limit the growth. A weaker guest moiety was shown to be less efficient in stabilizing the SNPs. Also steric repulsion is important for achieving SNP stability. SNPs of controlled particle size and good stability (up to seven days) were prepared by fine-tuning the ratio of multivalent and monovalent interactions. Finally, reversibility of the SNPs was confirmed by oxidizing the Fc guest moieties in the core of the SNPs. PMID:26733345

Measurement of intercolumnar forces between parallel guanosine four-stranded helices.

PubMed Central

Mariani, P; Saturni, L

1996-01-01

The deoxyguanosine-5'-monophosphate in aqueous solution self-associates into stable structures, which include hexagonal and cholesteric columnar phases. The structural unit is a four-stranded helix, composed of a stacked array of Hoogsteen-bonded guanosine quartets. We have measured by osmotic stress method the force per unit length versus interaxial distance between helices in the hexagonal phase under various ionic conditions. Two contributions have been recognized: the first one is purely electrostatic, is effective at large distances, and shows a strong dependence on the salt concentration of the solution. The second contribution is short range, dominates at interaxial separations smaller than about 30-32 A, and rises steeply as the columns approach each other, preventing the coalescence of the helices. This repulsion has an exponential nature and shows a magnitude and a decay length insensitive to the ionic strength of the medium. Because these features are distinctive of the hydration force detected between phospholipid bilayers or between several linear macromolecules (DNA, polysaccharides, collagen), we conclude that the dominant force experienced by deoxyguanosine helices approaching contact is hydration repulsion. The observed decay length of about 0.7 A has been rationalized to emerge from the coupling between the 3-A decay length of water solvent and the helically ordered structure of the hydrophilic groups on the opposing surfaces. The present results agree with recent measurements, also showing the dependence of the hydration force decay on the structure of interacting surfaces and confirm the correlations between force and structure. Images FIGURE 1 PMID:8744324

First-principles simulations of electrostatic interactions between dust grains

NASA Astrophysics Data System (ADS)

Itou, H.; Amano, T.; Hoshino, M.

2014-12-01

We investigated the electrostatic interaction between two identical dust grains of an infinite mass immersed in homogeneous plasma by employing first-principles N-body simulations combined with the Ewald method. We specifically tested the possibility of an attractive force due to overlapping Debye spheres (ODSs), as was suggested by Resendes et al. [Phys. Lett. A 239, 181-186 (1998)]. Our simulation results demonstrate that the electrostatic interaction is repulsive and even stronger than the standard Yukawa potential. We showed that the measured electric field acting on the grain is highly consistent with a model electrostatic potential around a single isolated grain that takes into account a correction due to the orbital motion limited theory. Our result is qualitatively consistent with the counterargument suggested by Markes and Williams [Phys. Lett. A 278, 152-158 (2000)], indicating the absence of the ODS attractive force.

Modified interactions among globular proteins below isoelectric point in the presence of mono-, di- and tri-valent ions: A small angle neutron scattering study

NASA Astrophysics Data System (ADS)

Das, Kaushik; Kundu, Sarathi; Mehan, Sumit; Aswal, V. K.

2016-02-01

Both short range attraction and long range electrostatic repulsion exist among globular protein Bovine Serum Albumin in solution below its isoelectric point (pI ≈ 4.8). At pD ≈ 4.0, below pI, protein has a net positive surface charge although local charge inhomogeneity presents. Small angle neutron scattering study reveals that in the presence of both mono-(Na+) and di-(Ni2+) valent ions attractive interaction increases and repulsive interaction decreases with the increase of salt concentration. However, for tri-valent (Fe3+) ions, both attractive and repulsive interaction increases with increasing salt concentration but the relative strength of repulsion is more than the attraction.

Dark matter, long-range forces, and large-scale structure

NASA Technical Reports Server (NTRS)

Gradwohl, Ben-Ami; Frieman, Joshua A.

1992-01-01

If the dark matter in galaxies and clusters is nonbaryonic, it can interact with additional long-range fields that are invisible to experimental tests of the equivalence principle. We discuss the astrophysical and cosmological implications of a long-range force coupled only to the dark matter and find rather tight constraints on its strength. If the force is repulsive (attractive), the masses of galaxy groups and clusters (and the mean density of the universe inferred from them) have been systematically underestimated (overestimated). We explore the consequent effects on the two-point correlation function, large-scale velocity flows, and microwave background anisotropies, for models with initial scale-invariant adiabatic perturbations and cold dark matter.

Casimir effect for perfect electromagnetic conductors (PEMCs): a sum rule for attractive/repulsive forces

NASA Astrophysics Data System (ADS)

Rode, Stefan; Bennett, Robert; Yoshi Buhmann, Stefan

2018-04-01

We discuss the Casimir effect for boundary conditions involving perfect electromagnetic conductors, which interpolate between perfect electric conductors and perfect magnetic conductors. Based on the corresponding reciprocal Green’s tensor we construct the Green’s tensor for two perfectly reflecting plates with magnetoelectric coupling (non-reciprocal media) within the framework of macroscopic quantum electrodynamics. We calculate the Casimir force between two arbitrary perfect electromagnetic conductor plates, resulting in a universal analytic expression that connects the attractive Casimir force with the repulsive Boyer force. We relate the results to a duality symmetry of electromagnetism.

Determination of Hamaker constants of polymeric nanoparticles in organic solvents by asymmetrical flow field-flow fractionation.

PubMed

Noskov, Sergey; Scherer, Christian; Maskos, Michael

2013-01-25

Interaction forces between all objects are either of repulsive or attractive nature. Concerning attractive interactions, the determination of dispersion forces are of special interest since they appear in all colloidal systems and have a crucial influence on the properties and processes in these systems. One possibility to link theory and experiment is the description of the London-Van der Waals forces in terms of the Hamaker constant, which leads to the challenging problem of calculating the van der Waals interaction energies between colloidal particles. Hence, the determination of a Hamaker constant for a given material is needed when interfacial phenomena such as adhesion are discussed in terms of the total potential energy between particles and substrates. In this work, the asymmetrical flow field-flow fractionation (AF-FFF) in combination with a Newton algorithm based iteration process was used for the determination of Hamaker constants of different nanoparticles in toluene. Copyright © 2012 Elsevier B.V. All rights reserved.

Examining the origins of the hydration force between lipid bilayers using all-atom simulations.

PubMed

Gentilcore, Anastasia N; Michaud-Agrawal, Naveen; Crozier, Paul S; Stevens, Mark J; Woolf, Thomas B

2010-05-01

Using 237 all-atom double bilayer simulations, we examined the thermodynamic and structural changes that occur as a phosphatidylcholine lipid bilayer stack is dehydrated. The simulated system represents a micropatch of lipid multilayer systems that are studied experimentally using surface force apparatus, atomic force microscopy and osmotic pressure studies. In these experiments, the hydration level of the system is varied, changing the separation between the bilayers, in order to understand the forces that the bilayers feel as they are brought together. These studies have found a curious, strongly repulsive force when the bilayers are very close to each other, which has been termed the "hydration force," though the origins of this force are not clearly understood. We computationally reproduce this repulsive, relatively free energy change as bilayers come together and make qualitative conclusions as to the enthalpic and entropic origins of the free energy change. This analysis is supported by data showing structural changes in the waters, lipids and salts that have also been seen in experimental work. Increases in solvent ordering as the bilayers are dehydrated are found to be essential in causing the repulsion as the bilayers come together.

Binary collision approximations for the memory function for density fluctuations in equilibrium atomic liquids

NASA Astrophysics Data System (ADS)

Noah, Joyce E.

Time correlation functions of density fluctuations of liquids at equilibrium can be used to relate the microscopic dynamics of a liquid to its macroscopic transport properties. Time correlation functions are especially useful since they can be generated in a variety of ways, from scattering experiments to computer simulation to analytic theory. The kinetic theory of fluctuations in equilibrium liquids is an analytic theory for calculating correlation functions using memory functions. In this work, we use a diagrammatic formulation of the kinetic theory to develop a series of binary collision approximations for the collisional part of the memory function. We define binary collisions as collisions between two distinct density fluctuations whose identities are fixed during the duration of a collsion. R approximations are for the short time part of the memory function, and build upon the work of Ranganathan and Andersen. These approximations have purely repulsive interactions between the fluctuations. The second type of approximation, RA approximations, is for the longer time part of the memory function, where the density fluctuations now interact via repulsive and attractive forces. Although RA approximations are a natural extension of R approximations, they permit two density fluctuations to become trapped in the wells of the interaction potential, leading to long-lived oscillatory behavior, which is unphysical. Therefore we consider S approximations which describe binary particles which experience the random effect of the surroundings while interacting via repulsive or repulsive and attractive interactions. For each of these approximations for the memory function we numerically solve the kinetic equation to generate correlation functions. These results are compared to molecular dynamics results for the correlation functions. Comparing the successes and failures of the different approximations, we conclude that R approximations give more accurate intermediate and long time results while RA and S approximations do particularly well at predicting the short time behavior. Lastly, we also develop a series of non-graphically derived approximations and use an optimization procedure to determine the underlying memory function from the simulation data. These approaches provide valuable information about the memory function that will be used in the development of future kinetic theories.

Evaluating the potential energy landscape over single molecules at room temperature with lateral force microscopy

NASA Astrophysics Data System (ADS)

Weymouth, Alfred J.; Riegel, Elisabeth; Matencio, Sonia; Giessibl, Franz J.

2018-04-01

One of the challenges of AFM, in contrast to STM, is that the measured signal includes both long-range and short-range components. The most accurate method for removing long-range components is to measure both on and off an adsorbate and to subtract the difference. This on-off method is challenging at room temperature due to thermal drift. By moving to a non-contact scheme in which the lateral component of the force interaction is probed, the measurement is dominated by short-range interactions. We use frequency-modulation lateral force microscopy to measure individual PTCDA molecules adsorbed on Ag/Si(111)-( √{3 }×√{3 } ). By fitting the data to a model potential, we can extract the depth and width of the potential. When the tip is closer to the sample, a repulsive feature can be observed in the data.

Interaction, coalescence, and collapse of localized patterns in a quasi-one-dimensional system of interacting particles

NASA Astrophysics Data System (ADS)

Dessup, Tommy; Coste, Christophe; Saint Jean, Michel

2017-01-01

We study the path toward equilibrium of pairs of solitary wave envelopes (bubbles) that modulate a regular zigzag pattern in an annular channel. We evidence that bubble pairs are metastable states, which spontaneously evolve toward a stable single bubble. We exhibit the concept of topological frustration of a bubble pair. A configuration is frustrated when the particles between the two bubbles are not organized in a modulated staggered row. For a nonfrustrated (NF) bubble pair configuration, the bubbles interaction is attractive, whereas it is repulsive for a frustrated (F) configuration. We describe a model of interacting solitary wave that provides all qualitative characteristics of the interaction force: It is attractive for NF systems and repulsive for F systems and decreases exponentially with the bubbles distance. Moreover, for NF systems, the bubbles come closer and eventually merge as a single bubble, in a coalescence process. We also evidence a collapse process, in which one bubble shrinks in favor of the other one, overcoming an energetic barrier in phase space. This process is relevant for both NF systems and F systems. In NF systems, the coalescence prevails at low temperature, whereas thermally activated jumps make the collapse prevail at high temperature. In F systems, the path toward equilibrium involves a collapse process regardless of the temperature.

Modeling the mechanical properties of DNA nanostructures.

PubMed

Arbona, Jean Michel; Aimé, Jean-Pierre; Elezgaray, Juan

2012-11-01

We discuss generalizations of a previously published coarse-grained description [Mergell et al., Phys. Rev. E 68, 021911 (2003)] of double stranded DNA (dsDNA). The model is defined at the base-pair level and includes the electrostatic repulsion between neighbor helices. We show that the model reproduces mechanical and elastic properties of several DNA nanostructures (DNA origamis). We also show that electrostatic interactions are necessary to reproduce atomic force microscopy measurements on planar DNA origamis.

Molecular Self-Assembly Driven by London Dispersion Forces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Guo; Cooper, Valentino R; Cho, Jun-Hyung

2011-01-01

The nature and strength of intermolecular interactions are crucial to a variety of kinetic and dynamic processes at surfaces. Whereas strong chemisorption bonds are known to facilitate molecular binding, the importance of the weaker yet ubiquitous van der Waals (vdW) interactions remains elusive in most cases. Here we use first-principles calculations combined with kinetic Monte Carlo simulations to unambiguously demonstrate the vital role that vdW interactions play in molecular self-assembly, using styrene nanowire growth on silicon as a prototypical example. We find that, only when the London dispersion forces are included, accounting for the attractive parts of vdW interactions, canmore » the effective intermolecular interaction be reversed from being repulsive to attractive. Such attractive interactions, in turn, ensure the preferred growth of long wires under physically realistic conditions as observed experimentally. We further propose a cooperative scheme, invoking the application of an electric field and the selective creation of Si dangling bonds, to drastically improve the ordered arrangement of the molecular structures. The present study represents a significant step forward in the fundamental understanding and precise control of molecular self-assembly guided by London dispersion forces.« less

Controlling Casimir force via coherent driving field

NASA Astrophysics Data System (ADS)

Ahmad, Rashid; Abbas, Muqaddar; Ahmad, Iftikhar; Qamar, Sajid

2016-04-01

A four level atom-field configuration is used to investigate the coherent control of Casimir force between two identical plates made up of chiral atomic media and separated by vacuum of width d. The electromagnetic chirality-induced negative refraction is obtained via atomic coherence. The behavior of Casimir force is investigated using Casimir-Lifshitz formula. It is noticed that Casimir force can be switched from repulsive to attractive and vice versa via coherent control of the driving field. This switching feature provides new possibilities of using the repulsive Casimir force in the development of new emerging technologies, such as, micro-electro-mechanical and nano-electro-mechanical systems, i.e., MEMS and NEMS, respectively.

Coulomb-Driven Relativistic Electron Beam Compression

NASA Astrophysics Data System (ADS)

Lu, Chao; Jiang, Tao; Liu, Shengguang; Wang, Rui; Zhao, Lingrong; Zhu, Pengfei; Xiang, Dao; Zhang, Jie

2018-01-01

Coulomb interaction between charged particles is a well-known phenomenon in many areas of research. In general, the Coulomb repulsion force broadens the pulse width of an electron bunch and limits the temporal resolution of many scientific facilities such as ultrafast electron diffraction and x-ray free-electron lasers. Here we demonstrate a scheme that actually makes use of the Coulomb force to compress a relativistic electron beam. Furthermore, we show that the Coulomb-driven bunch compression process does not introduce additional timing jitter, which is in sharp contrast to the conventional radio-frequency buncher technique. Our work not only leads to enhanced temporal resolution in electron-beam-based ultrafast instruments that may provide new opportunities in probing material systems far from equilibrium, but also opens a promising direction for advanced beam manipulation through self-field interactions.

Coulomb-Driven Relativistic Electron Beam Compression.

PubMed

Lu, Chao; Jiang, Tao; Liu, Shengguang; Wang, Rui; Zhao, Lingrong; Zhu, Pengfei; Xiang, Dao; Zhang, Jie

2018-01-26

Coulomb interaction between charged particles is a well-known phenomenon in many areas of research. In general, the Coulomb repulsion force broadens the pulse width of an electron bunch and limits the temporal resolution of many scientific facilities such as ultrafast electron diffraction and x-ray free-electron lasers. Here we demonstrate a scheme that actually makes use of the Coulomb force to compress a relativistic electron beam. Furthermore, we show that the Coulomb-driven bunch compression process does not introduce additional timing jitter, which is in sharp contrast to the conventional radio-frequency buncher technique. Our work not only leads to enhanced temporal resolution in electron-beam-based ultrafast instruments that may provide new opportunities in probing material systems far from equilibrium, but also opens a promising direction for advanced beam manipulation through self-field interactions.

Electrostatics at the nanoscale.

PubMed

Walker, David A; Kowalczyk, Bartlomiej; de la Cruz, Monica Olvera; Grzybowski, Bartosz A

2011-04-01

Electrostatic forces are amongst the most versatile interactions to mediate the assembly of nanostructured materials. Depending on experimental conditions, these forces can be long- or short-ranged, can be either attractive or repulsive, and their directionality can be controlled by the shapes of the charged nano-objects. This Review is intended to serve as a primer for experimentalists curious about the fundamentals of nanoscale electrostatics and for theorists wishing to learn about recent experimental advances in the field. Accordingly, the first portion introduces the theoretical models of electrostatic double layers and derives electrostatic interaction potentials applicable to particles of different sizes and/or shapes and under different experimental conditions. This discussion is followed by the review of the key experimental systems in which electrostatic interactions are operative. Examples include electroactive and "switchable" nanoparticles, mixtures of charged nanoparticles, nanoparticle chains, sheets, coatings, crystals, and crystals-within-crystals. Applications of these and other structures in chemical sensing and amplification are also illustrated.

Quantifying the adhesion and interaction forces between Pseudomonas aeruginosa and natural organic matter.

PubMed

Abu-Lail, Laila I; Liu, Yatao; Atabek, Arzu; Camesano, Terri A

2007-12-01

Atomic force microscopy (AFM) was used to characterize interactions between natural organic matter (NOM), and glass or bacteria. Poly(methacrylic acid) (PMA), soil humic Acid (SHA), and Suwannee River humic Acid (SRHA), were adsorbed to silica AFM probes. Adhesion forces (Fadh) for the interaction of organic-probes and glass slides correlated with organic molecular weight (MW), but not with radius of the organic aggregate (R), charge density (Q), or zeta potential (zeta). Two Pseudomonas aeruginosa strains with different lipopolysaccharides (LPS) were chosen: PAO1 (A+B+), whose LPS have common antigen (A-band) + O-antigen (B-band); and mutant AK1401 (A+B-). Fadh between bacteria and organics correlated with organic MW, R, and Q, but not zeta. PAO1 had lower Fadh with silica than NOM, which was attributed to negative charges from the B-band polymers causing electrostatic repulsion. AK1401 adhered stronger to silica than to the organics, perhaps because the absence of the B-band exposed underlying positively charged proteins. DLVO calculations could not explain the differences in the two bacteria or predict qualitative or quantitative trends in interaction forces in these systems. Molecular-level information from AFM studies can bring us closer to understanding the complex nature of bacterial-NOM interactions.

Direct observation of growth and collapse of a Bose-Einstein condensate with attractive interactions

NASA Astrophysics Data System (ADS)

Gerton, Jordan M.; Strekalov, Dmitry; Prodan, Ionut; Hulet, Randall G.

2000-12-01

Quantum theory predicts that Bose-Einstein condensation of a spatially homogeneous gas with attractive interactions is precluded by a conventional phase transition into either a liquid or solid. When confined to a trap, however, such a condensate can form, provided that its occupation number does not exceed a limiting value. The stability limit is determined by a balance between the self-attractive forces and a repulsion that arises from position-momentum uncertainty under conditions of spatial confinement. Near the stability limit, self-attraction can overwhelm the repulsion, causing the condensate to collapse. Growth of the condensate is therefore punctuated by intermittent collapses that are triggered by either macroscopic quantum tunnelling or thermal fluctuation. Previous observations of growth and collapse dynamics have been hampered by the stochastic nature of these mechanisms. Here we report direct observations of the growth and subsequent collapse of a 7Li condensate with attractive interactions, using phase-contrast imaging. The success of the measurement lies in our ability to reduce the stochasticity in the dynamics by controlling the initial number of condensate atoms using a two-photon transition to a diatomic molecular state.

The role of atomic level steric effects and attractive forces in protein folding.

PubMed

Lammert, Heiko; Wolynes, Peter G; Onuchic, José N

2012-02-01

Protein folding into tertiary structures is controlled by an interplay of attractive contact interactions and steric effects. We investigate the balance between these contributions using structure-based models using an all-atom representation of the structure combined with a coarse-grained contact potential. Tertiary contact interactions between atoms are collected into a single broad attractive well between the C(β) atoms between each residue pair in a native contact. Through the width of these contact potentials we control their tolerance for deviations from the ideal structure and the spatial range of attractive interactions. In the compact native state dominant packing constraints limit the effects of a coarse-grained contact potential. During folding, however, the broad attractive potentials allow an early collapse that starts before the native local structure is completely adopted. As a consequence the folding transition is broadened and the free energy barrier is decreased. Eventually two-state folding behavior is lost completely for systems with very broad attractive potentials. The stabilization of native-like residue interactions in non-perfect geometries early in the folding process frequently leads to structural traps. Global mirror images are a notable example. These traps are penalized by the details of the repulsive interactions only after further collapse. Successful folding to the native state requires simultaneous guidance from both attractive and repulsive interactions. Copyright © 2011 Wiley Periodicals, Inc.

Three-Body Forces and the Limit of Oxygen Isotopes

NASA Astrophysics Data System (ADS)

Otsuka, Takaharu; Suzuki, Toshio; Holt, Jason D.; Schwenk, Achim; Akaishi, Yoshinori

2010-07-01

The limit of neutron-rich nuclei, the neutron drip line, evolves regularly from light to medium-mass nuclei except for a striking anomaly in the oxygen isotopes. This anomaly is not reproduced in shell-model calculations derived from microscopic two-nucleon forces. Here, we present the first microscopic explanation of the oxygen anomaly based on three-nucleon forces that have been established in few-body systems. This leads to repulsive contributions to the interactions among excess neutrons that change the location of the neutron drip line from O28 to the experimentally observed O24. Since the mechanism is robust and general, our findings impact the prediction of the most neutron-rich nuclei and the synthesis of heavy elements in neutron-rich environments.

Tip induced mechanical deformation of epitaxial graphene grown on reconstructed 6H-SiC(0001) surface during scanning tunneling and atomic force microscopy studies.

PubMed

Meza, José Antonio Morán; Lubin, Christophe; Thoyer, François; Cousty, Jacques

2015-01-26

The structural and mechanical properties of an epitaxial graphene (EG) monolayer thermally grown on top of a 6H-SiC(0001) surface were studied by combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM). Experimental STM, dynamic STM and AFM images of EG on 6H-SiC(0001) show a lattice with a 1.9 nm period corresponding to the (6 × 6) quasi-cell of the SiC surface. The corrugation amplitude of this (6 × 6) quasi-cell, measured from AFM topographies, increases with the setpoint value of the frequency shift Δf (15-20 Hz, repulsive interaction). Excitation variations map obtained simultaneously with the AFM topography shows that larger dissipation values are measured in between the topographical bumps of the (6 × 6) quasi-cell. These results demonstrate that the AFM tip deforms the graphene monolayer. During recording in dynamic STM mode, a frequency shift (Δf) map is obtained in which Δf values range from 41 to 47 Hz (repulsive interaction). As a result, we deduced that the STM tip, also, provokes local mechanical distortions of the graphene monolayer. The origin of these tip-induced distortions is discussed in terms of electronic and mechanical properties of EG on 6H-SiC(0001).

Nonlinear tapping dynamics of multi-walled carbon nanotube tipped atomic force microcantilevers

NASA Astrophysics Data System (ADS)

Lee, S. I.; Howell, S. W.; Raman, A.; Reifenberger, R.; Nguyen, C. V.; Meyyappan, M.

2004-05-01

The nonlinear dynamics of an atomic force microcantilever (AFM) with an attached multi-walled carbon nanotube (MWCNT) tip is investigated experimentally and theoretically. We present the experimental nonlinear frequency response of a MWCNT tipped microcantilever in the tapping mode. Several unusual features in the response distinguish it from those traditionally observed for conventional tips. The MWCNT tipped AFM probe is apparently immune to conventional imaging instabilities related to the coexistence of attractive and repulsive tapping regimes. A theoretical interaction model for the system using an Euler elastica MWCNT model is developed and found to predict several unusual features of the measured nonlinear response.

Measurements of Attractive Forces between Proteins and End-Grafted Poly(Ethylene Glycol) Chains

NASA Astrophysics Data System (ADS)

Sheth, S. R.; Leckband, D.

1997-08-01

The surface force apparatus was used to measure directly the molecular forces between streptavidin and lipid bilayers displaying grafted Mr 2,000 poly(ethylene glycol) (PEG). These measurements provide direct evidence for the formation of relatively strong attractive forces between PEG and protein. At low compressive loads, the forces were repulsive, but they became attractive when the proteins were pressed into the polymer layer at higher loads. The adhesion was sufficiently robust that separation of the streptavidin and PEG uprooted anchored polymer from the supporting membrane. These interactions altered the properties of the grafted chains. After the onset of the attraction, the polymer continued to bind protein for several hours. The changes were not due to protein denaturation. These data demonstrate directly that the biological activity of PEG is not due solely to properties of simple polymers such as the excluded volume. It is also coupled to the competitive interactions between solvent and other materials such as proteins for the chain segments and to the ability of this material to adopt higher order intrachain structures.

Origins of the Non-DLVO Force between Glass Surfaces in Aqueous Solution.

PubMed

Adler, Joshua J.; Rabinovich, Yakov I.; Moudgil, Brij M.

2001-05-15

Direct measurement of surface forces has revealed that silica surfaces seem to have a short-range repulsion that is not accounted for in classical DLVO theory. The two leading hypotheses for the origin of the non-DLVO force are (i) structuring of water at the silica interface or (ii) water penetration into the surface resulting in a gel layer. In this article, the interaction of silica surfaces will be reviewed from the perspective of the non-DLVO force origin. In an attempt to more accurately describe the behavior of silica and glass surfaces, alternative models of how surfaces with gel layers should interact are proposed. It is suggested that a lessened van der Waals attraction originating from a thin gel layer may explain both the additional stability and the coagulation behavior of silica. It is important to understand the mechanisms underlying the existence of the non-DLVO force which is likely to have a major influence on the adsorption of polymers and surfactants used to modify the silica surface for practical applications in the ceramic, mineral, and microelectronic industries. Copyright 2001 Academic Press.

Casimir repulsion in sphere-plate geometry

NASA Astrophysics Data System (ADS)

Pirozhenko, Irina G.; Bordag, Michael

2013-04-01

The electromagnetic vacuum energy is considered in the presence of a perfectly conducting plane and a ball with dielectric permittivity ɛ and magnetic permeability μ, μ≠1. The attention is focused on the Casimir repulsion in this system caused by the magnetic permeability of the sphere. In the case of a perfectly permeable sphere, μ=∞, the vacuum energy is estimated numerically. The short- and long-distance asymptotes corresponding to the repulsive force and respective low-temperature corrections and high-temperature limits are found for a wide range of μ. The constraints on the Casimir repulsion in this system are discussed.

Neural cryptography with feedback.

PubMed

Ruttor, Andreas; Kinzel, Wolfgang; Shacham, Lanir; Kanter, Ido

2004-04-01

Neural cryptography is based on a competition between attractive and repulsive stochastic forces. A feedback mechanism is added to neural cryptography which increases the repulsive forces. Using numerical simulations and an analytic approach, the probability of a successful attack is calculated for different model parameters. Scaling laws are derived which show that feedback improves the security of the system. In addition, a network with feedback generates a pseudorandom bit sequence which can be used to encrypt and decrypt a secret message.

Self-assembled morphologies of an amphiphilic Y-shaped weak polyelectrolyte in a thin film.

PubMed

Mu, Dan; Li, Jian-Quan; Feng, Sheng-Yu

2017-11-29

Different from the self-assembly of neutral polymers, polyelectrolytes self-assemble into smaller aggregates with a more loosely assembled structure, which results from the repulsive forces acting between similar electrical compositions with the introduction of ions. The Y-shaped weak polyelectrolytes self-assemble into a core-shell type cylindrical structure with a hexagonal arrangement in a thin film, whose thickness is smaller than the gyration radius of the polymer chain. The corresponding formation mechanism consists of enrichment of the same components, adjustment of the shape of the aggregate, and the subsequent separation into individual aggregates. With the increase in the thickness of the thin film until it exceeds the gyration radius of the polymer chain, combined with the greater freedom of movement along the direction of thin film thickness, the self-assembled structure changes into a micellar structure. Under confinement, the repulsive force to the polymeric components is weakened by the repulsive forces among polyelectrolyte components with like charges, and this helps in generating aggregates with more uniform size and density distribution. In particular, when the repulsive force between the walls and the core forming components is greater than that between the walls and the shell forming components, such asymmetric confinement produces a crossed-cylindrical structure with nearly perpendicular arrangement of two cylinder arrays. Similarly, a novel three-crossed cylinder morphology is self-assembled upon removal of confinement.

Probing lipid membrane electrostatics

NASA Astrophysics Data System (ADS)

Yang, Yi

The electrostatic properties of lipid bilayer membranes play a significant role in many biological processes. Atomic force microscopy (AFM) is highly sensitive to membrane surface potential in electrolyte solutions. With fully characterized probe tips, AFM can perform quantitative electrostatic analysis of lipid membranes. Electrostatic interactions between Silicon nitride probes and supported zwitterionic dioleoylphosphatidylcholine (DOPC) bilayer with a variable fraction of anionic dioleoylphosphatidylserine (DOPS) were measured by AFM. Classical Gouy-Chapman theory was used to model the membrane electrostatics. The nonlinear Poisson-Boltzmann equation was numerically solved with finite element method to provide the potential distribution around the AFM tips. Theoretical tip-sample electrostatic interactions were calculated with the surface integral of both Maxwell and osmotic stress tensors on tip surface. The measured forces were interpreted with theoretical forces and the resulting surface charge densities of the membrane surfaces were in quantitative agreement with the Gouy-Chapman-Stern model of membrane charge regulation. It was demonstrated that the AFM can quantitatively detect membrane surface potential at a separation of several screening lengths, and that the AFM probe only perturbs the membrane surface potential by <2%. One important application of this technique is to estimate the dipole density of lipid membrane. Electrostatic analysis of DOPC lipid bilayers with the AFM reveals a repulsive force between the negatively charged probe tips and the zwitterionic lipid bilayers. This unexpected interaction has been analyzed quantitatively to reveal that the repulsion is due to a weak external field created by the internai membrane dipole moment. The analysis yields a dipole moment of 1.5 Debye per lipid with a dipole potential of +275 mV for supported DOPC membranes. This new ability to quantitatively measure the membrane dipole density in a noninvasive manner will be useful in identifying the biological effects of the dipole potential. Finally, heterogeneous model membranes were studied with fluid electric force microscopy (FEFM). Electrostatic mapping was demonstrated with 50 nm resolution. The capabilities of quantitative electrostatic measurement and lateral charge density mapping make AFM a unique and powerful probe of membrane electrostatics.

Body size affects the strength of social interactions and spatial organization of a schooling fish (Pseudomugil signifer)

NASA Astrophysics Data System (ADS)

Romenskyy, Maksym; Herbert-Read, James E.; Ward, Ashley J. W.; Sumpter, David J. T.

2017-04-01

While a rich variety of self-propelled particle models propose to explain the collective motion of fish and other animals, rigorous statistical comparison between models and data remains a challenge. Plausible models should be flexible enough to capture changes in the collective behaviour of animal groups at their different developmental stages and group sizes. Here, we analyse the statistical properties of schooling fish (Pseudomugil signifer) through a combination of experiments and simulations. We make novel use of a Boltzmann inversion method, usually applied in molecular dynamics, to identify the effective potential of the mean force of fish interactions. Specifically, we show that larger fish have a larger repulsion zone, but stronger attraction, resulting in greater alignment in their collective motion. We model the collective dynamics of schools using a self-propelled particle model, modified to include varying particle speed and a local repulsion rule. We demonstrate that the statistical properties of the fish schools are reproduced by our model, thereby capturing a number of features of the behaviour and development of schooling fish.

Dynamics and interactions of particles in a thermophoretic trap

NASA Astrophysics Data System (ADS)

Foster, Benjamin; Fung, Frankie; Fieweger, Connor; Usatyuk, Mykhaylo; Gaj, Anita; DeSalvo, B. J.; Chin, Cheng

2017-08-01

We investigate dynamics and interactions of particles levitated and trapped by the thermophoretic force in a vacuum cell. Our analysis is based on footage taken by orthogonal cameras that are able to capture the three dimensional trajectories of the particles. In contrast to spherical particles, which remain stationary at the center of the cell, here we report new qualitative features of the motion of particles with non-spherical geometry. Singly levitated particles exhibit steady spinning around their body axis and rotation around the symmetry axis of the cell. When two levitated particles approach each other, repulsive or attractive interactions between the particles are observed. Our levitation system offers a wonderful platform to study interaction between particles in a microgravity environment.

Quasi-one-dimensional spin-orbit- and Rabi-coupled bright dipolar Bose-Einstein-condensate solitons

NASA Astrophysics Data System (ADS)

Chiquillo, Emerson

2018-01-01

We study the formation of stable bright solitons in quasi-one-dimensional (quasi-1D) spin-orbit- (SO-) and Rabi-coupled two pseudospinor dipolar Bose-Einstein condensates (BECs) of 164Dy atoms in the presence of repulsive contact interactions. As a result of the combined attraction-repulsion effect of both interactions and the addition of SO and Rabi couplings, two kinds of ground states in the form of self-trapped bright solitons can be formed, a plane-wave soliton (PWS) and a stripe soliton (SS). These quasi-1D solitons cannot exist in a condensate with purely repulsive contact interactions and SO and Rabi couplings (no dipole). Neglecting the repulsive contact interactions, our findings also show the possibility of creating PWSs and SSs. When the strengths of the two interactions are close to each other, the SS develops an oscillatory instability indicating a possibility of a breather solution, eventually leading to its destruction. We also obtain a phase diagram showing regions where the solution is a PWS or SS.

Dielectrophoretic levitation in the presence of shear flow: implications for colloidal fouling of filtration membranes.

PubMed

Molla, Shahnawaz; Bhattacharjee, Subir

2007-10-09

The ability of dielectrophoretic (DEP) forces created using a microelectrode array to levitate particles in a colloidal suspension is studied experimentally and theoretically. The experimental system employs microfabricated electrode arrays on a glass substrate to apply repulsive DEP forces on polystyrene latex particles suspended in an aqueous medium. A numerical model based on the convection-diffusion-migration equation is presented to calculate the concentration distribution of colloidal particles in shear flow under the influence of a repulsive DEP force field. The results obtained from the numerical simulations are compared against trajectory analysis results and experimental data. The results indicate that by incorporating ac electric field-induced DEP forces in a shear flow, particle accumulation and deposition on the flow channel surfaces can be significantly reduced or even completely averted. The mathematical model is then used to indicate how the deposition behavior is modified in the presence of a permeable substrate, representative of tangential flow membrane filtration operations. The results indicate that the repulsive dielectrophoretic (DEP) forces imparted to the particles suspended in the feed can be employed to mitigate membrane fouling in a cross-flow filtration process.

Repulsive force actuated rotary micromirror

NASA Astrophysics Data System (ADS)

He, Siyuan; Ben Mrad, Ridha

2004-09-01

In this paper, a novel repulsive force based rotary micromirror is proposed. A repulsive force is produced in the rotary micromirror and the mirror plate is pushed up and away from the substrate. Therefore the rotation angle of the micromirror is not limited to the space underneath the mirror plate and thus the "pull-in" effect is completely circumvented. The novel rotary micromirror can achieve a large rotation angle with a large mirror plate. In addition the novel micromirror has a very simple structure and can be fabricated by standard surface micromachining technology. Numerical simulation is used to verify the working principle of the novel micromirror. A prototype of the novel rotary micromirror is fabricated by a commercially available surface microfabrication process called MUMPs. The prototype has a mirror size of 300μm x 300μm. The experimental measurements show that the prototype can achieve a mechanical rotation of 2.25 degrees (an optical angle of 4.5 degrees) at a driving voltage of 170 volts. A conventional surface micromachined attractive force based rotary micromirror of the same size can only achieve an angle of 0.1~0.2 degree.

Casimir Repulsion between Metallic Objects in Vacuum

DTIC Science & Technology

2010-08-27

levitation , as the particle is unstable to displacements away from the symmetry axis. DOI: 10.1103/PhysRevLett.105.090403 PACS numbers: 03.70.+k, 03.65.w...force. The geometry consists of an elongated metal particle centered above a metal plate with a hole. We prove that this geometry has a repulsive regime...ever be repulsive? In this Letter, we answer this question in the affirmative by showing that a small elongated metal particle centered above a thin

Casimir repulsion between metallic objects in vacuum.

PubMed

Levin, Michael; McCauley, Alexander P; Rodriguez, Alejandro W; Reid, M T Homer; Johnson, Steven G

2010-08-27

We give an example of a geometry in which two metallic objects in vacuum experience a repulsive Casimir force. The geometry consists of an elongated metal particle centered above a metal plate with a hole. We prove that this geometry has a repulsive regime using a symmetry argument and confirm it with numerical calculations for both perfect and realistic metals. The system does not support stable levitation, as the particle is unstable to displacements away from the symmetry axis.

Interaction of evaporating and condensing particles in the free-molecular regime

NASA Astrophysics Data System (ADS)

Kogan, M. N.; Bobrov, I. N.; Cercignani, C.; Frezzotti, A.

1995-07-01

In a previous paper it was shown that repulsive/attractive forces arise between evaporating/ condensing particles in the free-molecular regime. Here we obtain explicit expressions for these forces in the case of spherical particles with equal temperatures. The temperature of the surrounding vapor is, generally speaking, different from that of the particles. Numerical results are obtained for different values of the ratios between particle and vapor temperatures and pressures, of the particles radii and of the evaporation coefficients. In the case when the evaporation coefficient equals unity, an exact expression is obtained for the force between particles of different radii. A simple model describing coagulation processes and taking the above-mentioned forces into account is proposed. It is shown that for large values of the vapor supersaturation, the influence of these forces on the coagulation rate may be very pronounced.

Micelle-induced depletion interaction and resultant structure in charged colloidal nanoparticle system

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ray, D.; Aswal, V. K., E-mail: vkaswal@barc.gov.in; Kohlbrecher, J.

2015-04-28

The evolution of the interaction and the resultant structure in the mixed system of anionic silica nanoparticles (Ludox LS30) and non-ionic surfactant decaethylene glycol monododecylether (C12E10), undergoing phase separation, have been studied using small-angle neutron scattering and dynamic light scattering. The measurements have been carried out for a fixed concentration of nanoparticle (1 wt. %) with varying concentration of surfactant (0 to 1 wt. %), in the absence and presence of an electrolyte. It is found that the micelles of non-ionic surfactant adsorb on the nanoparticle in the absence of electrolyte (form stable system), whereas these micelles become non-adsorbing in the presence of electrolytemore » (show phase separation). The phase separation arises because of C12E10 micelles, causing depletion interaction between nanoparticles and leading to their aggregation. The interaction is modeled by double Yukawa potential accounting for attractive depletion as well as repulsive electrostatic forces. Both the interactions (attraction and repulsion) are found to be of long-range. The nanoparticle aggregation (phase separation) is governed by the increase in the magnitude and the range of the depletion attraction with the increase in the surfactant concentration. The nanoparticle aggregates formed are quite large in size (order of micron) and are characterized by the surface fractal having simple cubic packing of nanoparticles within the aggregates.« less

Interaction between Air Bubbles and Superhydrophobic Surfaces in Aqueous Solutions.

PubMed

Shi, Chen; Cui, Xin; Zhang, Xurui; Tchoukov, Plamen; Liu, Qingxia; Encinas, Noemi; Paven, Maxime; Geyer, Florian; Vollmer, Doris; Xu, Zhenghe; Butt, Hans-Jürgen; Zeng, Hongbo

2015-07-07

Superhydrophobic surfaces are usually characterized by a high apparent contact angle of water drops in air. Here we analyze the inverse situation: Rather than focusing on water repellency in air, we measure the attractive interaction of air bubbles and superhydrophobic surfaces in water. Forces were measured between microbubbles with radii R of 40-90 μm attached to an atomic force microscope cantilever and submerged superhydrophobic surfaces. In addition, forces between macroscopic bubbles (R = 1.2 mm) at the end of capillaries and superhydrophobic surfaces were measured. As superhydrophobic surfaces we applied soot-templated surfaces, nanofilament surfaces, micropillar arrays with flat top faces, and decorated micropillars. Depending on the specific structure of the superhydrophobic surfaces and the presence and amount of entrapped air, different interactions were observed. Soot-templated surfaces in the Cassie state showed superaerophilic behavior: Once the electrostatic double-layer force and a hydrodynamic repulsion were overcome, bubbles jumped onto the surface and fully merged with the entrapped air. On nanofilaments and micropillar arrays we observed in addition the formation of sessile bubbles with finite contact angles below 90° or the attachment of bubbles, which retained their spherical shape.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pratt, Lawrence R.; Chaudhari, Mangesh I.; Rempe, Susan B.

Here this review focuses on the striking recent progress in solving for hydrophobic interactions between small inert molecules. We discuss several new understandings. First, the inverse temperature phenomenology of hydrophobic interactions, i.e., strengthening of hydrophobic bonds with increasing temperature, is decisively exhibited by hydrophobic interactions between atomic-scale hard sphere solutes in water. Second, inclusion of attractive interactions associated with atomic-size hydrophobic reference cases leads to substantial, nontrivial corrections to reference results for purely repulsive solutes. Hydrophobic bonds are weakened by adding solute dispersion forces to treatment of reference cases. The classic statistical mechanical theory for those corrections is not accuratemore » in this application, but molecular quasi-chemical theory shows promise. Lastly, because of the masking roles of excluded volume and attractive interactions, comparisons that do not discriminate the different possibilities face an interpretive danger.« less

Arginine "Magic": Guanidinium Like-Charge Ion Pairing from Aqueous Salts to Cell Penetrating Peptides.

PubMed

Vazdar, Mario; Heyda, Jan; Mason, Philip E; Tesei, Giulio; Allolio, Christoph; Lund, Mikael; Jungwirth, Pavel

2018-06-19

It is a textbook knowledge that charges of the same polarity repel each other. For two monovalent ions in the gas phase at a close contact this repulsive interaction amounts to hundreds of kilojoules per mole. In aqueous solutions, however, this Coulomb repulsion is strongly attenuated by a factor equal to the dielectric constant of the medium. The residual repulsion, which now amounts only to units of kilojoules per mole, may be in principle offset by attractive interactions. Probably the smallest cationic pair, where a combination of dispersion and cavitation forces overwhelms the Coulomb repulsion, consists of two guanidinium ions in water. Indeed, by a combination of molecular dynamics with electronic structure calculations and electrophoretic, as well as spectroscopic, experiments, we have demonstrated that aqueous guanidinium cations form (weakly) thermodynamically stable like-charge ion pairs. The importance of pairing of guanidinium cations in aqueous solutions goes beyond a mere physical curiosity, since it has significant biochemical implications. Guanidinium chloride is known to be an efficient and flexible protein denaturant. This is due to the ability of the orientationally amphiphilic guanidinium cations to disrupt various secondary structural motifs of proteins by pairing promiscuously with both hydrophobic and hydrophilic groups, including guanidinium-containing side chains of arginines. The fact that the cationic guanidinium moiety forms the dominant part of the arginine side chain implies that the like-charge ion pairing may also play a role for interactions between peptides and proteins. Indeed, arginine-arginine pairing has been frequently found in structural protein databases. In particular, when strengthened by a presence of negatively charged glutamate, aspartate, or C-terminal carboxylic groups, this binding motif helps to stabilize peptide or protein dimers and is also found in or near active sites of several enzymes. The like-charge pairing of the guanidinium side-chain groups may also hold the key to the understanding of the arginine "magic", that is, the extraordinary ability of arginine-rich polypeptides to passively penetrate across cellular membranes. Unlike polylysines, which are also highly cationic but lack the ease in crossing membranes, polyarginines do not exhibit mutual repulsion. Instead, they accumulate at the membrane, weaken it, and might eventually cross in a concerted, "train-like" manner. This behavior of arginine-rich cell penetrating peptides can be exploited when devising smart strategies how to deliver in a targeted way molecular cargos into the cell.

Raman non-coincidence effect of boroxol ring: The interplay between repulsion and attraction forces in the glassy, supercooled and liquid state

NASA Astrophysics Data System (ADS)

Kalampounias, Angelos G.; Papatheodorou, George N.

2018-06-01

Temperature dependent Raman spectra of boric oxide have been measured in a temperature range covering the glassy, supercooled and liquid state. The shift of the isotropic band assigned to boroxol rings relative to the anisotropic component upon heating the glass is measured and attributed to the Raman non-coincidence effect. The measured shift is associated with the competition between attraction and repulsion forces with increasing temperature. The relation of dephasing and orientational relaxation times to the non-coincidence effect of the condensed phases has been examined. We discuss our results in the framework of the current phenomenological status of the field in an attempt to separate the attraction and repulsion contributions corresponding to the observed non-coincidence effect.

Baryonic Force for Accelerated Cosmic Expansion and Generalized U1b Gauge Symmetry in Particle-Cosmology

NASA Astrophysics Data System (ADS)

Khan, Mehbub; Hao, Yun; Hsu, Jong-Ping

2018-01-01

Based on baryon charge conservation and a generalized Yang-Mills symmetry for Abelian (and non-Abelian) groups, we discuss a new baryonic gauge field and its linear potential for two point-like baryon charges. The force between two point-like baryons is repulsive, extremely weak and independent of distance. However, for two extended baryonic systems, we have a dominant linear force α r. Thus, only in the later stage of the cosmic evolution, when two baryonic galaxies are separated by an extremely large distance, the new repulsive baryonic force can overcome the gravitational attractive force. Such a model provides a gauge-field-theoretic understanding of the late-time accelerated cosmic expansion. The baryonic force can be tested by measuring the accelerated Wu-Doppler frequency shifts of supernovae at different distances.

Transferable model of water with variable molecular size

NASA Astrophysics Data System (ADS)

Kiss, Péter T.; Baranyai, András

2011-06-01

By decreasing the steepness of the repulsive wing in the intermolecular potential, one can extend the applicability of a water model to the high pressure region. Exploiting this trivial possibility, we published a polarizable model of water which provided good estimations not only of gas clusters, ambient liquid, hexagonal ice, but ice VII at very high pressures as well [A. Baranyai and P. Kiss, J. Chem. Phys. 133, 144109 (2010), 10.1063/1.3490660]. This straightforward method works well provided the closest O-O distance is reasonably shorter in the high pressure phase than in hexagonal ice. If these O-O distances are close to each other and we fit the interactions to obtain an accurate picture of hexagonal ice, we underestimate the density of the high-pressure phases. This can be overcome if models use contracted molecules under high external pressure.In this paper we present a method, which is capable to describe the contraction of water molecules under high pressure by using two simple repulsion-attraction functions. These functions represent the dispersion interaction under low pressure and high pressure. The switch function varies between 0 and 1 and portions the two repulsions among the individual particles. The argument of the switch function is a virial-type expression, which can be interpreted as a net force compressing the molecule. We calculated the properties of gas clusters, densities, and internal energies of ambient water, hexagonal ice, ice III, ice VI, and ice VII phases and obtained excellent match of experimental data.

Construction and Self-Assembly of Single-Chain Polymer Nanoparticles via Coordination Association and Electrostatic Repulsion in Water.

PubMed

Zhu, Zhengguang; Xu, Na; Yu, Qiuping; Guo, Lei; Cao, Hui; Lu, Xinhua; Cai, Yuanli

2015-08-01

Simultaneous coordination-association and electrostatic-repulsion interactions play critical roles in the construction and stabilization of enzymatic function metal centers in water media. These interactions are promising for construction and self-assembly of artificial aqueous polymer single-chain nanoparticles (SCNPs). Herein, the construction and self-assembly of dative-bonded aqueous SCNPs are reported via simultaneous coordination-association and electrostatic-repulsion interactions within single chains of histamine-based hydrophilic block copolymer. The electrostatic-repulsion interactions are tunable through adjusting the imidazolium/imidazole ratio in response to pH, and in situ Cu(II)-coordination leads to the intramolecular association and single-chain collapse in acidic water. SCNPs are stabilized by the electrostatic repulsion of dative-bonded block and steric shielding of nonionic water-soluble block, and have a huge specific surface area of function metal centers accessible to substrates in acidic water. Moreover, SCNPs can assemble into micelles, networks, and large particles programmably in response to the solution pH. These unique media-sensitive phase-transformation behaviors provide a general, facile, and versatile platform for the fabrication of enzyme-inspired smart aqueous catalysts. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Depletion forces on circular and elliptical obstacles induced by active matter

NASA Astrophysics Data System (ADS)

Leite, L. R.; Lucena, D.; Potiguar, F. Q.; Ferreira, W. P.

2016-12-01

Depletion forces exerted by self-propelled particles on circular and elliptical passive objects are studied using numerical simulations. We show that a bath of active particles can induce repulsive and attractive forces which are sensitive to the shape and orientation of the passive objects (either horizontal or vertical ellipses). The resultant force on the passive objects due to the active particles is studied as a function of the shape and orientation of the passive objects, magnitude of the angular noise, and distance between the passive objects. By increasing the distance between obstacles the magnitude of the repulsive depletion force increases, as long as such a distance is less than one active particle diameter. For longer distances, the magnitude of the force always decreases with increasing distance. We also found that attractive forces may arise for vertical ellipses at high enough area fraction.

Depletion forces on circular and elliptical obstacles induced by active matter.

PubMed

Leite, L R; Lucena, D; Potiguar, F Q; Ferreira, W P

2016-12-01

Depletion forces exerted by self-propelled particles on circular and elliptical passive objects are studied using numerical simulations. We show that a bath of active particles can induce repulsive and attractive forces which are sensitive to the shape and orientation of the passive objects (either horizontal or vertical ellipses). The resultant force on the passive objects due to the active particles is studied as a function of the shape and orientation of the passive objects, magnitude of the angular noise, and distance between the passive objects. By increasing the distance between obstacles the magnitude of the repulsive depletion force increases, as long as such a distance is less than one active particle diameter. For longer distances, the magnitude of the force always decreases with increasing distance. We also found that attractive forces may arise for vertical ellipses at high enough area fraction.

D-amino acids inhibit initial bacterial adhesion: thermodynamic evidence.

PubMed

Xing, Su-Fang; Sun, Xue-Fei; Taylor, Alicia A; Walker, Sharon L; Wang, Yi-Fu; Wang, Shu-Guang

2015-04-01

Bacterial biofilms are structured communities of cells enclosed in a self-produced hydrated polymeric matrix that can adhere to inert or living surfaces. D-Amino acids were previously identified as self-produced compounds that mediate biofilm disassembly by causing the release of the protein component of the polymeric matrix. However, whether exogenous D-amino acids could inhibit initial bacterial adhesion is still unknown. Here, the effect of the exogenous amino acid D-tyrosine on initial bacterial adhesion was determined by combined use of chemical analysis, force spectroscopic measurement, and theoretical predictions. The surface thermodynamic theory demonstrated that the total interaction energy increased with more D-tyrosine, and the contribution of Lewis acid-base interactions relative to the change in the total interaction energy was much greater than the overall nonspecific interactions. Finally, atomic force microscopy analysis implied that the hydrogen bond numbers and adhesion forces decreased with the increase in D-tyrosine concentrations. D-Tyrosine contributed to the repulsive nature of the cell and ultimately led to the inhibition of bacterial adhesion. This study provides a new way to regulate biofilm formation by manipulating the contents of D-amino acids in natural or engineered systems. © 2014 Wiley Periodicals, Inc.

Auxiliary-field quantum Monte Carlo simulations of neutron matter in chiral effective field theory.

PubMed

Wlazłowski, G; Holt, J W; Moroz, S; Bulgac, A; Roche, K J

2014-10-31

We present variational Monte Carlo calculations of the neutron matter equation of state using chiral nuclear forces. The ground-state wave function of neutron matter, containing nonperturbative many-body correlations, is obtained from auxiliary-field quantum Monte Carlo simulations of up to about 340 neutrons interacting on a 10(3) discretized lattice. The evolution Hamiltonian is chosen to be attractive and spin independent in order to avoid the fermion sign problem and is constructed to best reproduce broad features of the chiral nuclear force. This is facilitated by choosing a lattice spacing of 1.5 fm, corresponding to a momentum-space cutoff of Λ=414  MeV/c, a resolution scale at which strongly repulsive features of nuclear two-body forces are suppressed. Differences between the evolution potential and the full chiral nuclear interaction (Entem and Machleidt Λ=414  MeV [L. Coraggio et al., Phys. Rev. C 87, 014322 (2013).

On coagulation mechanisms of charged nanoparticles produced by combustion of hydrocarbon and metallized fuels

DOE Office of Scientific and Technical Information (OSTI.GOV)

Savel'ev, A. M.; Starik, A. M.

2009-02-15

The contributions of van der Waals, Coulomb, and polarization interactions between nanometersized particles to the particle coagulation rate in both free-molecular and continuum regimes are analyzed for particle charges of various magnitudes and signs. Analytical expressions are obtained for the coagulation rate constant between particles whose interaction in the free-molecular regime is described by a singular potential. It is shown that van der Waals and polarization forces significantly increase the coagulation rate between a neutral and a charged particle (by a factor of up to 10) and can even suppress the Coulomb repulsion between like-charged particles of widely different sizes.

[On Two Opposing (Bio)surfaces as Comprehended in Terms of an Extension of the Coulomb-Amontons Law of Friction, with Its Virtual Usefulness for Biotribology in Nanoscale].

PubMed

Gadomski, A; Hladyszowski, J

2015-01-01

An extension of the Coulomb-Amontons law is proposed in terms of an interaction-detail involving renormalization (simplified) n-th level scheme. The coefficient of friction is obtained in a general exponential (nonlinear) form, characteristic of virtually infinite (or, many body) level of the interaction map. Yet, its application for a hydration repulsion bilayered system, prone to facilitated lubrication, is taken as linearly confined, albeit with an inclusion of a decisive repelling force/pressure factor. Some perspectives toward related systems, fairly outside biotribological issues, have been also addressed.

Tunable Stable Levitation Based on Casimir Interaction between Nanostructures

NASA Astrophysics Data System (ADS)

Liu, Xianglei; Zhang, Zhuomin M.

2016-03-01

Quantum levitation enabled by repulsive Casimir force has been desirable due to the potential exciting applications in passive-suspension devices and frictionless bearings. In this paper, dynamically tunable stable levitation is theoretically demonstrated based on the configuration of dissimilar gratings separated by an intervening fluid using exact scattering theory. The levitation position is insensitive to temperature variations and can be actively tuned by adjusting the lateral displacement between the two gratings. This work investigates the possibility of applying quantum Casimir interactions into macroscopic mechanical devices working in a noncontact and low-friction environment for controlling the position or transducing lateral movement into vertical displacement at the nanoscale.

Interaction of a parabolic-shaped pulse pair in a passively mode-locked Yb-doped fiber laser

NASA Astrophysics Data System (ADS)

Wang, Da-Shuai; Wu, Ge; Gao, Bo; Tian, Xiao-Jian

2013-01-01

We numerically investigate the formation and interaction of a parabolic-shaped pulse pair in a passively mode-locked Yb-doped fiber laser. Based on a lumped model, the parabolic-shaped pulse pair is obtained by controlling the inter-cavity average dispersion and gain saturation energy, Moreover, pulse repulsive and attractive motion are also achieved with different pulse separations. Simulation results show that the phase shift plays an important role in pulse interaction, and the interaction is determined by the inter-cavity average dispersion and gain saturation energy, i.e., the strength of the interaction is proportional to the gain saturation energy, a stronger gain saturation energy will result in a higher interaction intensity. On the contrary, the increase of the inter-cavity dispersion will counterbalance some interaction force. The results also show that the interaction of a parabolic-shaped pulse pair has a larger interaction distance compared to conventional solitons.

Simultaneous shape repulsion and global assimilation in the perception of aspect ratio

PubMed Central

Sweeny, Timothy D.; Grabowecky, Marcia; Suzuki, Satoru

2012-01-01

Although local interactions involving orientation and spatial frequency are well understood, less is known about spatial interactions involving higher level pattern features. We examined interactive coding of aspect ratio, a prevalent two-dimensional feature. We measured perception of two simultaneously flashed ellipses by randomly post-cueing one of them and having observers indicate its aspect ratio. Aspect ratios interacted in two ways. One manifested as an aspect-ratio-repulsion effect. For example, when a slightly tall ellipse and a taller ellipse were simultaneously flashed, the less tall ellipse appeared flatter and the taller ellipse appeared even taller. This repulsive interaction was long range, occurring even when the ellipses were presented in different visual hemifields. The other interaction manifested as a global assimilation effect. An ellipse appeared taller when it was a part of a global vertical organization than when it was a part of a global horizontal organization. The repulsion and assimilation effects temporally dissociated as the former slightly strengthened, and the latter disappeared when the ellipse-to-mask stimulus onset asynchrony was increased from 40 to 140 ms. These results are consistent with the idea that shape perception emerges from rapid lateral and hierarchical neural interactions. PMID:21248223

Statistical Analyses of Hydrophobic Interactions: A Mini-Review

DOE PAGES

Pratt, Lawrence R.; Chaudhari, Mangesh I.; Rempe, Susan B.

2016-07-14

Here this review focuses on the striking recent progress in solving for hydrophobic interactions between small inert molecules. We discuss several new understandings. First, the inverse temperature phenomenology of hydrophobic interactions, i.e., strengthening of hydrophobic bonds with increasing temperature, is decisively exhibited by hydrophobic interactions between atomic-scale hard sphere solutes in water. Second, inclusion of attractive interactions associated with atomic-size hydrophobic reference cases leads to substantial, nontrivial corrections to reference results for purely repulsive solutes. Hydrophobic bonds are weakened by adding solute dispersion forces to treatment of reference cases. The classic statistical mechanical theory for those corrections is not accuratemore » in this application, but molecular quasi-chemical theory shows promise. Lastly, because of the masking roles of excluded volume and attractive interactions, comparisons that do not discriminate the different possibilities face an interpretive danger.« less

Direct quantitative measurement of the C═O⋅⋅⋅H–C bond by atomic force microscopy

PubMed Central

Kawai, Shigeki; Nishiuchi, Tomohiko; Kodama, Takuya; Spijker, Peter; Pawlak, Rémy; Meier, Tobias; Tracey, John; Kubo, Takashi; Meyer, Ernst; Foster, Adam S.

2017-01-01

The hydrogen atom—the smallest and most abundant atom—is of utmost importance in physics and chemistry. Although many analysis methods have been applied to its study, direct observation of hydrogen atoms in a single molecule remains largely unexplored. We use atomic force microscopy (AFM) to resolve the outermost hydrogen atoms of propellane molecules via very weak C═O⋅⋅⋅H–C hydrogen bonding just before the onset of Pauli repulsion. The direct measurement of the interaction with a hydrogen atom paves the way for the identification of three-dimensional molecules such as DNAs and polymers, building the capabilities of AFM toward quantitative probing of local chemical reactivity. PMID:28508080

A Monte Carlo (N,V,T) study of the stability of charged interfaces: A simulation on a hypersphere

NASA Astrophysics Data System (ADS)

Delville, A.; Pellenq, R. J.-M.; Caillol, J. M.

1997-05-01

We have used an exact expression of the Coulombic interactions derived on a hypersphere of an Euclidian space of dimension four to determine the swelling behavior of two infinite charged plates neutralized by exchangeable counterions. Monte Carlo simulations in the (N,V,T) ensemble allows for a derivation of short-ranged hard core repulsions and long-ranged electrostatic forces, which are the two components of the interionic forces in the context of the primitive model. Comparison with numerical results obtained by a classical Euclidian method illustrates the efficiency of the hyperspherical approach, especially at strong coupling between the charged particles, i.e., for divalent counterions and small plate separation.

Large-scale compensation of errors in pairwise-additive empirical force fields: comparison of AMBER intermolecular terms with rigorous DFT-SAPT calculations.

PubMed

Zgarbová, Marie; Otyepka, Michal; Sponer, Jirí; Hobza, Pavel; Jurecka, Petr

2010-09-21

The intermolecular interaction energy components for several molecular complexes were calculated using force fields available in the AMBER suite of programs and compared with Density Functional Theory-Symmetry Adapted Perturbation Theory (DFT-SAPT) values. The extent to which such comparison is meaningful is discussed. The comparability is shown to depend strongly on the intermolecular distance, which means that comparisons made at one distance only are of limited value. At large distances the coulombic and van der Waals 1/r(6) empirical terms correspond fairly well with the DFT-SAPT electrostatics and dispersion terms, respectively. At the onset of electronic overlap the empirical values deviate from the reference values considerably. However, the errors in the force fields tend to cancel out in a systematic manner at equilibrium distances. Thus, the overall performance of the force fields displays errors an order of magnitude smaller than those of the individual interaction energy components. The repulsive 1/r(12) component of the van der Waals expression seems to be responsible for a significant part of the deviation of the force field results from the reference values. We suggest that further improvement of the force fields for intermolecular interactions would require replacement of the nonphysical 1/r(12) term by an exponential function. Dispersion anisotropy and its effects are discussed. Our analysis is intended to show that although comparing the empirical and non-empirical interaction energy components is in general problematic, it might bring insights useful for the construction of new force fields. Our results are relevant to often performed force-field-based interaction energy decompositions.

The shape and dynamics of local attraction

NASA Astrophysics Data System (ADS)

Strömbom, D.; Siljestam, M.; Park, J.; Sumpter, D. J. T.

2015-11-01

Moving animal groups, such as flocks of birds or schools of fish, exhibit a varity of self-organized complex dynamical behaviors and shapes. This kind of flocking behavior has been studied using self-propelled particle models, in which the "particles" interact with their nearest neighbors through repulsion, attraction and alignment responses. In particular, it has been shown that models based on attraction alone can generate a range of dynamic groups in 2D, with periodic boundary conditions, and in the absence of repulsion. Here we investigate the effects of changing these conditions on the type of groups observed in the model. We show that replacing the periodic boundary conditions with a weak global attaction term in 2D, and extending the model to 3D does not significantly change the type of groups observed. We also provide a description of how attraction strength and blind angle determine the groups generated in the 3D version of the model. Finally, we show that adding repulsion do change the type of groups oberved, making them appear and behave more like real moving animal groups. Our results suggest that many biological instances of collective motion may be explained without assuming that animals explicitly align with each other. Instead, complex collective motion is explained by the interplay of attraction and repulsion forces. Supplementary material in the form of four mp4 files available from the Journal web page at http://dx.doi.org/10.1140/epjst/e2015-50093-5

The evolution processes of DNA sequences, languages and carols

NASA Astrophysics Data System (ADS)

Hauck, Jürgen; Henkel, Dorothea; Mika, Klaus

2001-04-01

The sequences of bases A, T, C and G of about 100 enolase, secA and cytochrome DNA were analyzed for attractive or repulsive interactions by the numbers T 1,T 2,T 3; r of nearest, next-nearest and third neighbor bases of the same kind and the concentration r=other bases/analyzed base. The area of possible T1, T2 values is limited by the linear borders T 2=2T 1-2, T 2=0 or T1=0 for clustering, attractive or repulsive interactions and the border T2=-2 T1+2(2- r) for a variation from repulsive to attractive interactions at r⩽2. Clustering is preferred by most bases in sequences of enolases and secA’ s. Major deviations with repulsive interactions of some bases are observed for archaea bacteria in secA and for highly developed animals and the human species in enolase sequences. The borders of the structure map for enthalpy stabilized structures with maximum interactions are approached in few cases. Most letters of the natural languages and some music notes are at the borders of the structure map.

Casimir forces between defects in one-dimensional quantum liquids

NASA Astrophysics Data System (ADS)

Recati, A.; Fuchs, J. N.; Peça, C. S.; Zwerger, W.

2005-08-01

We discuss the effective interactions between two localized perturbations in one-dimensional quantum liquids. For noninteracting fermions, the interactions exhibit Friedel oscillations, giving rise to a Ruderman-Kittel-Kasuya-Yosida-type interaction familiar from impurity spins in metals. In the interacting case, at low energies, a Luttinger-liquid description applies. In the case of repulsive fermions, the Friedel oscillations of the interacting system are replaced, at long distances, by a universal Casimir-type interaction which depends only on the sound velocity and decays inversely with the separation. The Casimir-type interaction between localized perturbations embedded in a fermionic environment gives rise to a long-range coupling between quantum dots in ultracold Fermi gases, opening an alternative to couple qubits with neutral atoms. We also briefly discuss the case of bosonic quantum liquids in which the interaction between weak impurities turns out to be short ranged, decaying exponentially on the scale of the healing length.

DNA Replication and Cell Cycle Progression Regulatedby Long Range Interaction between Protein Complexes bound to DNA.

PubMed

Matsson, L

2001-12-01

A nonstationary interaction that controlsDNA replication and the cell cycle isderived from many-body physics in achemically open T cell. The model predictsa long range force F'(ξ) =- (κ/2) ξ(1 - ξ)(2 - ξ)between thepre-replication complexes (pre-RCs) boundby the origins in DNA, ξ = ϕ/N being the relativedisplacement of pre-RCs, ϕ the number of pre-RCs, N the number of replicons to be replicated,and κ the compressibilitymodulus in the lattice of pre-RCs whichbehaves dynamically like an elasticallybraced string. Initiation of DNAreplication is induced at the thresholdϕ = N by a switch ofsign of F''(ξ), fromattraction (-) and assembly in the G(1) phase (0<ϕ

High-temperature atomic superfluidity in lattice Bose-Fermi mixtures.

PubMed

Illuminati, Fabrizio; Albus, Alexander

2004-08-27

We consider atomic Bose-Fermi mixtures in optical lattices and study the superfluidity of fermionic atoms due to s-wave pairing induced by boson-fermion interactions. We prove that the induced fermion-fermion coupling is always attractive if the boson-boson on-site interaction is repulsive, and predict the existence of an enhanced BEC-BCS crossover as the strength of the lattice potential is varied. We show that for direct on-site fermion-fermion repulsion, the induced attraction can give rise to superfluidity via s-wave pairing at striking variance with the case of pure systems of fermionic atoms with direct repulsive interactions.

Small-Angle Neutron Scattering Study of Interplay of Attractive and Repulsive Interactions in Nanoparticle-Polymer System.

PubMed

Kumar, Sugam; Aswal, Vinod K; Kohlbrecher, Joachim

2016-02-16

The phase behavior of nanoparticle (silica)-polymer (polyethylene glycol) system without and with an electrolyte (NaCl) has been studied. It is observed that nanoparticle-polymer system behaves very differently in the presence of electrolyte. In the absence of electrolyte, the nanoparticle-polymer system remains in one-phase even at very high polymer concentrations. On the other hand, a re-entrant phase behavior is found in the presence of electrolyte, where one-phase (individual) system undergoes two-phase (nanoparticle aggregation) and then back to one-phase with increasing polymer concentration. The regime of two-phase system has been tuned by varying the electrolyte concentration. The polymer concentration range over which the two-phase system exists is significantly enhanced with the increase in the electrolyte concentration. These systems have been characterized by small-angle neutron scattering (SANS) experiments of contrast-marching the polymer to the solvent. The data are modeled using a two-Yukawa potential accounting for both attractive and repulsive parts of the interaction between nanoparticles. The phase behavior of nanoparticle-polymer system is explained by interplay of attractive (polymer-induced attractive depletion between nanoparticles) and repulsive (nanoparticle-nanoparticle electrostatic repulsion and polymer-polymer repulsion) interactions present in the system. In the absence of electrolyte, the strong electrostatic repulsion between nanoparticles dominates over the polymer-induced depletion attraction and the nanoparticle system remains in one-phase. With addition of electrolyte, depletion attraction overcomes electrostatic repulsion at some polymer concentration, resulting into nanoparticle aggregation and two-phase system. Further addition of polymer increases the polymer-polymer repulsion which eventually reduces the strength of depletion and hence re-entrant phase behavior. The effects of varying electrolyte concentration on the phase behavior of nanoparticle-polymer system are understood in terms of modifications in nanoparticle-nanoparticle and polymer-polymer interactions. The nanoparticle aggregates in two-phase systems are found to have surface fractal morphology.

Tunable interactions between paramagnetic colloidal particles driven in a modulated ratchet potential.

PubMed

Straube, Arthur V; Tierno, Pietro

2014-06-14

We study experimentally and theoretically the interactions between paramagnetic particles dispersed in water and driven above the surface of a stripe patterned magnetic garnet film. An external rotating magnetic field modulates the stray field of the garnet film and generates a translating potential landscape which induces directed particle motion. By varying the ellipticity of the rotating field, we tune the inter-particle interactions from net repulsive to net attractive. For attractive interactions, we show that pairs of particles can approach each other and form stable doublets which afterwards travel along the modulated landscape at a constant mean speed. We measure the strength of the attractive force between the moving particles and propose an analytically tractable model that explains the observations and is in quantitative agreement with experiment.

Gauge field back reaction on a black hole

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hochberg, D.; Kephart, T.W.

1993-02-15

The order-[h bar] fluctuations of gauge fields in the vicinity of a black hole can create a repulsive antigravity region extending out beyond the renormalized Schwarzschild horizon. If the strength of this repulsive force increases as higher orders in the back reaction are included, the formation of a wormholelike object could occur.

Electron Pairing, Repulsion, and Correlation: A Simplistic Approach

ERIC Educational Resources Information Center

Olsson, Lars-Fride; Kloo, Lars

2004-01-01

The interplay between a nucleus and an electron pair is explained through a basic application of an electrostatic and balanced model to determine the correlated and repulsive movements of the electron pair. The stable correlation depends on the positive charge produced by the combined force, which in turn establishes a negative potential energy.

The Simple Metals and New Models of the Interacting-Electron-Gas Type: I. Anomalous Plasmon Dispersion Relations in Heavy Alkali Metals

NASA Astrophysics Data System (ADS)

Okuda, Takashi; Horio, Kohji; Ohmura, Yoshihiro; Mizuno, Yukio

2018-06-01

The well-known interacting-electron-gas model of metallic states is modified by replacing the Coulomb interaction by a truncated one to weaken the repulsive force between electrons at short distances. The new model is applied to the so-called simple metals and is found far superior to the old one. Most of the calculations are carried out successfully on the basis of the random-phase-approximation (RPA), which is known much too poor for the old familiar model. In the present paper the numerical value of the new parameter peculiar to the new model is determined systematically with the help of the observed plasmon spectrum for each metal.

Quantum liquid droplets in a mixture of Bose-Einstein condensates

NASA Astrophysics Data System (ADS)

Cabrera, C. R.; Tanzi, L.; Sanz, J.; Naylor, B.; Thomas, P.; Cheiney, P.; Tarruell, L.

2018-01-01

Quantum droplets are small clusters of atoms self-bound by the balance of attractive and repulsive forces. Here, we report on the observation of droplets solely stabilized by contact interactions in a mixture of two Bose-Einstein condensates. We demonstrate that they are several orders of magnitude more dilute than liquid helium by directly measuring their size and density via in situ imaging. We show that the droplets are stablized against collapse by quantum fluctuations and that they require a minimum atom number to be stable. Below that number, quantum pressure drives a liquid-to-gas transition that we map out as a function of interaction strength. These ultradilute isotropic liquids remain weakly interacting and constitute an ideal platform to benchmark quantum many-body theories.

Tuning of electrostatic vs. depletion interaction in deciding the phase behavior of nanoparticle-polymer system

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kumar, Sugam, E-mail: sugam@barc.gov.in; Aswal, V. K.; Kohlbrecher, J.

2015-06-24

Nanoparticle-polymer system interestingly show a re-entrant phase behavior where charge stabilized silica nanoparticles (phase I) undergo particle clustering (phase II) and then back to individual particles (phase I) as a function of polymer concentration. Such phase behavior arises as a result of dominance of various interactions (i) nanoparticle-nanoparticle electrostatic repulsion (ii) polymer induced attractive depletion between nanoparticles and (iii) polymer-polymer repulsion, at different concentration regimes. Small-angle neutron scattering (SANS) has been used to study the evolution of interaction during this re-entrant phase behavior of nanoparticles by contrast-marching the polymer. The SANS data have been modeled using a two-Yukawa potential accountingmore » for both attractive and repulsive parts of the interaction between nanoparticles. The degree of both of these parts has been separately tuned by varying the polymer concentration and ionic strength of the solution. Both of these parts are found to have long-range nature. At low polymer concentrations, the electrostatic repulsion dominates over the depletion attraction. The magnitude and the range of the depletion interaction increase with the polymer concentration leading to nanoparticle clustering. At higher polymer concentrations, the increased polymer-polymer repulsion reduces the strength of depletion leading to re-entrant phase behavior. The clusters formed under depletion attraction are found to have surface fractal morphology.« less

Fluctuation spectra and force generation in nonequilibrium systems.

PubMed

Lee, Alpha A; Vella, Dominic; Wettlaufer, John S

2017-08-29

Many biological systems are appropriately viewed as passive inclusions immersed in an active bath: from proteins on active membranes to microscopic swimmers confined by boundaries. The nonequilibrium forces exerted by the active bath on the inclusions or boundaries often regulate function, and such forces may also be exploited in artificial active materials. Nonetheless, the general phenomenology of these active forces remains elusive. We show that the fluctuation spectrum of the active medium, the partitioning of energy as a function of wavenumber, controls the phenomenology of force generation. We find that, for a narrow, unimodal spectrum, the force exerted by a nonequilibrium system on two embedded walls depends on the width and the position of the peak in the fluctuation spectrum, and oscillates between repulsion and attraction as a function of wall separation. We examine two apparently disparate examples: the Maritime Casimir effect and recent simulations of active Brownian particles. A key implication of our work is that important nonequilibrium interactions are encoded within the fluctuation spectrum. In this sense, the noise becomes the signal.

Repulsive Casimir effect from extra dimensions and Robin boundary conditions: From branes to pistons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Elizalde, E.; Odintsov, S. D.; Institucio Catalana de Recerca i Estudis Avanccats

2009-03-15

We evaluate the Casimir energy and force for a massive scalar field with general curvature coupling parameter, subject to Robin boundary conditions on two codimension-one parallel plates, located on a (D+1)-dimensional background spacetime with an arbitrary internal space. The most general case of different Robin coefficients on the two separate plates is considered. With independence of the geometry of the internal space, the Casimir forces are seen to be attractive for special cases of Dirichlet or Neumann boundary conditions on both plates and repulsive for Dirichlet boundary conditions on one plate and Neumann boundary conditions on the other. For Robinmore » boundary conditions, the Casimir forces can be either attractive or repulsive, depending on the Robin coefficients and the separation between the plates, what is actually remarkable and useful. Indeed, we demonstrate the existence of an equilibrium point for the interplate distance, which is stabilized due to the Casimir force, and show that stability is enhanced by the presence of the extra dimensions. Applications of these properties in braneworld models are discussed. Finally, the corresponding results are generalized to the geometry of a piston of arbitrary cross section.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Van Vleet, Mary J.; Misquitta, Alston J.; Stone, Anthony J.

Short-range repulsion within inter-molecular force fields is conventionally described by either Lennard-Jones or Born-Mayer forms. Despite their widespread use, these simple functional forms are often unable to describe the interaction energy accurately over a broad range of inter-molecular distances, thus creating challenges in the development of ab initio force fields and potentially leading to decreased accuracy and transferability. Herein, we derive a novel short-range functional form based on a simple Slater-like model of overlapping atomic densities and an iterated stockholder atom (ISA) partitioning of the molecular electron density. We demonstrate that this Slater-ISA methodology yields a more accurate, transferable, andmore » robust description of the short-range interactions at minimal additional computational cost compared to standard Lennard-Jones or Born-Mayer approaches. Lastly, we show how this methodology can be adapted to yield the standard Born-Mayer functional form while still retaining many of the advantages of the Slater-ISA approach.« less

Pair interactions of heavy vortices in quantum fluids

NASA Astrophysics Data System (ADS)

Pshenichnyuk, Ivan A.

2018-02-01

The dynamics of quantum vortex pairs carrying heavy doping matter trapped inside their cores is studied. The nonlinear classical matter field formalism is used to build a universal mathematical model of a heavy vortex applicable to different types of quantum mixtures. It is shown how the usual vortex dynamics typical for undoped pairs qualitatively changes when heavy dopants are used: heavy vortices with opposite topological charges (chiralities) attract each other, while vortices with the same charge are repelled. The force responsible for such behavior appears as a result of superposition of vortices velocity fields in the presence of doping substance and can be considered as a special realization of the Magnus effect. The force is evaluated quantitatively and its inverse proportionality to the distance is demonstrated. The mechanism described in this paper gives an example of how a light nonlinear classical field may realize repulsive and attractive interactions between embedded heavy impurities.

Protonmotive force: development of electrostatic drivers for synthetic molecular motors.

PubMed

Crowley, James D; Steele, Ian M; Bosnich, Brice

2006-12-04

Ferrocene has been investigated as a platform for developing protonmotive electrostatic drivers for molecular motors. When two 3-pyridine groups are substituted to the (rapidly rotating) cyclopentadienyl (Cp) rings of ferrocene, one on each Cp, it is shown that the (Cp) eclipsed, pi-stacked rotameric conformation is preferred both in solution and in the solid state. Upon quaternization of both of the pyridines substituents, either by protonation or by alkylation, it is shown that the preferred rotameric conformation is one where the pyridinium groups are rotated away from the fully pi-stacked conformation. Electrostatic calculations indicate that the rotation is caused by the electrostatic repulsion between the charges. Consistently, when the pi-stacking energy is increased pi-stacked population increases, and conversely when the electrostatic repulsion is increased pi-stacked population is decreased. This work serves to provide an approximate estimate of the amount of torque that the electrostatically driven ferrocene platform can generate when incorporated into a molecular motor. The overall conclusion is that the electrostatic interaction energy between dicationic ferrocene dipyridyl systems is similar to the pi-stacking interaction energy and, consequently, at least tricationic systems are required to fully uncouple the pi-stacked pyridine substituents.

Tension amplification in tethered layers of bottle-brush polymers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Leuty, Gary M.; Tsige, Mesfin; Grest, Gary S.

2016-02-26

In this paper, molecular dynamics simulations of a coarse-grained bead–spring model have been used to study the effects of molecular crowding on the accumulation of tension in the backbone of bottle-brush polymers tethered to a flat substrate. The number of bottle-brushes per unit surface area, Σ, as well as the lengths of the bottle-brush backbones N bb (50 ≤ N bb ≤ 200) and side chains N sc (50 ≤ N sc ≤ 200) were varied to determine how the dimensions and degree of crowding of bottle-brushes give rise to bond tension amplification along the backbone, especially near the substrate.more » From these simulations, we have identified three separate regimes of tension. For low Σ, the tension is due solely to intramolecular interactions and is dominated by the side chain repulsion that governs the lateral brush dimensions. With increasing Σ, the interactions between bottle-brush polymers induce compression of the side chains, transmitting increasing tension to the backbone. For large Σ, intermolecular side chain repulsion increases, forcing side chain extension and reorientation in the direction normal to the surface and transmitting considerable tension to the backbone.« less

Influence of smectite suspension structure on sheet orientation in dry sediments: XRD and AFM applications.

PubMed

Zbik, Marek S; Frost, Ray L

2010-06-15

The structure-building phenomena within clay aggregates are governed by forces acting between clay particles. Measurements of such forces are important to understand in order to manipulate the aggregate structure for applications such as dewatering of mineral processing tailings. A parallel particle orientation is required when conducting XRD investigation on the oriented samples and conduct force measurements acting between basal planes of clay mineral platelets using atomic force microscopy (AFM). To investigate how smectite clay platelets were oriented on silicon wafer substrate when dried from suspension range of methods like SEM, XRD and AFM were employed. From these investigations, we conclude that high clay concentrations and larger particle diameters (up to 5 microm) in suspension result in random orientation of platelets in the substrate. The best possible laminar orientation in the clay dry film, represented in the XRD 001/020 intensity ratio of 47 was obtained by drying thin layers from 0.02 wt.% clay suspensions of the natural pH. Conducted AFM investigations show that smectite studied in water based electrolytes show very long-range repulsive forces lower in strength than electrostatic forces from double-layer repulsion. It was suggested that these forces may have structural nature. Smectite surface layers rehydrate in water environment forms surface gel with spongy and cellular texture which cushion approaching AFM probe. This structural effect can be measured in distances larger than 1000 nm from substrate surface and when probe penetrate this gel layer, structural linkages are forming between substrate and clay covered probe. These linkages prevent subsequently smooth detachments of AFM probe on way back when retrieval. This effect of tearing new formed structure apart involves larger adhesion-like forces measured in retrieval. It is also suggested that these effect may be enhanced by the nano-clay particles interaction. 2010 Elsevier Inc. All rights reserved.

Dendrimer Interactions with Lipid Bilayer: Comparison of Force Field and Effect of Implicit vs Explicit Solvation.

PubMed

Kanchi, Subbarao; Gosika, Mounika; Ayappa, K G; Maiti, Prabal K

2018-06-13

The understanding of dendrimer interactions with cell membranes has great importance in drug/gene delivery based therapeutics. Although molecular simulations have been used to understand the nature of dendrimer interactions with lipid membranes, its dependency on available force field parameters is poorly understood. In this study, we have carried out fully atomistic molecular dynamics (MD) simulations of a protonated G3 poly(amido amine) (PAMAM) dendrimer-dimyristoylphosphatidylcholine (DMPC) lipid bilayer complex using three different force fields (FFs) namely, CHARMM, GAFF, and GROMOS in the presence of explicit water to understand the structure of the lipid-dendrimer complex and nature of their interaction. CHARMM and GAFF dendrimers initially in contact with the lipid head groups were found to move away from the lipid bilayer during the course of simulation; however, the dendrimer remained strongly bound to the lipid head groups with the GROMOS FF. Potential of the mean force (PMF) computations of the dendrimer along the bilayer normal showed a repulsive barrier (∼20 kcal/mol) between dendrimer and lipid bilayer in the case of CHARMM and GAFF force fields. In contrast, an attractive interaction (∼40 kcal/mol) is obtained with the GROMOS force field, consistent with experimental observations of membrane binding observed with lower generation G3 PAMAM dendrimers. This difference with the GROMOS dendrimer is attributed to the strong dendrimer-lipid interaction and lowered surface hydration of the dendrimer. Assessing the role of solvent, we find that the CHARMM and GAFF dendrimers strongly bind to the lipid bilayer with an implicit solvent (Generalized Born) model, whereas binding is not observed with explicit water (TIP3P). The opposing nature of dendrimer-membrane interactions in the presence of explicit and implicit solvents demonstrates that hydration effects play an important role in modulating the dendrimer-lipid interaction warranting a case for refinement of the existing dendrimer/lipid force fields.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Akaishi, Yoshinori; College of Science and Technology, Nihon University, Funabashi 274-8501; Myint, Khin Swe

The overbinding problem of {sub {lambda}}{sup 5}He is solved by introducing a concept of coherent {lambda}-{sigma} coupling which is equivalent to a {lambda}NN three-body force. This three-body force is coherently enhanced in the 0{sup +} states of {sub {lambda}}{sup 4}H and {sub {lambda}}{sup 4}He. The 0{sup +}-1{sup +} splitting in these hypernuclei is mainly due to coherent {lambda}-{sigma} coupling and partly due to the {lambda}N spin-spin interaction. A {lambda}NN three-body potential is derived from the coupled-channel treatment. The origin of the repulsive and attractive nature of the three-body force is discussed. Coherent {lambda}-{sigma} coupling becomes more important in neutron-rich hypernucleimore » and especially in neutron-star matter at high densities. The possible existence of ''hyperheavy hydrogen'', {sub {lambda}}{sup 6}H, is suggested.« less

Electrostatics of colloids in mixtures

NASA Astrophysics Data System (ADS)

Samin, Sela; Tsori, Yoav

2013-03-01

We examine the force between two charged colloids immersed in salty aqueous mixtures close to the coexistence curve. In an initially water-poor phase, the short-range solvation-related forces promote the condensation of a water-rich phase at a distance in the range 1-100nm. This leads to a strong long-range attraction between the colloids and hence to a deep metastable or globally stable energetic state. Our calculations are in good agreement with recent experiments on the reversible aggregation of colloids in critical mixtures. The specific nature of the solvation energy of ions can lead to some surprising effects, whereby positively charged surfaces attract while negatively charged surfaces repel. For hydrophilic anions and hydrophobic cations, a repulsive interaction is predicted between oppositely charged and hydrophilic colloids even though both the electrostatic and adsorption forces alone are attractive.

Forces between functionalized silica nanoparticles in solution

NASA Astrophysics Data System (ADS)

Lane, J. Matthew D.; Ismail, Ahmed E.; Chandross, Michael; Lorenz, Christian D.; Grest, Gary S.

2009-05-01

To prevent the flocculation and phase separation of nanoparticles in solution, nanoparticles are often functionalized with short chain surfactants. Here we present fully atomistic molecular dynamics simulations which characterize how these functional coatings affect the interactions between nanoparticles and with the surrounding solvent. For 5-nm-diameter silica nanoparticles coated with poly(ethylene oxide) (PEO) oligomers in water, we determined the hydrodynamic drag on two approaching nanoparticles moving through solvent and on a single nanoparticle as it approaches a planar surface. In most circumstances, macroscale fluid theory accurately predicts the drag on these nanoscale particles. Good agreement is seen with Brenner’s analytical solutions for wall separations larger than the soft nanoparticle radius. For two approaching coated nanoparticles, the solvent-mediated (velocity independent) and lubrication (velocity-dependent) forces are purely repulsive and do not exhibit force oscillations that are typical of uncoated rigid spheres.

Tunable Electron-Electron Interactions in LaAlO 3 / SrTiO 3 Nanostructures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cheng, Guanglei; Tomczyk, Michelle; Tacla, Alexandre B.

The interface between the two complex oxides LaAlO 3 and SrTiO 3 has remarkable properties that can be locally reconfigured between conducting and insulating states using a conductive atomic force microscope. Prior investigations of “sketched” quantum dot devices revealed a phase in which electrons form pairs, implying a strongly attractive electron-electron interaction. Here, we show that these devices with strong electron-electron interactions can exhibit a gate-tunable transition from a pair-tunneling regime to a single-electron (Andreev bound state) tunneling regime where the interactions become repulsive. The electron-electron interaction sign change is associated with a Lifshitz transition where the d xz andmore » d yz bands start to become occupied. This electronically tunable electron-electron interaction, combined with the nanoscale reconfigurability of this system, provides an interesting starting point towards solid-state quantum simulation.« less

Liquid drops attract or repel by the inverted Cheerios effect.

PubMed

Karpitschka, Stefan; Pandey, Anupam; Lubbers, Luuk A; Weijs, Joost H; Botto, Lorenzo; Das, Siddhartha; Andreotti, Bruno; Snoeijer, Jacco H

2016-07-05

Solid particles floating at a liquid interface exhibit a long-ranged attraction mediated by surface tension. In the absence of bulk elasticity, this is the dominant lateral interaction of mechanical origin. Here, we show that an analogous long-range interaction occurs between adjacent droplets on solid substrates, which crucially relies on a combination of capillarity and bulk elasticity. We experimentally observe the interaction between droplets on soft gels and provide a theoretical framework that quantitatively predicts the interaction force between the droplets. Remarkably, we find that, although on thick substrates the interaction is purely attractive and leads to drop-drop coalescence, for relatively thin substrates a short-range repulsion occurs, which prevents the two drops from coming into direct contact. This versatile interaction is the liquid-on-solid analog of the "Cheerios effect." The effect will strongly influence the condensation and coarsening of drops on soft polymer films, and has potential implications for colloidal assembly and mechanobiology.

Tunable Electron-Electron Interactions in LaAlO 3 / SrTiO 3 Nanostructures

DOE PAGES

Cheng, Guanglei; Tomczyk, Michelle; Tacla, Alexandre B.; ...

2016-12-01

The interface between the two complex oxides LaAlO 3 and SrTiO 3 has remarkable properties that can be locally reconfigured between conducting and insulating states using a conductive atomic force microscope. Prior investigations of “sketched” quantum dot devices revealed a phase in which electrons form pairs, implying a strongly attractive electron-electron interaction. Here, we show that these devices with strong electron-electron interactions can exhibit a gate-tunable transition from a pair-tunneling regime to a single-electron (Andreev bound state) tunneling regime where the interactions become repulsive. The electron-electron interaction sign change is associated with a Lifshitz transition where the d xz andmore » d yz bands start to become occupied. This electronically tunable electron-electron interaction, combined with the nanoscale reconfigurability of this system, provides an interesting starting point towards solid-state quantum simulation.« less

Repulsive four-body interactions of α particles and quasistable nuclear α -particle condensates in heavy self-conjugate nuclei

NASA Astrophysics Data System (ADS)

Bai, Dong; Ren, Zhongzhou

2018-05-01

We study the effects of repulsive four-body interactions of α particles on nuclear α -particle condensates in heavy self-conjugate nuclei using a semianalytic approach, and find that the repulsive four-body interactions could decrease the critical number of α particles, beyond which quasistable α -particle condensate states can no longer exist, even if these four-body interactions make only tiny contributions to the total energy of the Hoyle-like state of 16O. Explicitly, we study eight benchmark parameter sets, and find that the critical number Ncr decreases by |Δ Ncr|˜1 -4 from Ncr˜11 with vanishing four-body interactions. We also discuss the effects of four-body interactions on energies and radii of α -particle condensates. Our study can be useful for future experiments to study α -particle condensates in heavy self-conjugate nuclei. Also, the experimental determination of Ncr will eventually help establish a better understanding on the α -particle interactions, especially the four-body interactions.

Cooperativity of self-organized Brownian motors pulling on soft cargoes.

PubMed

Orlandi, Javier G; Blanch-Mercader, Carles; Brugués, Jan; Casademunt, Jaume

2010-12-01

We study the cooperative dynamics of Brownian motors moving along a one-dimensional track when an external load is applied to the leading motor, mimicking molecular motors pulling on membrane-bound cargoes in intracellular traffic. Due to the asymmetric loading, self-organized motor clusters form spontaneously. We model the motors with a two-state noise-driven ratchet formulation and study analytically and numerically the collective velocity-force and efficiency-force curves resulting from mutual interactions, mostly hard-core repulsion and weak (nonbinding) attraction. We analyze different parameter regimes including the limits of weak noise, mean-field behavior, rigid coupling, and large numbers of motors, for the different interactions. We present a general framework to classify and quantify cooperativity. We show that asymmetric loading leads generically to enhanced cooperativity beyond the simple superposition of the effects of individual motors. For weakly attracting interactions, the cooperativity is mostly enhanced, including highly coordinated motion of motors and complex nonmonotonic velocity-force curves, leading to self-regulated clusters. The dynamical scenario is enriched by resonances associated to commensurability of different length scales. Large clusters exhibit synchronized dynamics and bidirectional motion. Biological implications are discussed.

Cooperativity of self-organized Brownian motors pulling on soft cargoes

NASA Astrophysics Data System (ADS)

Orlandi, Javier G.; Blanch-Mercader, Carles; Brugués, Jan; Casademunt, Jaume

2010-12-01

We study the cooperative dynamics of Brownian motors moving along a one-dimensional track when an external load is applied to the leading motor, mimicking molecular motors pulling on membrane-bound cargoes in intracellular traffic. Due to the asymmetric loading, self-organized motor clusters form spontaneously. We model the motors with a two-state noise-driven ratchet formulation and study analytically and numerically the collective velocity-force and efficiency-force curves resulting from mutual interactions, mostly hard-core repulsion and weak (nonbinding) attraction. We analyze different parameter regimes including the limits of weak noise, mean-field behavior, rigid coupling, and large numbers of motors, for the different interactions. We present a general framework to classify and quantify cooperativity. We show that asymmetric loading leads generically to enhanced cooperativity beyond the simple superposition of the effects of individual motors. For weakly attracting interactions, the cooperativity is mostly enhanced, including highly coordinated motion of motors and complex nonmonotonic velocity-force curves, leading to self-regulated clusters. The dynamical scenario is enriched by resonances associated to commensurability of different length scales. Large clusters exhibit synchronized dynamics and bidirectional motion. Biological implications are discussed.

Electrostatic interaction between stereocilia: II. Influence on the mechanical properties of the hair bundle.

PubMed

Dolgobrodov, S G; Lukashkin, A N; Russell, I J

2000-12-01

This paper is based on our model [Dolgobrodov et al., 2000. Hear. Res., submitted for publication] in which we examine the significance of the polyanionic surface layers of stereocilia for electrostatic interaction between them. We analyse how electrostatic forces modify the mechanical properties of the sensory hair bundle. Different charge distribution profiles within the glycocalyx are considered. When modelling a typical experiment on bundle stiffness measurements, applying an external force to the tallest row of stereocilia shows that the asymptotic stiffness of the hair bundle for negative displacements is always larger than the asymptotic stiffness for positive displacements. This increase in stiffness is monotonic for even charge distribution and shows local minima when the negative charge is concentrated in a thinner layer within the cell coat. The minima can also originate from the co-operative effect of electrostatic repulsion and inter-ciliary links with non-linear mechanical properties. Existing experimental observations are compared with the predictions of the model. We conclude that the forces of electrostatic interaction between stereocilia may influence the mechanical properties of the hair bundle and, being strongly non-linear, contribute to the non-linear phenomena, which have been recorded from the auditory periphery.

Exchange-Correlation Effects for Noncovalent Interactions in Density Functional Theory.

PubMed

Otero-de-la-Roza, A; DiLabio, Gino A; Johnson, Erin R

2016-07-12

In this article, we develop an understanding of how errors from exchange-correlation functionals affect the modeling of noncovalent interactions in dispersion-corrected density-functional theory. Computed CCSD(T) reference binding energies for a collection of small-molecule clusters are decomposed via a molecular many-body expansion and are used to benchmark density-functional approximations, including the effect of semilocal approximation, exact-exchange admixture, and range separation. Three sources of error are identified. Repulsion error arises from the choice of semilocal functional approximation. This error affects intermolecular repulsions and is present in all n-body exchange-repulsion energies with a sign that alternates with the order n of the interaction. Delocalization error is independent of the choice of semilocal functional but does depend on the exact exchange fraction. Delocalization error misrepresents the induction energies, leading to overbinding in all induction n-body terms, and underestimates the electrostatic contribution to the 2-body energies. Deformation error affects only monomer relaxation (deformation) energies and behaves similarly to bond-dissociation energy errors. Delocalization and deformation errors affect systems with significant intermolecular orbital interactions (e.g., hydrogen- and halogen-bonded systems), whereas repulsion error is ubiquitous. Many-body errors from the underlying exchange-correlation functional greatly exceed in general the magnitude of the many-body dispersion energy term. A functional built to accurately model noncovalent interactions must contain a dispersion correction, semilocal exchange, and correlation components that minimize the repulsion error independently and must also incorporate exact exchange in such a way that delocalization error is absent.

Nuclear force from lattice QCD.

PubMed

Ishii, N; Aoki, S; Hatsuda, T

2007-07-13

The nucleon-nucleon (NN) potential is studied by lattice QCD simulations in the quenched approximation, using the plaquette gauge action and the Wilson quark action on a 32(4) [approximately (4.4 fm)(4)] lattice. A NN potential V(NN)(r) is defined from the equal-time Bethe-Salpeter amplitude with a local interpolating operator for the nucleon. By studying the NN interaction in the (1)S(0) and (3)S(1) channels, we show that the central part of V(NN)(r) has a strong repulsive core of a few hundred MeV at short distances (r approximately < 0.5 fm) surrounded by an attractive well at medium and long distances. These features are consistent with the known phenomenological features of the nuclear force.

Sorption of organic phosphates and its effects on aggregation of hematite nanoparticles in monovalent and bivalent solutions.

PubMed

Xu, Chen-Yang; Li, Jiu-Yu; Xu, Ren-Kou; Hong, Zhi-Neng

2017-03-01

Sorption of organic phosphates-myo-inositol hexakisphosphate (IHP) and glycerol phosphate (GP) and its effects on the early stage of hematite aggregation kinetics were investigated at different pH and electrolyte composition. KH 2 PO 4 (KP) was taken as an inorganic P source for comparison. Results indicated that for all types of P, the sorption amounts decreased with increasing solution pH. Sorption amount of IHP was almost two times that of KP, while those of GP and KP were close. Both organic P and inorganic P interacted with hematite via ligand exchange through their phosphate groups, which conveyed negative charges to mineral surface and significantly decreased the zeta potential of hematite. In Na + solution, critical coagulation concentrations (CCCs) of hematite suspensions increased with increasing P concentration and followed the order of KP < GP < IHP at pH 5.5. Compared with KP, the organic P could more effectively stabilize the hematite suspension not only through increasing the negative charges and electrostatic repulsive force, but also through steric repulsion between P-sorbed hematite nanoparticles. When the pH was increased from 5.5 to 10.0, the CCCs of the hematite suspensions with GP and IHP decreased mainly because of the great reductions in organic P sorption amounts and consequent decreases in electrostatic and steric repulsive forces. However, enhanced aggregation was observed in the presence of IHP at pH 4.5 and above in low Ca 2+ solutions. The precipitation of calcium phytate formed net-like structure, which served as bridges to bind hematite nanoparticles and resulted in enhanced aggregation. These results have important implications for assessing the fate and transport of organic P and hematite nanoparticles in soil and aquatic environments.

Incorporating water-release and lateral protein interactions in modeling equilibrium adsorption for ion-exchange chromatography.

PubMed

Thrash, Marvin E; Pinto, Neville G

2006-09-08

The equilibrium adsorption of two albumin proteins on a commercial ion exchanger has been studied using a colloidal model. The model accounts for electrostatic and van der Waals forces between proteins and the ion exchanger surface, the energy of interaction between adsorbed proteins, and the contribution of entropy from water-release accompanying protein adsorption. Protein-surface interactions were calculated using methods previously reported in the literature. Lateral interactions between adsorbed proteins were experimentally measured with microcalorimetry. Water-release was estimated by applying the preferential interaction approach to chromatographic retention data. The adsorption of ovalbumin and bovine serum albumin on an anion exchanger at solution pH>pI of protein was measured. The experimental isotherms have been modeled from the linear region to saturation, and the influence of three modulating alkali chlorides on capacity has been evaluated. The heat of adsorption is endothermic for all cases studied, despite the fact that the net charge on the protein is opposite that of the adsorbing surface. Strong repulsive forces between adsorbed proteins underlie the endothermic heat of adsorption, and these forces intensify with protein loading. It was found that the driving force for adsorption is the entropy increase due to the release of water from the protein and adsorbent surfaces. It is shown that the colloidal model predicts protein adsorption capacity in both the linear and non-linear isotherm regions, and can account for the effects of modulating salt.

Effect of the heterogeneity of metamaterials on the Casimir-Lifshitz interaction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Azari, Arash; Golestanian, Ramin; Miri, MirFaez

2010-09-15

The Casimir-Lifshitz interaction between metamaterials is studied using a model that takes into account the structural heterogeneity of the dielectric and magnetic properties of the bodies. A recently developed perturbation theory for the Casimir-Lifshitz interaction between arbitrary material bodies is generalized to include nonuniform magnetic permeability profiles and used to study the interaction between the magneto-dielectric heterostructures within the leading order. The metamaterials are modeled as two-dimensional arrays of domains with varying permittivity and permeability. In the case of two semi-infinite bodies with flat boundaries, the patterned structure of the material properties is found to cause the normal Casimir-Lifshitz forcemore » to develop an oscillatory behavior when the distance between the two bodies is comparable to the wavelength of the patterned features in the metamaterials. The nonuniformity also leads to the emergence of lateral Casimir-Lifshitz forces, which tend to strengthen as the gap size becomes smaller. Our results suggest that the recent studies on Casimir-Lifshitz forces between metamaterials, which have been performed with the aim of examining the possibility of observing the repulsive force, should be revisited to include the effect of the patterned structure at the wavelength of several hundred nanometers that coincides with the relevant gap size in the experiments.« less

Computer Simulation of Energy Parameters and Magnetic Effects in Fe-Si-C Ternary Alloys

NASA Astrophysics Data System (ADS)

Ridnyi, Ya. M.; Mirzoev, A. A.; Mirzaev, D. A.

2018-06-01

The paper presents ab initio simulation with the WIEN2k software package of the equilibrium structure and properties of silicon and carbon atoms dissolved in iron with the body-centered cubic crystal system of the lattice. Silicon and carbon atoms manifest a repulsive interaction in the first two nearest neighbors, in the second neighbor the repulsion being stronger than in the first. In the third and next-nearest neighbors a very weak repulsive interaction occurs and tends to zero with increasing distance between atoms. Silicon and carbon dissolution reduces the magnetic moment of iron atoms.

Charge interaction between particle-laden fluid interfaces.

PubMed

Xu, Hui; Kirkwood, John; Lask, Mauricio; Fuller, Gerald

2010-03-02

Experiments are described where two oil/water interfaces laden with charged particles move at close proximity relative to one another. The particles on one of the interfaces were observed to be attracted toward the point of closest approach, forming a denser particle monolayer, while the particles on the opposite interface were repelled away from this point, forming a particle depletion zone. Such particle attraction/repulsion was observed even if one of the interfaces was free of particles. This phenomenon can be explained by the electrostatic interaction between the two interfaces, which causes surface charges (charged particles and ions) to redistribute in order to satisfy surface electric equipotential at each interface. In a forced particle oscillation experiment, we demonstrated the control of charged particle positions on the interface by manipulating charge interaction between interfaces.

CPT symmetry and antimatter gravity in general relativity

NASA Astrophysics Data System (ADS)

Villata, M.

2011-04-01

The gravitational behavior of antimatter is still unknown. While we may be confident that antimatter is self-attractive, the interaction between matter and antimatter might be either attractive or repulsive. We investigate this issue on theoretical grounds. Starting from the CPT invariance of physical laws, we transform matter into antimatter in the equations of both electrodynamics and gravitation. In the former case, the result is the well-known change of sign of the electric charge. In the latter, we find that the gravitational interaction between matter and antimatter is a mutual repulsion, i.e. antigravity appears as a prediction of general relativity when CPT is applied. This result supports cosmological models attempting to explain the Universe accelerated expansion in terms of a matter-antimatter repulsive interaction.

Instability of Bose-Einstein condensation into the one-particle ground state on quantum graphs under repulsive perturbations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bolte, Jens, E-mail: jens.bolte@rhul.ac.uk; Kerner, Joachim, E-mail: joachim.kerner@fernuni-hagen.de

In this paper we investigate Bose-Einstein condensation into the one-particle ground state in interacting quantum many-particle systems on graphs. We extend previous results obtained for particles on an interval and show that even arbitrarily small repulsive two-particle interactions destroy the condensate in the one-particle ground state present in the non-interacting Bose gas. Our results also cover singular two-particle interactions, such as the well-known Lieb-Liniger model, in the thermodynamic limit.

Effect of interactions for one-dimensional asymmetric exclusion processes under periodic and bath-adapted coupling environment

NASA Astrophysics Data System (ADS)

Midha, Tripti; Kolomeisky, Anatoly B.; Gupta, Arvind Kumar

2018-04-01

Stimulated by the effect of the nearest neighbor interactions in vehicular traffic and motor proteins, we study a 1D driven lattice gas model, in which the nearest neighbor particle interactions are taken in accordance with the thermodynamic concepts. The non-equilibrium steady-state properties of the system are analyzed under both open and periodic boundary conditions using a combination of cluster mean-field analysis and Monte Carlo simulations. Interestingly, the fundamental diagram of current versus density shows a complex behavior with a unimodal dependence for attractions and weak repulsions that turns into the bimodal behavior for stronger repulsive interactions. Specific details of system-reservoir coupling for the open system have a strong effect on the stationary phases. We produce the steady-state phase diagrams for the bulk-adapted coupling to the reservoir using the minimum and maximum current principles. The strength and nature of interaction energy has a striking influence on the number of stationary phases. We observe that interactions lead to correlations having a strong impact on the system dynamical properties. The correlation between any two sites decays exponentially as the distance between the sites increases. Moreover, they are found to be short-range for repulsions and long-range for attractions. Our results also suggest that repulsions and attractions asymmetrically modify the dynamics of interacting particles in exclusion processes.

Red mud carbonation using carbon dioxide: Effects of carbonate and calcium ions on goethite surface properties and settling.

PubMed

Liang, Gaojie; Chen, Wenmi; Nguyen, Anh V; Nguyen, Tuan A H

2018-05-01

Carbonation using CO 2 appears as an attractive solution for disposing of red mud suspensions, an aluminum industry hazardous waste since it also offers an option for CO 2 sequestration. Here we report the novel findings that CO 3 2- together with Ca 2+ can significantly affect the surface properties and settling of goethite, a major component of red mud. Specifically, their effects on the goethite surface chemistry, colloidal interaction forces and settling in alkaline solutions are investigated. The surface potential becomes more negative by the formation of carbonate inner-sphere complexes on goethite surface. It is consistent with the strong repulsion, decreased particle size and settling velocity with increased carbonate concentrations as measured by atomic force microscopy, particle size analysis, and particle settling. Adding Ca 2+ that forms outer-sphere complexes with pre-adsorbed carbonate changes goethite surface charge negligibly. Changing repulsion to the attraction between goethite surfaces by increasing calcium dosage indicates the surface bridging, in accordance with the increased settling velocity. The adverse effect of carbonate on goethite flocculation is probably due to its specific chemisorption and competition with flocculants. By forming outer-sphere complexes together with the flocculant-calcium bridging effect, calcium ions can eliminate the negative influence of carbonate and improve the flocculation of goethite particles. These findings contribute to a better understanding of goethite particle interaction with salt ions and flocculants in controlling the particle behavior in the handling processes, including the red mud carbonation. Copyright © 2018 Elsevier Inc. All rights reserved.

Quantifying Tip-Sample Interactions in Vacuum Using Cantilever-Based Sensors: An Analysis

NASA Astrophysics Data System (ADS)

Dagdeviren, Omur E.; Zhou, Chao; Altman, Eric I.; Schwarz, Udo D.

2018-04-01

Atomic force microscopy is an analytical characterization method that is able to image a sample's surface topography at high resolution while simultaneously probing a variety of different sample properties. Such properties include tip-sample interactions, the local measurement of which has gained much popularity in recent years. To this end, either the oscillation frequency or the oscillation amplitude and phase of the vibrating force-sensing cantilever are recorded as a function of tip-sample distance and subsequently converted into quantitative values for the force or interaction potential. Here, we theoretically and experimentally show that the force law obtained from such data acquired under vacuum conditions using the most commonly applied methods may deviate more than previously assumed from the actual interaction when the oscillation amplitude of the probe is of the order of the decay length of the force near the surface, which may result in a non-negligible error if correct absolute values are of importance. Caused by approximations made in the development of the mathematical reconstruction procedures, the related inaccuracies can be effectively suppressed by using oscillation amplitudes sufficiently larger than the decay length. To facilitate efficient data acquisition, we propose a technique that includes modulating the drive amplitude at a constant height from the surface while monitoring the oscillation amplitude and phase. Ultimately, such an amplitude-sweep-based force spectroscopy enables shorter data acquisition times and increased accuracy for quantitative chemical characterization compared to standard approaches that vary the tip-sample distance. An additional advantage is that since no feedback loop is active while executing the amplitude sweep, the force can be consistently recovered deep into the repulsive regime.

Self-Organization of Polymer Brush Layers in a Poor Solvent

NASA Astrophysics Data System (ADS)

Karim, A.; Tsukruk, V. V.; Douglas, J. F.; Satija, S. K.; Fetters, L. J.; Reneker, D. H.; Foster, M. D.

1995-10-01

Synthesis of densely grafted polymer brushes from good solvent polymer solutions is difficult when the surface interaction is only weakly attractive because of the strong steric repulsion between the polymer chains. To circumvent this difficulty we graft polymer layers in a poor solvent to exploit attractive polymer-polymer interactions which largely nullify the repulsive steric interactions. This simple strategy gives rise to densely grafted and homogeneous polymer brush layers. Model end-grafted polystyrene chains (M_w = 105,000) are prepared in the poor solvent cyclohexane (9.5 °C) where the chains are chemically attached to the surface utilizing a trichlorosilane end-group. Polished silicon wafers were then exposed to the reactive polymer solutions for a series of “induction times” tau_I and the evolving layer was characterized by X-ray reflectivity and atomic force microscopy. Distinct morphologies were found depending on tau_I. For short tau_I, corresponding to a grafting density less than 5 mg/m^2, the grafted layer forms an inhomogeneous island-like structure. At intermediate tau_I, where the coverage becomes percolating, a surface pattern develops which appears similar to spinodal decomposition in bulk solution. Finally, after sufficiently long tau_I, a dense and nearly homogeneous layer with a sharp interface is formed which does not exhibit surface pattern formation. The stages of brush growth are discussed qualitatively in terms of a random deposition model.

Effective Electrostatic Interactions Between Two Overall Neutral Surfaces with Quenched Charge Heterogeneity Over Atomic Length Scale

NASA Astrophysics Data System (ADS)

Zhou, S.

2017-12-01

Using Monte Carlo results as a reference, a classical density functional theory ( CDFT) is shown to reliably predict the forces between two heterogeneously charged surfaces immersed in an electrolyte solution, whereas the Poisson-Boltzmann ( PB) theory is demonstrated to deteriorate obviously for the same system even if the system parameters considered fall within the validity range of the PB theory in the homogeneously charged surfaces. By applying the tested CDFT, we study the effective electrostatic potential of mean force ( EPMF) between two face-face planar and hard surfaces of zero net charge on which positive and negative charges are separated and considered to present as discontinuous spots on the inside edges of the two surfaces. Main conclusions are summarized as follows: (i) strength of the EPMF in the surface charge separation case is very sensitively and positively correlated with the surface charge separation level and valency of the salt ion. Particularly, the charge separation level and the salt ion valency have a synergistic effect, which makes high limit of the EPMF strength in the surface charge separation case significantly go beyond that of the ideal homogeneously charged surface counterpart at average surface charge density similar to the average surface positive or negative charge density in the charge separation case. (ii) The surface charge distribution patterns mainly influence sign of the EPMF: symmetrical and asymmetrical patterns induce repulsive and attractive (at small distances) EPMF, respectively; but with low valency salt ions and low charge separation level the opposite may be the case. With simultaneous presence of both higher valency cation and anion, the EPMF can be repulsive at intermediate distances for asymmetrical patterns. (iii) Salt ion size has a significant impact, which makes the EPMF tend to become more and more repulsive with the ion diameter regardless of the surface charge distribution patterns and the valency of the salt ion; whereas if the 1:1 type electrolyte and the symmetrical patterns are considered, then the opposite may be the case. All of these findings can be explained self-consistently from several perspectives: an excess adsorption of the salt ions (induced by the surface charge separation) serving to raise the osmotic pressure between the plates, configuration fine-tuning in the thinner ion adsorption layer driven by the energy decrease principle, direct Coulombic interactions operating between charged objects on the two face-to-face plates involved, and net charge strength in the ion adsorption layer responsible for the net electrostatic repulsion.

Long-Range Repulsion Between Spatially Confined van der Waals Dimers

NASA Astrophysics Data System (ADS)

Sadhukhan, Mainak; Tkatchenko, Alexandre

2017-05-01

It is an undisputed textbook fact that nonretarded van der Waals (vdW) interactions between isotropic dimers are attractive, regardless of the polarizability of the interacting systems or spatial dimensionality. The universality of vdW attraction is attributed to the dipolar coupling between fluctuating electron charge densities. Here, we demonstrate that the long-range interaction between spatially confined vdW dimers becomes repulsive when accounting for the full Coulomb interaction between charge fluctuations. Our analytic results are obtained by using the Coulomb potential as a perturbation over dipole-correlated states for two quantum harmonic oscillators embedded in spaces with reduced dimensionality; however, the long-range repulsion is expected to be a general phenomenon for spatially confined quantum systems. We suggest optical experiments to test our predictions, analyze their relevance in the context of intermolecular interactions in nanoscale environments, and rationalize the recent observation of anomalously strong screening of the lateral vdW interactions between aromatic hydrocarbons adsorbed on metal surfaces.

Effect of a core-softened O-O interatomic interaction on the shock compression of fused silica

NASA Astrophysics Data System (ADS)

Izvekov, Sergei; Weingarten, N. Scott; Byrd, Edward F. C.

2018-03-01

Isotropic soft-core potentials have attracted considerable attention due to their ability to reproduce thermodynamic, dynamic, and structural anomalies observed in tetrahedral network-forming compounds such as water and silica. The aim of the present work is to assess the relevance of effective core-softening pertinent to the oxygen-oxygen interaction in silica to the thermodynamics and phase change mechanisms that occur in shock compressed fused silica. We utilize the MD simulation method with a recently published numerical interatomic potential derived from an ab initio MD simulation of liquid silica via force-matching. The resulting potential indicates an effective shoulder-like core-softening of the oxygen-oxygen repulsion. To better understand the role of the core-softening we analyze two derivative force-matching potentials in which the soft-core is replaced with a repulsive core either in the three-body potential term or in all the potential terms. Our analysis is further augmented by a comparison with several popular empirical models for silica that lack an explicit core-softening. The first outstanding feature of shock compressed glass reproduced with the soft-core models but not with the other models is that the shock compression values at pressures above 20 GPa are larger than those observed under hydrostatic compression (an anomalous shock Hugoniot densification). Our calculations indicate the occurrence of a phase transformation along the shock Hugoniot that we link to the O-O repulsion core-softening. The phase transformation is associated with a Hugoniot temperature reversal similar to that observed experimentally. With the soft-core models, the phase change is an isostructural transformation between amorphous polymorphs with no associated melting event. We further examine the nature of the structural transformation by comparing it to the Hugoniot calculations for stishovite. For stishovite, the Hugoniot exhibits temperature reversal and associated phase transformation, which is a transition to a disordered phase (liquid or dense amorphous), regardless of whether or not the model accounts for core-softening. The onset pressures of the transformation predicted by different models show a wide scatter within 60-110 GPa; for potentials without core-softening, the onset pressure is much higher than 110 GPa. Our results show that the core-softening of the interaction in the oxygen subsystem of silica is the key mechanism for the structural transformation and thermodynamics in shock compressed silica. These results may provide an important contribution to a unified picture of anomalous response to shock compression observed in other network-forming oxides and single-component systems with core-softening of effective interactions.

Massive antigravity field and incomplete black hole evaporation

NASA Astrophysics Data System (ADS)

Massa, Corrado

2008-04-01

If gravity is a mixture of the ordinary attractive force carried by the massless graviton, and of a repulsive force carried by a particle with nonzero mass, an evaporating black hole might leave a stable remnant.

Mapping repulsive to attractive interaction in driven-dissipative quantum systems

NASA Astrophysics Data System (ADS)

Li, Andy C. Y.; Koch, Jens

2017-11-01

Repulsive and attractive interactions usually lead to very different physics. Striking exceptions exist in the dynamics of driven-dissipative quantum systems. For the example of a photonic Bose-Hubbard dimer, we establish a one-to-one mapping relating cases of onsite repulsion and attraction. We prove that the mapping is valid for an entire class of Markovian open quantum systems with a time-reversal-invariant Hamiltonian and physically meaningful inverse-sign Hamiltonian. To underline the broad applicability of the mapping, we illustrate the one-to-one correspondence between the nonequilibrium dynamics in a geometrically frustrated spin lattice and those in a non-frustrated partner lattice.

Coulomb repulsion in short polypeptides.

PubMed

Norouzy, Amir; Assaf, Khaleel I; Zhang, Shuai; Jacob, Maik H; Nau, Werner M

2015-01-08

Coulomb repulsion between like-charged side chains is presently viewed as a major force that impacts the biological activity of intrinsically disordered polypeptides (IDPs) by determining their spatial dimensions. We investigated short synthetic models of IDPs, purely composed of ionizable amino acid residues and therefore expected to display an extreme structural and dynamic response to pH variation. Two synergistic, custom-made, time-resolved fluorescence methods were applied in tandem to study the structure and dynamics of the acidic and basic hexapeptides Asp6, Glu6, Arg6, Lys6, and His6 between pH 1 and 12. (i) End-to-end distances were obtained from the short-distance Förster resonance energy transfer (sdFRET) from N-terminal 5-fluoro-l-tryptophan (FTrp) to C-terminal Dbo. (ii) End-to-end collision rates were obtained for the same peptides from the collision-induced fluorescence quenching (CIFQ) of Dbo by FTrp. Unexpectedly, the very high increase of charge density at elevated pH had no dynamical or conformational consequence in the anionic chains, neither in the absence nor in the presence of salt, in conflict with the common view and in partial conflict with accompanying molecular dynamics simulations. In contrast, the cationic peptides responded to ionization but with surprising patterns that mirrored the rich individual characteristics of each side chain type. The contrasting results had to be interpreted, by considering salt screening experiments, N-terminal acetylation, and simulations, in terms of an interplay of local dielectric constant and peptide-length dependent side chain charge-charge repulsion, side chain functional group solvation, N-terminal and side chain charge-charge repulsion, and side chain-side chain as well as side chain-backbone interactions. The common picture that emerged is that Coulomb repulsion between water-solvated side chains is efficiently quenched in short peptides as long as side chains are not in direct contact with each other or the main chain.

Electrostatic repulsive out-of-plane actuator using conductive substrate.

PubMed

Wang, Weimin; Wang, Qiang; Ren, Hao; Ma, Wenying; Qiu, Chuankai; Chen, Zexiang; Fan, Bin

2016-10-07

A pseudo-three-layer electrostatic repulsive out-of-plane actuator is proposed. It combines the advantages of two-layer and three-layer repulsive actuators, i.e., fabrication requirements and fill factor. A theoretical model for the proposed actuator is developed and solved through the numerical calculation of Schwarz-Christoffel mapping. Theoretical and simulated results show that the pseudo-three-layer actuator offers higher performance than the two-layer and three-layer actuators with regard to the two most important characteristics of actuators, namely, driving force and theoretical stroke. Given that the pseudo-three-layer actuator structure is compatible with both the parallel-plate actuators and these two types of repulsive actuators, a 19-element two-layer repulsive actuated deformable mirror is operated in pseudo-three-layer electrical connection mode. Theoretical and experimental results demonstrate that the pseudo-three-layer mode produces a larger displacement of 0-4.5 μm for a dc driving voltage of 0-100 V, when compared with that in two-layer mode.

Electrostatic repulsive out-of-plane actuator using conductive substrate

PubMed Central

Wang, Weimin; Wang, Qiang; Ren, Hao; Ma, Wenying; Qiu, Chuankai; Chen, Zexiang; Fan, Bin

2016-01-01

A pseudo-three-layer electrostatic repulsive out-of-plane actuator is proposed. It combines the advantages of two-layer and three-layer repulsive actuators, i.e., fabrication requirements and fill factor. A theoretical model for the proposed actuator is developed and solved through the numerical calculation of Schwarz-Christoffel mapping. Theoretical and simulated results show that the pseudo-three-layer actuator offers higher performance than the two-layer and three-layer actuators with regard to the two most important characteristics of actuators, namely, driving force and theoretical stroke. Given that the pseudo-three-layer actuator structure is compatible with both the parallel-plate actuators and these two types of repulsive actuators, a 19-element two-layer repulsive actuated deformable mirror is operated in pseudo-three-layer electrical connection mode. Theoretical and experimental results demonstrate that the pseudo-three-layer mode produces a larger displacement of 0–4.5 μm for a dc driving voltage of 0–100 V, when compared with that in two-layer mode. PMID:27713542

Electron Pair Repulsion Responsible for the Peculiar Edge Effects and Surface Chemistry of Black Phosphorus.

PubMed

Kong, Xiang-Peng; Shen, Xiaomei; Jang, Joonkyung; Gao, Xingfa

2018-03-01

The electronic and optical properties of black phosphorus (black-P) are significantly modulated by fabricating the edges of this two-dimensional material. Electron lone pairs (ELPs) are ubiquitous in black-P, but their role in creating the edge effects of black-P is poorly understood. Using first-principle calculations, we report ELPs of black-P experience severe Coulomb repulsion and play a central role in creating the edge effects of black-P. We discover the outermost P atoms of the zigzag edges of black-PQDs are free of the Coulomb repulsion, but the P atoms of the armchair edges do experience the Coulomb repulsion. The Coulomb repulsion serves as a new chemical driving force to make electron donor-acceptor bonds with chemical groups bearing vacant orbitals. Our results provide insights into the mechanism responsible for the peculiar edge effects of black-P and highlight the opportunity to use the ELPs of black-P for their damage-free surface functionalization.

A multiphase ion-transport analysis of the electrostatic disjoining pressure: implications for binary droplet coalescence

NASA Astrophysics Data System (ADS)

Mason, Lachlan; Gebauer, Felix; Bart, Hans-Jörg; Stevens, Geoffrey; Harvie, Dalton

2016-11-01

Understanding the physics of emulsion coalescence is critical for the robust simulation of industrial solvent extraction processes, in which loaded organic and raffinate phases are separated via the coalescence of dispersed droplets. At the droplet scale, predictive collision-outcome models require an accurate description of the repulsive surface forces arising from electrical-double-layer interactions. The conventional disjoining-pressure treatment of double-layer forces, however, relies on assumptions which do not hold generally for deformable droplet collisions: namely, low interfacial curvature and negligible advection of ion species. This study investigates the validity bounds of the disjoining pressure approximation for low-inertia droplet interactions. A multiphase ion-transport model, based on a coupling of droplet-scale Nernst-Planck and Navier-Stokes equations, predicts ion-concentration fields that are consistent with the equilibrium Boltzmann distribution; indicating that the disjoining-pressure approach is valid for both static and dynamic interactions in low-Reynolds-number settings. The present findings support the development of coalescence kernels for application in macro-scale population balance modelling.

Brush in the bath of active particles: Anomalous stretching of chains and distribution of particles

NASA Astrophysics Data System (ADS)

Li, Hui-shu; Zhang, Bo-kai; Li, Jian; Tian, Wen-de; Chen, Kang

2015-12-01

The interaction between polymer brush and colloidal particles has been intensively studied in the last two decades. Here, we consider a flat chain-grafted substrate immersed in a bath of active particles. Simulations show that an increase in the self-propelling force causes an increase in the number of particles that penetrate into the brush. Anomalously, the particle density inside the main body of the brush eventually becomes higher than that outside the brush at very large self-propelling force. The grafted chains are further stretched due to the steric repulsion from the intruded particles. Upon the increase of the self-propelling force, distinct stretching behaviors of the chains were observed for low and high grafting densities. Surprisingly, we find a weak descent of the average end-to-end distance of chains at high grafting density and very large force which is reminiscent of the compression effect of a chain in the active bath.

Pedestrians’ behavior in emergency evacuation: Modeling and simulation

NASA Astrophysics Data System (ADS)

Wang, Lei; Zheng, Jie-Hui; Zhang, Xiao-Shuang; Zhang, Jian-Lin; Wang, Qiu-Zhen; Zhang, Qian

2016-11-01

The social force model has been widely used to simulate pedestrian evacuation by analyzing attractive, repulsive, driving, and fluctuating forces among pedestrians. Many researchers have improved its limitations in simulating behaviors of large-scale population. This study modifies the well-accepted social force model by considering the impacts of interaction among companions and further develops a comprehensive model by combining that with a multi-exit utility function. Then numerical simulations of evacuations based on the comprehensive model are implemented in the waiting hall of the Wulin Square Subway Station in Hangzhou, China. The results provide safety thresholds of pedestrian density and panic levels in different operation situations. In spite of the operation situation and the panic level, a larger friend-group size results in lower evacuation efficiency. Our study makes important contributions to building a comprehensive multi-exit social force model and to applying it to actual scenarios, which produces data to facilitate decision making in contingency plans and emergency treatment. Project supported by the National Natural Science Foundation of China (Grant No. 71471163).

A study on the distribution of adsorbed nanoparticles

NASA Astrophysics Data System (ADS)

Li, Ding

2008-02-01

We use Monte Carlo simulation to calculate the distributions of particles under adsorption force near planar and cylindrical surfaces, respectively. Both hard sphere interaction and repulsive Yukawa (screened coulomb) interaction are employed in our simulations. We study the influence of the inter-particle potentials. The difference between the MC simulation results and the analytical results of ideal gas model shows that the interaction between particles plays an important role in the density distribution under external fields. Moreover, the 2-dimensional constructions of particles close to the surface are studied and show relations of the interaction between particles. These results may indicate us how to improve the methods of building nanoparticle coatings and nano-scale patterns. Supported by 100 Persons Project of Chinese Academy of Sciences, National Natural Science Foundation of China (10474109, 10674146) and Major State Research Development Programme of China (2006CB933000, 2006CB708612)

Solute-mediated interactions between active droplets

NASA Astrophysics Data System (ADS)

Moerman, Pepijn G.; Moyses, Henrique W.; van der Wee, Ernest B.; Grier, David G.; van Blaaderen, Alfons; Kegel, Willem K.; Groenewold, Jan; Brujic, Jasna

2017-09-01

Concentration gradients play a critical role in embryogenesis, bacterial locomotion, as well as the motility of active particles. Particles develop concentration profiles around them by dissolution, adsorption, or the reactivity of surface species. These gradients change the surface energy of the particles, driving both their self-propulsion and governing their interactions. Here, we uncover a regime in which solute gradients mediate interactions between slowly dissolving droplets without causing autophoresis. This decoupling allows us to directly measure the steady-state, repulsive force, which scales with interparticle distance as F ˜1 /r2 . Our results show that the dissolution process is diffusion rather than reaction rate limited, and the theoretical model captures the dependence of the interactions on droplet size and solute concentration, using a single fit parameter, l =16 ±3 nm , which corresponds to the length scale of a swollen micelle. Our results shed light on the out-of-equilibrium behavior of particles with surface reactivity.

Potential of mean force of DNA guided assemblies past Debye-Hückel regime

NASA Astrophysics Data System (ADS)

Girard, Martin; Seo, Soyoung; Li, Yaohua; Mirkin, Chad; Olvera de La Cruz, Monica

Many of the bioinspired systems make use of biopolymers such as polypeptides or DNA. The latter is widely used in self-assembled systems, from colloidal crystals to origami construction. In these systems, salt is commonly required to screen the electrostatic repulsion between the strands. In the classical Debye-Hückel picture, salt ions are point particles and the screening distance is a decreasing monotonic function of salt concentration. This picture breaks down at moderate salt concentrations, where the behavior becomes non-monotonic. In this talk, we will show results for potential of mean force of DNA grafted colloids obtained through multiscale molecular dynamics. In this picture, the highly charged DNA causes non-trivial behavior at moderate salt concentrations (c 0 . 3 - 0 . 7 M), namely increase of repulsion for non-complementary DNA strands while repulsion decreases for complementary strands. We will show spatial cluster distribution as function of size and charge as well as implications for experimental systems.

Effects of Temperature on Aggregation Kinetics of Graphene Oxide in Aqueous Solutions

NASA Astrophysics Data System (ADS)

Wang, M.; Gao, B.; Tang, D.; Sun, H.; Yin, X.; Yu, C.

2017-12-01

Temperature may play an important role in controlling graphene oxide (GO) stability in aqueous solutions, but it has been overlooked in the literature. In this work, laboratory experiments were conducted to determine the effects of temperature (6, 25, and 40 °C) on GO aggregation kinetics under different combinations of ionic strength, cation type, humic acid (HA) concentration by monitoring GO hydrodynamic radii and attachment efficiencies. The results showed that, without HA, temperature increase promoted GO aggregation in both monovalent (Na+ and K+) and divalent (Ca2+) solutions. This phenomenon might be caused by multiple processes including enhanced collision frequency, enhanced cation dehydration, and reduced electrostatic repulsion. The presence of HA introduced steric repulsion forces that enhanced GO stability and temperature showed different effects GO aggregation kinetics in monovalent and divalent electrolytes. In monovalent electrolytes, cold temperature diminished the steric repulsion of HA-coated GO. As a result, the fastest increasing rate of GO hydrodynamic radius and the smallest critical coagulation concentration value appeared at the lowest temperature (6 °C). Conversely, in divalent electrolyte solutions with HA, high temperate favored GO aggregation, probably because the interactions between Ca2+ and HA increased with temperature resulting in lower HA coating on GO. Findings of this work emphasized the importance of temperature as well as solution chemistry on the stability and fate of GO nanoparticles in aquatic environment.

Assembly of Nanowire Arrays: Exploring Interparticle Interactions, Particle Orientation, and Mixed Particle Arrays

NASA Astrophysics Data System (ADS)

Kirby, David J.

This dissertation explores the fundamental interparticle and particle-substrate forces that contribute to nanowire assembly. Nanowires have a large aspect ratio which has made them favorable materials for applications in energy and sensing technologies. However, this anisotropy means that nanowires must be positioned and oriented during an assembly process. Within this work, the roles of gravity, van der Waals (VDW) attractions, and electrostatic repulsions are explored when different nanowire assemblies are created. Particles were synthesized by the template electrodeposition process so that stripes of different materials and therefore different VDW interactions could be patterned along the particle length. Electrostatic repulsions were provided by a small molecule coating or a porous silica shell to prevent aggregation during the assembly process. Chapters 2, 3, 5, 6, and 8 all used particles whose asymmetry was further adjusted by removal of a sacrificial segment to leave a partially etched nanowire (PEN), a rigid silica shell partially filled with a metal core. For these particles, the role of gravity was amplified due to the drastic density differences between the two segments. Topographic and high VDW surface interactions were patterned onto assembly substrates using photolithographic processing. These forces served as a passive template to direct nanowire assembly. The segment anisotropy of PENs allowed gravity to drive their sedimentation with the long axis perpendicular to the surface. The density difference between the two ends allowed them to convert between the horizontal and vertical orientation as they diffused on the substrate. Vertical arrays formed as particle concentrations increased while VDW attractions from neighboring PENs or the physical barrier of a microwell wall supported this structure. While vertical arrays were typically PENs, microwell walls were also able to enforce a vertical orientation on solid Au nanowires. These particles typically formed horizontal arrays on planar surfaces, but careful design of the microwell and nanowire dimensions enabled these particles to take on the vertical orientation. Solid nanowires and PENs with greater segment symmetry aligned parallel to the surface as gravity did not allow a conversion to the vertical orientation. When concentrated, these particles formed smectic row arrangements which were previously shown to originate from a balance of VDW attractions and electrostatic repulsions. Within rows of segmented particles, a preference was observed for like orientation of nearest neighbor particles (Chapter 6). With the aid of Monte Carlo simulations, it was determined that this observation was the result of small differences in VDW attractions between the two nanowire ends. Differences in VDW attraction were also applied to patterned surfaces (Chapter 7). Stripes of high VDW material (Au) were placed on a silica surface (a low VDW material). When relatively low surface concentrations were used, the high VDW regions collected Au nanowires and organized them into rows that were reminiscent of those observed on un-patterned surfaces at high particle concentrations. VDW and the gravitational force were explored as they combined to influence array orientation in binary PEN mixtures. Depending on the geometries of the particles combined, the contributions of gravity and interparticle interactions exhibited different balance in creating the final array. VDW and gravitational forces could also act as a force for reconfigurable nanowire assembly. In chapter 8, fluid flow was used to concentrate PENs and force them into horizontal arrangements. When fluid flow was stopped, van der Waals forces and gravity were responsible for a reorientation of the assembled particles into a standing array. These studies represent early steps into the future of nanowire assembly methods. I conclude this dissertation by discussing the implications of my work and providing perspective on their importance to the scientific community. I also offer suggestions for future work in nanowire assembly. These areas focus on the development of assembled nanowire devices, mixed nanowire assembly techniques, and potential stimuli responsive reconfigurable assemblies.

New coordination features; A bridging pyridine and the forced shortest non-covalent distance between two CO 3 2- species

DOE PAGES

Velasco, V.; Aguilà, D.; Barrios, L. A.; ...

2014-09-29

The aerobic reaction of the multidentate ligand 2,6-bis-(3-oxo-3-(2-hydroxyphenyl)-propionyl)-pyridine, H 4L, with Co (II) salts in strong basic conditions produces the clusters [Co 4(L) 2(OH)(py) 7]NO 3 (1) and [Co 8Na 4(L) 4(OH) 2(CO 3) 2(py) 10](BF 4) 2 (2). Analysis of their structure unveils unusual coordination features including a very rare bridging pyridine ligand or two trapped carbonate anions within one coordination cage, forced to stay at an extremely close distance (d O···O = 1.946 Å). This unprecedented non-bonding proximity represents a meeting point between long covalent interactions and “intermolecular” contacts. These original motifs have been analysed here through DFTmore » calculations, which have yielded interaction energies and the reduced repulsion energy experimented by both CO 3 2- anions when located in close proximity inside the coordination cage.« less

Fluid Surface Deformation by Objects in the Cheerios Effect

NASA Astrophysics Data System (ADS)

Nguyen, Khoi; Miller, Michael; Mandre, Shreyas; Mandre Lab Team

2012-11-01

Small objects floating on a fluid/air interface deform of the surface depending on material surface properties, density, and geometry. These objects attract each other through capillary interactions, a phenomenon dubbed the ``cheerios effect.'' The attractive force and torque exerted on these objects by the interface can be estimated if the meniscus deformation is known. In addition, the floating objects can also rotate due to such an interaction. We present a series of experiments focused on visualizing the the motions of the floating objects and the deformation of the interface. The experiments involve thin laser-cut acrylic pieces attracting each other on water in a large glass petri dish and a camera set-up to capture the process. Furthermore, optical distortion of a grid pattern is used to visualize the water surface deformation near the edge of the objects. This study of the deformation of the water surface around a floating object, of the attractive/repulsive forces, and of post-contact rotational dynamics are potentially instrumental in the study of colloidal self-assembly.

Magnetically leviated superconducting bearing

DOEpatents

Weinberger, Bernard R.; Lynds, Jr., Lahmer

1993-01-01

A magnetically levitated superconducting bearing includes a magnet (2) mounted on a shaft (12) that is rotatable around an axis of rotation and a Type II superconductor (6) supported on a stator (14) in proximity to the magnet (2). The superconductor (6) is positioned so that when it is cooled to its superconducting state in the presence of a magnetic field, it interacts with the magnet (2) to produce an attractive force that levitates the magnet (2) and supports a load on the shaft (12). The interaction between the superconductor (6) and magnet(2) also produces surface screening currents (8) that generate a repulsive force perpendicular to the load. The bearing also has means for maintaining the superconductor at a temperature below its critical temperature (16, 18). The bearing could also be constructed so the magnet (2) is supported on the stator (14) and the superconductor (6) is mounted on the shaft (12). The bearing can be operated by cooling the superconductor (6) to its superconducting state in the presence of a magnetic field.

pH profile of the adsorption of nucleotides onto montmorillonite. II - Adsorption and desorption of 5-prime-AMP in iron-calcium montmorillonite systems

NASA Technical Reports Server (NTRS)

Banin, A.; Lawless, J. G.; Mazzurco, J.; Church, F. M.; Margulies, L.; Orenberg, J. B.

1985-01-01

The interaction of 5-prime-AMP with montmorillonite saturated with various ratios of two metals found ubiquitously on the surface of earth, that is, iron and calcium, is investigated. Adsorption and desorption of the nucleotide were studied in the pH range of 2-12 at three levels of addition: 0.080, 0.268 and 0.803 mmole 5-prime-AMP per gram of clay. Two desorption stages were employed - H2O wash and NaOH extraction (pH = 12.0). 5-prime-AMP was preferentially adsorbed on the Fe-containing clays relative to the Ca clay. The nucleotide was fully recovered by the two desorption stages, mostly by the NaOH extraction. The evidence at hand indicates that 5-prime-AMP reaction with clay is affected by electrostatic interactions involving both attraction and repulsion forces. Some specific adsorption, possibly the result of covalent bonding and complex formation with the adsorbed ion, cannot be ruled out for iron but does not appear to operate for calcium. Changes in pH cause varying degrees of attaction and repulsion of 5-prime-AMP and may have been operating on the primitive earth, leading to sequences of adsorption and release of this biomolecule.

Flux quench in a system of interacting spinless fermions in one dimension

NASA Astrophysics Data System (ADS)

Nakagawa, Yuya O.; Misguich, Grégoire; Oshikawa, Masaki

2016-05-01

We study a quantum quench in a one-dimensional spinless fermion model (equivalent to the XXZ spin chain), where a magnetic flux is suddenly switched off. This quench is equivalent to imposing a pulse of electric field and therefore generates an initial particle current. This current is not a conserved quantity in the presence of a lattice and interactions, and we investigate numerically its time evolution after the quench, using the infinite time-evolving block decimation method. For repulsive interactions or large initial flux, we find oscillations that are governed by excitations deep inside the Fermi sea. At long times we observe that the current remains nonvanishing in the gapless cases, whereas it decays to zero in the gapped cases. Although the linear response theory (valid for a weak flux) predicts the same long-time limit of the current for repulsive and attractive interactions (relation with the zero-temperature Drude weight), larger nonlinearities are observed in the case of repulsive interactions compared with that of the attractive case.

Graphene Casimir Interactions and Some Possible Applications

NASA Astrophysics Data System (ADS)

Phan, Anh D.

Scientific development requires profound understandings of micromechanical and nanomechanical systems (MEMS/NEMS) due to their applications not only in the technological world, but also for scientific understanding. At the micro- or nano-scale, when two objects are brought close together, the existence of stiction or adhesion is inevitable and plays an important role in the behavior operation of these systems. Such effects are due to surface dispersion forces, such as the van der Waals or Casimir interactions. The scientific understanding of these forces is particularly important for low-dimensional materials. In addition, the discovery of materials, such as graphitic systems has provided opportunities for new classes of devices and challenging fundamental problems. Therefore, investigations of the van der Waals or Caismir forces in graphene-based systems, in particular, and the solution generating non-touching systems are needed. In this study, the Casimir force involving 2D graphene is investigated under various conditions. The Casimir interaction is usually studied in the framework of the Lifshitz theory. According to this theory, it is essential to know the frequency-dependent reflection coefficients of materials. Here, it is found that the graphene reflection coefficients strongly depend on the optical conductivity of graphene, which is described by the Kubo formalism. When objects are placed in vacuum, the Casimir force is attractive and leads to adhesion on the surface. We find that the Casimir repulsion can be obtained by replacing vacuum with a suitable liquid. Our studies show that bromobenzene is the liquid providing this effect. We also find that this long-range force is temperature dependent and graphene/bromobenzene/metal substrate configuration can be used to demonstrate merely thermal Casimir interaction at room temperature and micrometer distances. These findings would provide good guidance and predictions for practical studies.

Synergistic interactions of lipids and myelin basic protein

NASA Astrophysics Data System (ADS)

Hu, Yufang; Doudevski, Ivo; Wood, Denise; Moscarello, Mario; Husted, Cynthia; Genain, Claude; Zasadzinski, Joseph A.; Israelachvili, Jacob

2004-09-01

This report describes force measurements and atomic force microscope imaging of lipid-protein interactions that determine the structure of a model membrane system that closely mimics the myelin sheath. Our results suggest that noncovalent, mainly electrostatic and hydrophobic, interactions are responsible for the multilamellar structure and stability of myelin. We find that myelin basic protein acts as a lipid coupler between two apposed bilayers and as a lipid "hole-filler," effectively preventing defect holes from developing. From our protein-mediated-adhesion and force-distance measurements, we develop a simple quantitative model that gives a reasonably accurate picture of the molecular mechanism and adhesion of bilayer-bridging proteins by means of noncovalent interactions. The results and model indicate that optimum myelin adhesion and stability depend on the difference between, rather than the product of, the opposite charges on the lipid bilayers and myelin basic protein, as well as on the repulsive forces associated with membrane fluidity, and that small changes in any of these parameters away from the synergistically optimum values can lead to large changes in the adhesion or even its total elimination. Our results also show that the often-asked question of which membrane species, the lipids or the proteins, are the "important ones" may be misplaced. Both components work synergistically to provide the adhesion and overall structure. A better appreciation of the mechanism of this synergy may allow for a better understanding of stacked and especially myelin membrane structures and may lead to better treatments for demyelinating diseases such as multiple sclerosis. lipid-protein interactions | myelin membrane structure | membrane adhesion | membrane regeneration/healing | demyelinating diseases

Van-der-Waals interaction of atoms in dipolar Rydberg states

NASA Astrophysics Data System (ADS)

Kamenski, Aleksandr A.; Mokhnenko, Sergey N.; Ovsiannikov, Vitaly D.

2018-02-01

An asymptotic expression for the van-der-Waals constant C 6( n) ≈ -0.03 n 12 K p ( x) is derived for the long-range interaction between two highly excited hydrogen atoms A and B in their extreme Stark states of equal principal quantum numbers n A = n B = n ≫ 1 and parabolic quantum numbers n 1(2) = n - 1, n 2(1) = m = 0 in the case of collinear orientation of the Stark-state dipolar electric moments and the interatomic axis. The cubic polynomial K 3( x) in powers of reciprocal values of the principal quantum number x = 1/ n and quadratic polynomial K 2( y) in powers of reciprocal values of the principal quantum number squared y = 1/ n 2 were determined on the basis of the standard curve fitting polynomial procedure from the calculated data for C 6( n). The transformation of attractive van-der-Waals force ( C 6 > 0) for low-energy states n < 23 into repulsive force ( C 6 < 0) for all higher-energy states of n ≥ 23, is observed from the results of numerical calculations based on the second-order perturbation theory for the operator of the long-range interaction between neutral atoms. This transformation is taken into account in the asymptotic formulas (in both cases of p = 2, 3) by polynomials K p tending to unity at n → ∞ ( K p (0) = 1). The transformation from low- n attractive van-der-Waals force into high- n repulsive force demonstrates the gradual increase of the negative contribution to C 6( n) from the lower-energy two-atomic states, of the A(B)-atom principal quantum numbers n'A(B) = n-Δ n (where Δ n = 1, 2, … is significantly smaller than n for the terms providing major contribution to the second-order series), which together with the states of n″B(A) = n+Δ n make the joint contribution proportional to n 12. So, the hydrogen-like manifold structure of the energy spectrum is responsible for the transformation of the power-11 asymptotic dependence C 6( n) ∝ n 11of the low-angular-momenta Rydberg states in many-electron atoms into the power-12 dependence C 6( n) ∝ n 12 for the dipolar states of the Rydberg manifold.

Spatial Extent of Charge Repulsion Regulates Assembly Pathways for Lysozyme Amyloid Fibrils

PubMed Central

Hill, Shannon E.; Miti, Tatiana; Richmond, Tyson; Muschol, Martin

2011-01-01

Formation of large protein fibrils with a characteristic cross β-sheet architecture is the key indicator for a wide variety of systemic and neurodegenerative amyloid diseases. Recent experiments have strongly implicated oligomeric intermediates, transiently formed during fibril assembly, as critical contributors to cellular toxicity in amyloid diseases. At the same time, amyloid fibril assembly can proceed along different assembly pathways that might or might not involve such oligomeric intermediates. Elucidating the mechanisms that determine whether fibril formation proceeds along non-oligomeric or oligomeric pathways, therefore, is important not just for understanding amyloid fibril assembly at the molecular level but also for developing new targets for intervening with fibril formation. We have investigated fibril formation by hen egg white lysozyme, an enzyme for which human variants underlie non-neuropathic amyloidosis. Using a combination of static and dynamic light scattering, atomic force microscopy and circular dichroism, we find that amyloidogenic lysozyme monomers switch between three different assembly pathways: from monomeric to oligomeric fibril assembly and, eventually, disordered precipitation as the ionic strength of the solution increases. Fibril assembly only occurred under conditions of net repulsion among the amyloidogenic monomers while net attraction caused precipitation. The transition from monomeric to oligomeric fibril assembly, in turn, occurred as salt-mediated charge screening reduced repulsion among individual charged residues on the same monomer. We suggest a model of amyloid fibril formation in which repulsive charge interactions are a prerequisite for ordered fibril assembly. Furthermore, the spatial extent of non-specific charge screening selects between monomeric and oligomeric assembly pathways by affecting which subset of denatured states can form suitable intermolecular bonds and by altering the energetic and entropic requirements for the initial intermediates emerging along the monomeric vs. oligomeric assembly path. PMID:21483680

Particle Trapping Mechanisms Are Different in Spatially Ordered and Disordered Interacting Gels.

PubMed

Hansing, Johann; Netz, Roland R

2018-06-05

Using stochastic simulations, we study the influence of spatial disorder on the diffusion of a single particle through a gel that consists of rigid, straight fibers. The interaction between the particle and the gel fibers consists of an invariant short-range repulsion, the steric part, and an interaction part that can be attractive or repulsive and of varying range. The effect that spatial disorder of the gel structure has on the particle diffusivity depends crucially on the presence of nonsteric interactions. For attractive interactions, disorder slows down diffusion, because in disordered gels, the particle becomes strongly trapped in regions of locally increased fiber density. For repulsive interactions, the diffusivity is minimal for intermediate disorder strength, because highly disordered lattices exhibit abundant passageways of locally low fiber density. The comparison with experimental data on protein and fluorophore diffusion through various hydrogels is favorable. Our findings shed light on particle-diffusion mechanisms in biogels and thus on biological barrier properties, which can be helpful for the optimal design of synthetic diffusors as well as synthetic mucus constructs. Copyright © 2018 Biophysical Society. Published by Elsevier Inc. All rights reserved.

Nanoconfined ionic liquids: Disentangling electrostatic and viscous forces

NASA Astrophysics Data System (ADS)

Lhermerout, Romain; Perkin, Susan

2018-01-01

Recent reports of surface forces across nanoconfined ionic liquids have revealed the existence of an anomalously long-ranged interaction apparently of electrostatic origin. Ionic liquids are viscous, and therefore it is important to inspect rigorously whether the observed repulsive forces are indeed equilibrium forces or, rather, arise from the viscous force during drainage of the fluid between two confining surfaces. In this paper we present our direct measurements of surface forces between mica sheets approaching in the ionic liquid [C2C1Im ] [NTf2] , exploring three orders of magnitude in approach velocity. Trajectories are systematically fitted by solving the equation of motion, allowing us to disentangle the viscous and equilibrium contributions. First, we find that the drainage obeys classical hydrodynamics with a negative slip boundary condition in the range of the structural force, implying that a nanometer -thick portion of the liquid in the vicinity of the solid surface is composed of ordered molecules that do not contribute to the flow. Second, we show that a long-range static force must indeed be invoked, in addition to the viscous force, in order to describe the data quantitatively. This equilibrium interaction decays exponentially and with decay length in agreement with the screening length reported for the same system in previous studies. In those studies the decay was simply checked to be independent of velocity and measured at a low approach rate, rather than explicitly taking account of viscous effects: we explain why this gives indistinguishable outcomes for the screening length by noting that the viscous force is linear to very good approximation over a wide range of distances.

Understanding the surface properties and rheology of a silica suspension mediated by a comb-type poly(acrylic acid)/poly(ethylene oxide) (PAA/PEO) copolymer: effect of salinity.

PubMed

Yang, Dingzheng; Yan, Bin; Xiang, Li; Xu, Haolan; Wang, Xiaogang; Zeng, Hongbo

2018-06-13

Understanding the surface properties and rheology of colloidal suspensions in the presence of polymer additives with high salinity are of great importance in formulating construction materials and optimizing process conditions in the mining and petroleum industry. In this work, the surface properties and rheology of a model spherical silica aqueous suspension mediated by a comb-type poly(acrylic acid)/poly(ethylene oxide) (PAA/PEO) copolymer at various salt concentrations have been investigated. Adsorption measurements using a Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) align well with zeta potential tests and show that polymer adsorption on silica surfaces is enhanced at high salinity (i.e., 3 M NaCl) than at low salinity (i.e., 1 mM NaCl) due to the suppression of the electrical double layer. Surface Forces Apparatus (SFA) measurements reveal that for interactions between two mica surfaces (the basal plane of which has a similar structure as silica) at a high polymer concentration (e.g., 2 wt%), steric repulsion dominates in 1 mM NaCl while bridging attraction is observed in 3 M NaCl. Surface force measurements agree with rheological results on silica suspensions with 0.5 to 2 wt% of PAA/PEO addition, which shows a significant decrease in yield stress in 1 mM NaCl due to steric repulsion but an insignificant variation in yield stress in 3 M NaCl due to attractive bridging interactions. This work provides useful information regarding the surface properties and rheological properties of comb-type polymer-mediated silica suspensions under different salinity conditions, with implications on designing and processing complex colloidal suspensions with polymer additives for various applications.

A discrete particle model reproducing collective dynamics of a bee swarm.

PubMed

Bernardi, Sara; Colombi, Annachiara; Scianna, Marco

2018-02-01

In this article, we present a microscopic discrete mathematical model describing collective dynamics of a bee swarm. More specifically, each bee is set to move according to individual strategies and social interactions, the former involving the desire to reach a target destination, the latter accounting for repulsive/attractive stimuli and for alignment processes. The insects tend in fact to remain sufficiently close to the rest of the population, while avoiding collisions, and they are able to track and synchronize their movement to the flight of a given set of neighbors within their visual field. The resulting collective behavior of the bee cloud therefore emerges from non-local short/long-range interactions. Differently from similar approaches present in the literature, we here test different alignment mechanisms (i.e., based either on an Euclidean or on a topological neighborhood metric), which have an impact also on the other social components characterizing insect behavior. A series of numerical realizations then shows the phenomenology of the swarm (in terms of pattern configuration, collective productive movement, and flight synchronization) in different regions of the space of free model parameters (i.e., strength of attractive/repulsive forces, extension of the interaction regions). In this respect, constraints in the possible variations of such coefficients are here given both by reasonable empirical observations and by analytical results on some stability characteristics of the defined pairwise interaction kernels, which have to assure a realistic crystalline configuration of the swarm. An analysis of the effect of unconscious random fluctuations of bee dynamics is also provided. Copyright © 2018 Elsevier Ltd. All rights reserved.

Influence of long-range Coulomb interaction in velocity map imaging.

PubMed

Barillot, T; Brédy, R; Celep, G; Cohen, S; Compagnon, I; Concina, B; Constant, E; Danakas, S; Kalaitzis, P; Karras, G; Lépine, F; Loriot, V; Marciniak, A; Predelus-Renois, G; Schindler, B; Bordas, C

2017-07-07

The standard velocity-map imaging (VMI) analysis relies on the simple approximation that the residual Coulomb field experienced by the photoelectron ejected from a neutral or ion system may be neglected. Under this almost universal approximation, the photoelectrons follow ballistic (parabolic) trajectories in the externally applied electric field, and the recorded image may be considered as a 2D projection of the initial photoelectron velocity distribution. There are, however, several circumstances where this approximation is not justified and the influence of long-range forces must absolutely be taken into account for the interpretation and analysis of the recorded images. The aim of this paper is to illustrate this influence by discussing two different situations involving isolated atoms or molecules where the analysis of experimental images cannot be performed without considering long-range Coulomb interactions. The first situation occurs when slow (meV) photoelectrons are photoionized from a neutral system and strongly interact with the attractive Coulomb potential of the residual ion. The result of this interaction is the formation of a more complex structure in the image, as well as the appearance of an intense glory at the center of the image. The second situation, observed also at low energy, occurs in the photodetachment from a multiply charged anion and it is characterized by the presence of a long-range repulsive potential. Then, while the standard VMI approximation is still valid, the very specific features exhibited by the recorded images can be explained only by taking into consideration tunnel detachment through the repulsive Coulomb barrier.

Thin film modeling of crystal dissolution and growth in confinement.

PubMed

Gagliardi, Luca; Pierre-Louis, Olivier

2018-01-01

We present a continuum model describing dissolution and growth of a crystal contact confined against a substrate. Diffusion and hydrodynamics in the liquid film separating the crystal and the substrate are modeled within the lubrication approximation. The model also accounts for the disjoining pressure and surface tension. Within this framework, we obtain evolution equations which govern the nonequilibrium dynamics of the crystal interface. Based on this model, we explore the problem of dissolution under an external load, known as pressure solution. We find that in steady state, diverging (power-law) crystal-surface repulsions lead to flat contacts with a monotonic increase of the dissolution rate as a function of the load. Forces induced by viscous dissipation then surpass those due to disjoining pressure at large enough loads. In contrast, finite repulsions (exponential) lead to sharp pointy contacts with a dissolution rate independent of the load and the liquid viscosity. Ultimately, in steady state, the crystal never touches the substrate when pressed against it. This result is independent from the nature of the crystal-surface interaction due to the combined effects of viscosity and surface tension.

Partial synchronization in networks of non-linearly coupled oscillators: The Deserter Hubs Model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Freitas, Celso, E-mail: cbnfreitas@gmail.com; Macau, Elbert, E-mail: elbert.macau@inpe.br; Pikovsky, Arkady, E-mail: pikovsky@uni-potsdam.de

2015-04-15

We study the Deserter Hubs Model: a Kuramoto-like model of coupled identical phase oscillators on a network, where attractive and repulsive couplings are balanced dynamically due to nonlinearity of interactions. Under weak force, an oscillator tends to follow the phase of its neighbors, but if an oscillator is compelled to follow its peers by a sufficient large number of cohesive neighbors, then it actually starts to act in the opposite manner, i.e., in anti-phase with the majority. Analytic results yield that if the repulsion parameter is small enough in comparison with the degree of the maximum hub, then the fullmore » synchronization state is locally stable. Numerical experiments are performed to explore the model beyond this threshold, where the overall cohesion is lost. We report in detail partially synchronous dynamical regimes, like stationary phase-locking, multistability, periodic and chaotic states. Via statistical analysis of different network organizations like tree, scale-free, and random ones, we found a measure allowing one to predict relative abundance of partially synchronous stationary states in comparison to time-dependent ones.« less

Transport properties for a mixture of the ablation products C, C2, and C3

NASA Technical Reports Server (NTRS)

Biolsi, L.; Fenton, J.; Owenson, B.

1981-01-01

The ablation of carbon-phenolic heat shields upon entry into the atmosphere of one of the outer planets leads to the injection of large amounts of C, C2, and C3 into the shock layer. These species must be included in the calculation of transport properties in the shock layer. The kinetic theory of gases has been used to obtain accurate results for the transport properties of monatomic carbon. The Hulburt-Hirschelder potential, the most accurate general purpose atom-atom potential for states with an attractive minimum, was used to represent such states and repulsive states were represented by fitting quantum mechanical potential energy curves with the exponential repulsive potential. These results were orientation averaged according to the peripheral force model to obtain transport collision integrals for the C-C2 and C2-C2 interaction. Results for C3 were obtained by ignoring the presence of the central carbon atom. The thermal conductivity, viscosity, and diffusion coefficients for pure C, C2, and C3, and for mixtures of these gases, were then calculated from 1000 K - 25,000 K.

Thin film modeling of crystal dissolution and growth in confinement

NASA Astrophysics Data System (ADS)

Gagliardi, Luca; Pierre-Louis, Olivier

2018-01-01

We present a continuum model describing dissolution and growth of a crystal contact confined against a substrate. Diffusion and hydrodynamics in the liquid film separating the crystal and the substrate are modeled within the lubrication approximation. The model also accounts for the disjoining pressure and surface tension. Within this framework, we obtain evolution equations which govern the nonequilibrium dynamics of the crystal interface. Based on this model, we explore the problem of dissolution under an external load, known as pressure solution. We find that in steady state, diverging (power-law) crystal-surface repulsions lead to flat contacts with a monotonic increase of the dissolution rate as a function of the load. Forces induced by viscous dissipation then surpass those due to disjoining pressure at large enough loads. In contrast, finite repulsions (exponential) lead to sharp pointy contacts with a dissolution rate independent of the load and the liquid viscosity. Ultimately, in steady state, the crystal never touches the substrate when pressed against it. This result is independent from the nature of the crystal-surface interaction due to the combined effects of viscosity and surface tension.

The Discovery of Gravitational Repulsion by Johannes Droste

NASA Astrophysics Data System (ADS)

McGruder, Charles Hosewell; VanDerMeer, B. Wieb

2018-01-01

In 1687 Newton published his universal law of gravitation, which states that the gravitational force is always attractive. This law is based on our terrestrial experience with slowly moving bodies (v << c). In 1915 Einstein completed his theory of general relativity (also referred to as Einstein’s Theory of Gravitation), which is valid not just for slowly moving bodies but also for those with relativistic velocities. In 1916 Johannes Droste submitted a PhD thesis on general relativity to his advisor, H.A. Lorentz. In it he calculated the motion of a particle in what he called a “single center” and today we call the Schwarzschild field and found that highly relativistic particles experience gravitational repulsion. Thus, his thesis written in Dutch and never before translated contains the discovery of gravitational repulsion. Because of its historical importance we translate the entire section of his thesis containing the discovery of gravitational repulsion. We also translate his thesis in the hope of clearing up a major historical misconception. Namely, that David Hilbert in 1917 discovered gravitational repulsion. In fact, Hilbert rediscovered it, apparently completely independent of Droste’s work. Finally we note that one of the biggest mysteries of astrophysics is the question of how highly energetic particles in relativistic jets and cosmic rays are accelerated. It has been suggested that gravitational repulsion is the mechanism responsible for these phenomena. An historical understanding of gravitational repulsion is therefore pertinent.

Tuning of peptide assembly through force balance adjustment.

PubMed

Cao, Meiwen; Cao, Changhai; Zhang, Lijuan; Xia, Daohong; Xu, Hai

2013-10-01

Controlled self-assembly of amphiphilic tripeptides into distinct nanostructures is achieved via a controlled design of the molecular architecture. The tripeptide Ac-Phe-Phe-Lys-CONH2 (FFK), hardly soluble in water, forms long amyloid-like tubular structures with the aid of β-sheet hydrogen bonding and aromatic π-π stacking. Substitution of phenylalanine (F) with tyrosine (Y), that is, only a subtle structural variation in adding a hydroxyl group to the phenyl ring, results in great change in molecular self-assembly behavior. When one F is substituted with Y, the resulting molecules of FYK and YFK self-assemble into long thinner fibrils with high propensity for lateral association. When both Fs are substituted with Y, the resulting YYK molecule forms spherical aggregates. Introduction of hydroxyl groups into the molecule modifies aromatic interactions and introduces hydrogen bonding. Moreover, since the driving forces for peptide self-assembly including hydrogen bonding, electrostatic repulsion, and π-π stacking have high interdependence with each other, changes in aromatic interaction induce a Domino effect and cause a shift of force balance to a new state. This leads to significant variations in self-assembly behavior. Copyright © 2013 Elsevier Inc. All rights reserved.

Cooperative action of KIF1A Brownian motors with finite dwell time

NASA Astrophysics Data System (ADS)

Oriola, David; Casademunt, Jaume

2014-03-01

We study in detail the cooperative action of small groups of KIF1A motors in its monomeric (single-headed) form within an arrangement relevant to vesicle traffic or membrane tube extraction. It has been recently shown that under these circumstances, the presence of a finite dwell time in the motor cycle contributes to remarkably enhance collective force generation [D. Oriola and J. Casademunt, Phys. Rev. Lett. 111, 048103 (2013), 10.1103/PhysRevLett.111.048103]. We analyze this mechanism in detail by means of a two-state noise-driven ratchet model with hard-core repulsive interactions. We obtain staircase-shaped velocity-force curves and show that motors self-organize in clusters with a nontrivial force distribution that conveys a large part of the load to the central motors. Under heavy loads, large clusters adopt a synchronic mode of totally asymmetric steps. We also find a dramatic increase of the collective efficiency with the number of motors. Finally, we complete the study by addressing different interactions that impose spatial constraints such as rigid coupling and raft-induced confinement. Our results reinforce the hypothesis that the specificity of KIF1A to axonal vesicular transport may be deeply related to its high cooperativity.

Attractive versus repulsive interactions in the Bose-Einstein condensation dynamics of relativistic field theories

NASA Astrophysics Data System (ADS)

Berges, J.; Boguslavski, K.; Chatrchyan, A.; Jaeckel, J.

2017-10-01

We study the impact of attractive self-interactions on the nonequilibrium dynamics of relativistic quantum fields with large occupancies at low momenta. Our primary focus is on Bose-Einstein condensation and nonthermal fixed points in such systems. For a model system, we consider O (N ) -symmetric scalar field theories. We use classical-statistical real-time simulations as well as a systematic 1 /N expansion of the quantum (two-particle-irreducible) effective action to next-to-leading order. When the mean self-interactions are repulsive, condensation occurs as a consequence of a universal inverse particle cascade to the zero-momentum mode with self-similar scaling behavior. For attractive mean self-interactions, the inverse cascade is absent, and the particle annihilation rate is enhanced compared to the repulsive case, which counteracts the formation of coherent field configurations. For N ≥2 , the presence of a nonvanishing conserved charge can suppress number-changing processes and lead to the formation of stable localized charge clumps, i.e., Q balls.

The correlation between fragility, density, and atomic interaction in glass-forming liquids

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Lijin; Guan, Pengfei, E-mail: pguan@csrc.ac.cn; Wang, W. H.

2016-07-21

The fragility that controls the temperature-dependent viscous properties of liquids as the glass transition is approached, in various glass-forming liquids with different softness of the repulsive part of atomic interactions at different densities, is investigated by molecular dynamic simulations. We show that the landscape of fragility in purely repulsive systems can be separated into three regions denoted as R{sub I,} R{sub II}, and R{sub III}, respectively, with qualitatively disparate dynamic behaviors: R{sub I} which can be described by “softness makes strong glasses,” R{sub II} where fragility is independent of softness and can only be tuned by density, and R{sub III}more » with constant fragility, suggesting that density plays an unexpected role for understanding the repulsive softness dependence of fragility. What is more important is that we unify the long-standing inconsistence with respect to the repulsive softness dependence of fragility by observing that a glass former can be tuned more fragile if nonperturbative attraction is added into it. Moreover, we find that the vastly dissimilar influences of attractive interaction on fragility could be estimated from the structural properties of related zero-temperature glasses.« less

Multiscale contact mechanics model for RF-MEMS switches with quantified uncertainties

NASA Astrophysics Data System (ADS)

Kim, Hojin; Huda Shaik, Nurul; Xu, Xin; Raman, Arvind; Strachan, Alejandro

2013-12-01

We introduce a multiscale model for contact mechanics between rough surfaces and apply it to characterize the force-displacement relationship for a metal-dielectric contact relevant for radio frequency micro-electromechanicl system (MEMS) switches. We propose a mesoscale model to describe the history-dependent force-displacement relationships in terms of the surface roughness, the long-range attractive interaction between the two surfaces, and the repulsive interaction between contacting asperities (including elastic and plastic deformation). The inputs to this model are the experimentally determined surface topography and the Hamaker constant as well as the mechanical response of individual asperities obtained from density functional theory calculations and large-scale molecular dynamics simulations. The model captures non-trivial processes including the hysteresis during loading and unloading due to plastic deformation, yet it is computationally efficient enough to enable extensive uncertainty quantification and sensitivity analysis. We quantify how uncertainties and variability in the input parameters, both experimental and theoretical, affect the force-displacement curves during approach and retraction. In addition, a sensitivity analysis quantifies the relative importance of the various input quantities for the prediction of force-displacement during contact closing and opening. The resulting force-displacement curves with quantified uncertainties can be directly used in device-level simulations of micro-switches and enable the incorporation of atomic and mesoscale phenomena in predictive device-scale simulations.

Stability, diffusion and interactions of nonlinear excitations in a many body system

NASA Astrophysics Data System (ADS)

Coste, Christophe; Jean, Michel Saint; Dessup, Tommy

2017-04-01

When repelling particles are confined in a quasi-one-dimensional trap by a transverse potential, a configurational phase transition happens. All particles are aligned along the trap axis at large confinement, but below a critical transverse confinement they adopt a staggered row configuration (zigzag phase). This zigzag transition is a subcritical pitchfork bifurcation in extended systems and in systems with cyclic boundary conditions in the longitudinal direction. Among many evidences, phase coexistence is exhibited by localized nonlinear patterns made of a zigzag phase embedded in otherwise aligned particles. We give the normal form at the bifurcation and we show that these patterns can be described as solitary wave envelopes that we call bubbles. They are stable in a large temperature range and can diffuse as quasi-particles, with a diffusion coefficient that may be deduced from the normal form. The potential energy of a bubble is found to be lower than that of the homogeneous bifurcated phase, which explains their stability. We observe also metastable states, that are pairs of solitary wave envelopes which spontaneously evolve toward a stable single bubble. We evidence a strong effect of the discreteness of the underlying particles system and introduce the concept of topological frustration of a bubble pair. A configuration is frustrated when the particles between the two bubbles are not organized in a modulated staggered row. For a nonfrustrated (NF) bubble pair configuration, the bubbles interaction is attractive so that the bubbles come closer and eventually merge as a single bubble. In contrast, the bubbles interaction is found to be repulsive for a frustrated (F) configuration. We describe a model of interacting solitary wave that provides all qualitative characteristics of the interaction force: it is attractive for NF-systems, repulsive for F-systems, and decreases exponentially with the bubbles distance.

In-gap corner states in core-shell polygonal quantum rings.

PubMed

Sitek, Anna; Ţolea, Mugurel; Niţă, Marian; Serra, Llorenç; Gudmundsson, Vidar; Manolescu, Andrei

2017-01-10

We study Coulomb interacting electrons confined in polygonal quantum rings. We focus on the interplay of localization at the polygon corners and Coulomb repulsion. Remarkably, the Coulomb repulsion allows the formation of in-gap states, i.e., corner-localized states of electron pairs or clusters shifted to energies that were forbidden for non-interacting electrons, but below the energies of corner-side-localized states. We specify conditions allowing optical excitation to those states.

In-gap corner states in core-shell polygonal quantum rings

NASA Astrophysics Data System (ADS)

Sitek, Anna; Ţolea, Mugurel; Niţă, Marian; Serra, Llorenç; Gudmundsson, Vidar; Manolescu, Andrei

2017-01-01

We study Coulomb interacting electrons confined in polygonal quantum rings. We focus on the interplay of localization at the polygon corners and Coulomb repulsion. Remarkably, the Coulomb repulsion allows the formation of in-gap states, i.e., corner-localized states of electron pairs or clusters shifted to energies that were forbidden for non-interacting electrons, but below the energies of corner-side-localized states. We specify conditions allowing optical excitation to those states.

Liquid state isomorphism, Rosenfeld-Tarazona temperature scaling, and Riemannian thermodynamic geometry.

PubMed

Mausbach, Peter; Köster, Andreas; Vrabec, Jadran

2018-05-01

Aspects of isomorph theory, Rosenfeld-Tarazona temperature scaling, and thermodynamic geometry are comparatively discussed on the basis of the Lennard-Jones potential. The first two approaches approximate the high-density fluid state well when the repulsive interparticle interactions become dominant, which is typically the case close to the freezing line. However, previous studies of Rosenfeld-Tarazona scaling for the isochoric heat capacity and its relation to isomorph theory reveal deviations for the temperature dependence. It turns out that a definition of a state region in which repulsive interactions dominate is required for achieving consistent results. The Riemannian thermodynamic scalar curvature R allows for such a classification, indicating predominantly repulsive interactions by R>0. An analysis of the isomorphic character of the freezing line and the validity of Rosenfeld-Tarazona temperature scaling show that these approaches are consistent only in a small state region.

Coarse graining of NN inelastic interactions up to 3 GeV: Repulsive versus structural core

NASA Astrophysics Data System (ADS)

Fernández-Soler, P.; Ruiz Arriola, E.

2017-07-01

The repulsive short-distance core is one of the main paradigms of nuclear physics which even seems confirmed by QCD lattice calculations. On the other hand nuclear potentials at short distances are motivated by high energy behavior where inelasticities play an important role. We analyze NN interactions up to 3 GeV in terms of simple coarse grained complex and energy dependent interactions. We discuss two possible and conflicting scenarios which share the common feature of a vanishing wave function at the core location in the particular case of S waves. We find that the optical potential with a repulsive core exhibits a strong energy dependence whereas the optical potential with the structural core is characterized by a rather adiabatic energy dependence which allows one to treat inelasticity perturbatively. We discuss the possible implications for nuclear structure calculations of both alternatives.

Classical dimer model with anisotropic interactions on the square lattice

NASA Astrophysics Data System (ADS)

Otsuka, Hiromi

2009-07-01

We discuss phase transitions and the phase diagram of a classical dimer model with anisotropic interactions defined on a square lattice. For the attractive region, the perturbation of the orientational order parameter introduced by the anisotropy causes the Berezinskii-Kosterlitz-Thouless transitions from a dimer-liquid to columnar phases. According to the discussion by Nomura and Okamoto for a quantum-spin chain system [J. Phys. A 27, 5773 (1994)], we proffer criteria to determine transition points and also universal level-splitting conditions. Subsequently, we perform numerical diagonalization calculations of the nonsymmetric real transfer matrices up to linear dimension specified by L=20 and determine the global phase diagram. For the repulsive region, we find the boundary between the dimer-liquid and the strong repulsion phases. Based on the dispersion relation of the one-string motion, which exhibits a twofold “zero-energy flat band” in the strong repulsion limit, we give an intuitive account for the property of the strong repulsion phase.

Attraction between Opposing Planar Dipolar Polymer Brushes

DOE PAGES

Mahalik, J. P.; Sumpter, Bobby G.; Kumar, Rajeev

2017-08-01

In this paper, we use a field theory approach to study the effects of permanent dipoles on interpenetration and free energy changes as a function of distance between two identical planar polymer brushes. Melts (i.e., solvent-free) and solvated brushes made up of polymers grafted on nonadsorbing substrates are studied. In particular, the weak coupling limit of the dipolar interactions is considered, which leads to concentration-dependent pairwise interactions, and the effects of orientational order are neglected. It is predicted that a gradual increase in the dipole moment of the polymer segments can lead to attractive interactions between the brushes at intermediatemore » separation distances. Finally, because classical theory of polymer brushes based on the strong stretching limit (SSL) and the standard self-consistent field theory (SCFT) simulations using the Flory’s χ parameter always predicts repulsive interactions at all separations, our work highlights the importance of dipolar interactions in tailoring and accurately predicting forces between polar polymeric interfaces in contact with each other.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mahalik, J. P.; Sumpter, Bobby G.; Kumar, Rajeev

In this paper, we use a field theory approach to study the effects of permanent dipoles on interpenetration and free energy changes as a function of distance between two identical planar polymer brushes. Melts (i.e., solvent-free) and solvated brushes made up of polymers grafted on nonadsorbing substrates are studied. In particular, the weak coupling limit of the dipolar interactions is considered, which leads to concentration-dependent pairwise interactions, and the effects of orientational order are neglected. It is predicted that a gradual increase in the dipole moment of the polymer segments can lead to attractive interactions between the brushes at intermediatemore » separation distances. Finally, because classical theory of polymer brushes based on the strong stretching limit (SSL) and the standard self-consistent field theory (SCFT) simulations using the Flory’s χ parameter always predicts repulsive interactions at all separations, our work highlights the importance of dipolar interactions in tailoring and accurately predicting forces between polar polymeric interfaces in contact with each other.« less

Unified Description of Dynamics of a Repulsive Two-Component Fermi Gas

NASA Astrophysics Data System (ADS)

Grochowski, Piotr T.; Karpiuk, Tomasz; Brewczyk, Mirosław; Rzążewski, Kazimierz

2017-11-01

We study a binary spin mixture of a zero-temperature repulsively interacting Li 6 atoms using both the atomic-orbital and density-functional approaches. The gas is initially prepared in a configuration of two magnetic domains and we determine the frequency of the spin-dipole oscillations which are emerging after the repulsive barrier, initially separating the domains, is removed. We find, in agreement with recent experiment [G. Valtolina et al., Nat. Phys. 13, 704 (2017), 10.1038/nphys4108], the occurrence of a ferromagnetic instability in an atomic gas while the interaction strength between different spin states is increased, after which the system becomes ferromagnetic. The ferromagnetic instability is preceded by the softening of the spin-dipole mode.

Optimal plane-wave Hartree-Fock states for many-fermion systems

DOE Office of Scientific and Technical Information (OSTI.GOV)

de Llano, M.; Plastino, A.; Zabolitzky, J.G.

1979-12-01

The possibility of taking plane-wave orbitals of a Hartree-Fock determinant to fill k space differently from the ''normal'' Fermi sphere is investigated for several two-body potentials including the ''homework'' v/sub 0/, v/sub 1/, and v/sub 2/ - aken from the Reid nucleon-nucleon force - as well as a sum-of-Gaussians potential chosen to fit the deuteron binding and size. A random-search and random-walk numerical algorithm shows that, provided the potential strengths are made large enough, a single-shell ''abnormal'' occupation is always found to be lower in energy than the normal one if sufficient attraction is present in the two-body interaction. Nomore » abnormal occupation is possible for, among other pair interactions, the electron or charged-boson fluid, the repulsive square barrier, and a common form of the He-He interaction.« less

Virus removal in ceramic depth filters based on diatomaceous earth.

PubMed

Michen, Benjamin; Meder, Fabian; Rust, Annette; Fritsch, Johannes; Aneziris, Christos; Graule, Thomas

2012-01-17

Ceramic filter candles, based on the natural material diatomaceous earth, are widely used to purify water at the point-of-use. Although such depth filters are known to improve drinking water quality by removing human pathogenic protozoa and bacteria, their removal regarding viruses has rarely been investigated. These filters have relatively large pore diameters compared to the physical dimension of viruses. However, viruses may be retained by adsorption mechanisms due to intermolecular and surface forces. Here, we use three types of bacteriophages to investigate their removal during filtration and batch experiments conducted at different pH values and ionic strengths. Theoretical models based on DLVO-theory are applied in order to verify experimental results and assess surface forces involved in the adsorptive process. This was done by calculation of interaction energies between the filter surface and the viruses. For two small spherically shaped viruses (MS2 and PhiX174), these filters showed no significant removal. In the case of phage PhiX174, where attractive interactions were expected, due to electrostatic attraction of oppositely charged surfaces, only little adsorption was reported in the presence of divalent ions. Thus, we postulate the existence of an additional repulsive force between PhiX174 and the filter surface. It is hypothesized that such an additional energy barrier originates from either the phage's specific knobs that protrude from the viral capsid, enabling steric interactions, or hydration forces between the two hydrophilic interfaces of virus and filter. However, a larger-sized, tailed bacteriophage of the family Siphoviridae was removed by log 2 to 3, which is explained by postulating hydrophobic interactions.

The Pressure induced by salt crystallization in confinement.

PubMed

Desarnaud, J; Bonn, D; Shahidzadeh, N

2016-08-05

Salt crystallization is a major cause of weathering of rocks, artworks and monuments. Damage can only occur if crystals continue to grow in confinement, i.e. within the pore space of these materials, thus generating mechanical stress. We report the direct measurement, at the microscale, of the force exerted by growing alkali halide salt crystals while visualizing their spontaneous nucleation and growth. The experiments reveal the crucial role of the wetting films between the growing crystal and the confining walls for the development of the pressure. Our results suggest that the measured force originates from repulsion between the similarly charged confining wall and the salt crystal separated by a ~1.5 nm liquid film. Indeed, if the walls are made hydrophobic, no film is observed and no repulsive forces are detected. We also show that the magnitude of the induced pressure is system specific explaining why different salts lead to different amounts of damage to porous materials.

In situ investigation of the mobility of small gold clusters on cleaved MgO surfaces

NASA Technical Reports Server (NTRS)

Metois, J. J.; Heinemann, K.; Poppa, H.

1976-01-01

The mobility of small clusters of gold (about 10 A in diameter) on electron-beam-cleaved MgO surfaces was studied by in situ transmission electron microscopy under controlled vacuum and temperature conditions. During the first 10 min following a deposition at room temperature, over 10 per cent of the crystallites moved over short distances (about 20 A) discontinuously, with a velocity greater than 150 A/sec. Eighty per cent of the mobility events were characterized by the avoidance of proximity of other crystallites, and this was tentatively explained as the result of repulsive elastic forces between the interacting crystallites.

Scattering of water from the glycerol liquid-vacuum interface

NASA Technical Reports Server (NTRS)

Benjamin, I.; Wilson, M. A.; Pohorille, A.; Nathanson, G. M.

1995-01-01

Molecular dynamics calculations of the scattering of D2O from the glycerol surface at different collision energies are reported. The results for the trapping probabilities and energy transfer are in good agreement with experiments. The calculations demonstrate that the strong attractive forces between these two strongly hydrogen bonding molecules have only a minor effect on the initial collision dynamics. The trapping probability is influenced to a significant extent by the repulsive hard sphere-like initial encounter with the corrugated surface and, only at a later stage, by the efficiency of energy flow in the multiple interactions between the water and the surface molecules.

Effect of added mass on the interaction of bubbles in a low-Reynolds-number shear flow.

PubMed

Lavrenteva, Olga; Prakash, Jai; Nir, Avinoam

2016-02-01

Equal size air bubbles that are entrapped by a Taylor vortex of the secondary flow in a Couette device, thereby defying buoyancy, slowly form a stable ordered ring with equal separation distances between all neighbors. We present two models of the process dynamics based on force balance on a bubble in the presence of other bubbles positioned on the same streamline in a simple shear flow. The forces taken into account are the viscous resistance, the added mass force, and the inertia-induced repulsing force between two bubbles in a low-Reynolds-number shear flow obtained in Prakash et al. [J. Prakash et al., Phys. Rev. E 87, 043002 (2013)]. The first model of the process assumes that each bubble interacts solely with its nearest neighbors. The second model takes into account pairwise interactions among all the bubbles in the ring. The performed dynamic simulations were compared to the experimental results reported in Prakash et al. [J. Prakash et al., Phys. Rev. E 87, 043002 (2013)] and to the results of quasistationary models (ignoring the added mass effect) suggested in that paper. It is demonstrated that taking into account the effect of added mass, the models describe the major effect of the bubbles' ordering, provide good estimation of the relaxation time, and also predict nonmonotonic behavior of the separation distance between the bubbles, which exhibit over- and undershooting of equilibrium separations. The latter effects were observed in experiments, but are not predicted by the quasistationary models.

Repulsive tip tilting as the dominant mechanism for hydrogen bond-like features in atomic force microscopy imaging

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Alex J.; Sakai, Yuki; Kim, Minjung

2016-05-09

Experimental atomic force microscopy (AFM) studies have reported distinct features in regions with little electron density for various organic systems. These unexpected features have been proposed to be a direct visualization of intermolecular hydrogen bonding. Here, we apply a computational method using ab initio real-space pseudopotentials along with a scheme to account for tip tilting to simulate AFM images of the 8-hydroxyquinoline dimer and related systems to develop an understanding of the imaging mechanism for hydrogen bonds. We find that contrast for the observed “hydrogen bond” feature comes not from the electrostatic character of the bonds themselves but rather frommore » repulsive tip tilting induced by neighboring electron-rich atoms.« less

Protein adsorption onto CF(3)-terminated oligo(ethylene glycol) containing self-assembled monolayers (SAMs): the influence of ionic strength and electrostatic forces.

PubMed

Bonnet, Nelly; O'Hagan, David; Hähner, Georg

2010-05-07

Oligo(ethylene glycol) (OEG) containing self-assembled monolayers (SAMs) on gold are known for their protein resistant properties. The underlying molecular mechanisms and the contributions of the interactions involved, however, are still not completely understood. It is known that electrostatic, van der Waals, hydrophobic, and hydration forces all play a role in the interaction between proteins and surfaces, but it is difficult to study their influence separately and to quantify their contributions. In the present study we investigate five different OEG containing SAMs and the influence of the ionic strength and the electrostatic component on the amount of a negatively charged protein (fibrinogen) that adsorbs onto them. Atomic force microscopy (AFM) was employed to record force-distance curves with hydrophobic probes depending on the ion concentration, and the amount of the protein that adsorbs relative to a hydrophobic surface was quantified using ellipsometry. The findings suggest that electrostatic forces can create a very low energy barrier thus only slightly decreasing the number of negatively charged proteins in solution with sufficient energy to approach the surface closely, and have a rather small influence on the amount that adsorbs. The films we investigated were not protein resistant. This supports other studies, reporting that a strong short-range repulsion as for example caused by hydration forces is required to make these films resistant to the non-specific adsorption of proteins.

In-gap corner states in core-shell polygonal quantum rings

PubMed Central

Sitek, Anna; Ţolea, Mugurel; Niţă, Marian; Serra, Llorenç; Gudmundsson, Vidar; Manolescu, Andrei

2017-01-01

We study Coulomb interacting electrons confined in polygonal quantum rings. We focus on the interplay of localization at the polygon corners and Coulomb repulsion. Remarkably, the Coulomb repulsion allows the formation of in-gap states, i.e., corner-localized states of electron pairs or clusters shifted to energies that were forbidden for non-interacting electrons, but below the energies of corner-side-localized states. We specify conditions allowing optical excitation to those states. PMID:28071750

Slow relaxation mode in concentrated oil-in-water microemulsions consisting of repulsive droplets

NASA Astrophysics Data System (ADS)

Hattori, Y.; Ushiki, H.; Courbin, L.; Panizza, P.

2007-02-01

The present contribution reports on the observation of two diffusive relaxation modes in a concentrated microemulsion made of repulsive droplets. These two modes can be interpreted in the frame of Weissman’s and Pusey’s theoretical pioneering works. The fast mode is associated to the collective diffusion of droplets whereas the slow one corresponds to the relaxation of droplet concentration fluctuations associated with composition and/or size. We show that (i) repulsive interactions considerably slow down the latter and (ii) a generalized Stokes Einstein relationship between its coefficient of diffusion and the Newtonian viscosity of the solutions, similar to the Walden’s rule for electrolytes, holds for concentrated microemulsion systems made of repulsive droplets.

Small traveling clusters in attractive and repulsive Hamiltonian mean-field models.

PubMed

Barré, Julien; Yamaguchi, Yoshiyuki Y

2009-03-01

Long-lasting small traveling clusters are studied in the Hamiltonian mean-field model by comparing between attractive and repulsive interactions. Nonlinear Landau damping theory predicts that a Gaussian momentum distribution on a spatially homogeneous background permits the existence of traveling clusters in the repulsive case, as in plasma systems, but not in the attractive case. Nevertheless, extending the analysis to a two-parameter family of momentum distributions of Fermi-Dirac type, we theoretically predict the existence of traveling clusters in the attractive case; these findings are confirmed by direct N -body numerical simulations. The parameter region with the traveling clusters is much reduced in the attractive case with respect to the repulsive case.

Hierarchical Shared Control of Cane-Type Walking-Aid Robot

PubMed Central

Tao, Chunjing

2017-01-01

A hierarchical shared-control method of the walking-aid robot for both human motion intention recognition and the obstacle emergency-avoidance method based on artificial potential field (APF) is proposed in this paper. The human motion intention is obtained from the interaction force measurements of the sensory system composed of 4 force-sensing registers (FSR) and a torque sensor. Meanwhile, a laser-range finder (LRF) forward is applied to detect the obstacles and try to guide the operator based on the repulsion force calculated by artificial potential field. An obstacle emergency-avoidance method which comprises different control strategies is also assumed according to the different states of obstacles or emergency cases. To ensure the user's safety, the hierarchical shared-control method combines the intention recognition method with the obstacle emergency-avoidance method based on the distance between the walking-aid robot and the obstacles. At last, experiments validate the effectiveness of the proposed hierarchical shared-control method. PMID:29093805

Influence of electrostatic forces on particle propulsion in the evanescent field of silver ion-exchanged waveguides.

PubMed

Gebennikov, Dmytro; Mittler, Silvia

2013-02-26

The effect of electrostatic interaction between carboxylate- and amino-functionalized polystyrene particles and a charged waveguide surface on the propulsion speed in optical tweezers is considered to be a function of the pH and ionic strength. It was shown that with the variation of the pH of the aqueous solution in which the particles were immersed, a systematic change in propulsion speed with a maximum speed could be achieved. The appearance of a maximum speed was ascribed to changes in the particle-waveguide separation as a result of the combination of two forces: Coulomb repulsion/attraction and induced dipole forces. The highest maximum speed at low ionic strength was around 12 μm/s. Changes in the ionic strength of the solution influenced the gradient of the dielectric constant near the involved surfaces and also led to a slightly reduced hydrodynamic radius of the particles. The combination of these effects subsequently increased the maximum speed to about 23 μm/s.

Probing Atom-Surface Interactions by Diffraction of Bose-Einstein Condensates

NASA Astrophysics Data System (ADS)

Bender, Helmar; Stehle, Christian; Zimmermann, Claus; Slama, Sebastian; Fiedler, Johannes; Scheel, Stefan; Buhmann, Stefan Yoshi; Marachevsky, Valery N.

2014-01-01

In this article, we analyze the Casimir-Polder interaction of atoms with a solid grating and the repulsive interaction between the atoms and the grating in the presence of an external laser source. The Casimir-Polder potential is evaluated exactly in terms of Rayleigh reflection coefficients and via an approximate Hamaker approach. The laser-tuned repulsive interaction is given in terms of Rayleigh transmission coefficients. The combined potential landscape above the solid grating is probed locally by diffraction of Bose-Einstein condensates. Measured diffraction efficiencies reveal information about the shape of the potential landscape in agreement with the theory based on Rayleigh decompositions.

High ionic strength depresses muscle contractility by decreasing both force per cross-bridge and the number of strongly attached cross-bridges.

PubMed

Wang, Li; Bahadir, Anzel; Kawai, Masataka

2015-06-01

An increase in ionic strength (IS) lowers Ca(2+) activated tension in muscle fibres, however, its molecular mechanism is not well understood. In this study, we used single rabbit psoas fibres to perform sinusoidal analyses. During Ca(2+) activation, the effects of ligands (ATP, Pi, and ADP) at IS ranging 150-300 mM were studied on three rate constants to characterize elementary steps of the cross-bridge cycle. The IS effects were studied because a change in IS modifies the inter- and intra-molecular interactions, hence they may shed light on the molecular mechanisms of force generation. Both the ATP binding affinity (K1) and the ADP binding affinity (K 0) increased to 2-3x, and the Pi binding affinity (K5) decreased to 1/2, when IS was raised from 150 to 300 mM. The effect on ATP/ADP can be explained by stereospecific and hydrophobic interaction, and the effect on Pi can be explained by the electrostatic interaction with myosin. The increase in IS increased cross-bridge detachment steps (k2 and k-4), indicating that electrostatic repulsion promotes these steps. However, IS did not affect attachment steps (k-2 and k4). Consequently, the equilibrium constant of the detachment step (K2) increased by ~100%, and the force generation step (K4) decreased by ~30%. These effects together diminished the number of force-generating cross-bridges by 11%. Force/cross-bridge (T56) decreased by 26%, which correlates well with a decrease in the Debye length that limits the ionic atmosphere where ionic interactions take place. We conclude that the major effect of IS is a decrease in force/cross-bridge, but a decrease in the number of force generating cross-bridge also takes place. The stiffness during rigor induction did not change with IS, demonstrating that in-series compliance is not much affected by IS.

High Ionic Strength Depresses Muscle Contractility by Decreasing both Force per Cross-bridge and the Number of Strongly Attached Cross-bridges

PubMed Central

Wang, Li; Bahadir, Anzel; Kawai, Masataka

2015-01-01

An increase in ionic strength (IS) lowers Ca2+ activated tension in muscle fibres, however, its molecular mechanism is not well understood. In this study, we used single rabbit psoas fibres to perform sinusoidal analyses. During Ca2+ activation, the effects of ligands (ATP, Pi, and ADP) at IS ranging 150 mM – 300 mM were studied on three rate constants to characterize elementary steps of the cross-bridge cycle. The IS effects were studied because a change in IS modifies the inter- and intra-molecular interactions, hence they may shed light on the molecular mechanisms of force generation. Both the ATP binding affinity (K1) and the ADP binding affinity (K0) increased to 2-3x, and the Pi binding affinity (K5) decreased to 1/2, when IS was raised from 150 mM to 300 mM. The effect on ATP/ADP can be explained by stereospecific and hydrophobic interaction, and the effect on Pi can be explained by the electrostatic interaction with myosin. The increase in IS increased cross-bridge detachment steps (k2 and k−4), indicating that electrostatic repulsion promotes these steps. However, IS did not affect attachment steps (k−2 and k4). Consequently, the equilibrium constant of the detachment step (K2) increased by ~100%, and the force generation step (K4) decreased by ~30%. These effects together diminished the number of force-generating cross-bridges by 11%. Force/cross-bridge (T56) decreased by 26%, which correlates well with a decrease in the Debye length that limits the ionic atmosphere where ionic interactions take place. We conclude that the major effect of IS is a decrease in force/cross-bridge, but a decrease in the number of force generating cross-bridge also takes place. The stiffness during rigor induction did not change with IS, demonstrating that in-series compliance is not much affected by IS. PMID:25836331

One-loop quantum gravity repulsion in the early Universe.

PubMed

Broda, Bogusław

2011-03-11

Perturbative quantum gravity formalism is applied to compute the lowest order corrections to the classical spatially flat cosmological Friedmann-Lemaître-Robertson-Walker solution (for the radiation). The presented approach is analogous to the approach applied to compute quantum corrections to the Coulomb potential in electrodynamics, or rather to the approach applied to compute quantum corrections to the Schwarzschild solution in gravity. In the framework of the standard perturbative quantum gravity, it is shown that the corrections to the classical deceleration, coming from the one-loop graviton vacuum polarization (self-energy), have (UV cutoff free) opposite to the classical repulsive properties which are not negligible in the very early Universe. The repulsive "quantum forces" resemble those known from loop quantum cosmology.

Modeling and strain gauging of eddy current repulsion deicing systems

NASA Technical Reports Server (NTRS)

Smith, Samuel O.

1993-01-01

Work described in this paper confirms and extends work done by Zumwalt, et al., on a variety of in-flight deicing systems that use eddy current repulsion for repelling ice. Two such systems are known as electro-impulse deicing (EIDI) and the eddy current repulsion deicing strip (EDS). Mathematical models for these systems are discussed for their capabilities and limitations. The author duplicates a particular model of the EDS. Theoretical voltage, current, and force results are compared directly to experimental results. Dynamic strain measurements results are presented for the EDS system. Dynamic strain measurements near EDS or EIDI coils are complicated by the high magnetic fields in the vicinity of the coils. High magnetic fields induce false voltage signals out of the gages.

TASEP of interacting particles of arbitrary size

NASA Astrophysics Data System (ADS)

Narasimhan, S. L.; Baumgaertner, A.

2017-10-01

A mean-field description of the stationary state behaviour of interacting k-mers performing totally asymmetric exclusion processes (TASEP) on an open lattice segment is presented employing the discrete Takahashi formalism. It is shown how the maximal current and the phase diagram, including triple-points, depend on the strength of repulsive and attractive interactions. We compare the mean-field results with Monte Carlo simulation of three types interacting k-mers: monomers, dimers and trimers. (a) We find that the Takahashi estimates of the maximal current agree quantitatively with those of the Monte Carlo simulation in the absence of interaction as well as in both the the attractive and the strongly repulsive regimes. However, theory and Monte Carlo results disagree in the range of weak repulsion, where the Takahashi estimates of the maximal current show a monotonic behaviour, whereas the Monte Carlo data show a peaking behaviour. It is argued that the peaking of the maximal current is due to a correlated motion of the particles. In the limit of very strong repulsion the theory predicts a universal behavior: th maximal currents of k-mers correspond to that of non-interacting (k+1) -mers; (b) Monte Carlo estimates of the triple-points for monomers, dimers and trimers show an interesting general behaviour : (i) the phase boundaries α * and β* for entry and exit current, respectively, as function of interaction strengths show maxima for α* whereas β * exhibit minima at the same strength; (ii) in the attractive regime, however, the trend is reversed (β * > α * ). The Takahashi estimates of the triple-point for monomers show a similar trend as the Monte Carlo data except for the peaking of α * ; for dimers and trimers, however, the Takahashi estimates show an opposite trend as compared to the Monte Carlo data.

Two-dimensional assemblies of soft repulsive colloids confined at fluid interfaces

NASA Astrophysics Data System (ADS)

Isa, L.; Buttinoni, I.; Fernandez-Rodriguez, M. A.; Vasudevan, S. A.

2017-07-01

Colloidal systems are an excellent example of a materials class for which interrogating fundamental questions leads to answers of direct applied relevance. In our group, we in particular focus on two-dimensional assemblies of micro- and nano-particles confined at the interface between two fluids, e.g., oil-water. Here, we review our work on systems interacting through soft repulsive forces of different origin, i.e., electrostatic and steric. By starting from the paradigmatic case of charged colloids at an interface, we show how they are both offering great opportunities as model systems to investigate the structural and mechanical response of materials and as versatile patterning tools for surface nanostructuring. We then move to the case of deformable particles interacting via steric contacts. We first examine microgel particles, which we also demonstrate as very promising models for structural investigations and robust elements for tunable nanolithography. We conclude by briefly discussing the case of particles comprising a hard inorganic core and a deformable polymer shell, which maintain some of the advantageous features of microgel particles, but also enable the realization of two-dimensional functional materials. This article offers our perspective on a very active field of research, where many interesting developments are expected in the near future. Contribution to the Focus Issue Self-assemblies of Inorganic and Organic Nanomaterials edited by Marie-Paule Pileni.

Molecular Theory and Simulation of Water-Oil Contacts

NASA Astrophysics Data System (ADS)

Tan, Liang

The statistical mechanical theory of hydrophobic interactions was initiated decades ago for purely repulsive hydrophobic species, in fact, originally for hard-sphere solutes in liquid water. Systems which treat only repulsive solute-water interactions obviously differ from the real world situation. The issue of the changes to be expected from inclusion of realistic attractive solute-water interactions has been of specific interest also for decades. We consider the local molecular field (LMF) theory for the effects of solute attractive forces on hydrophobic interactions. The principal result of LMF theory is outlined, then tested by obtaining radial distribution functions (rdfs) for Ar atoms in water, with and without attractive interactions distinguished by the Weeks-Chandler-Andersen (WCA) separation. Change from purely repulsive atomic solute interactions to include realistic attractive interactions substantially diminishes the strength of hydrophobic bonds. Since attractions make a big contribution to hydrophobic interactions, Pratt-Chandler theory, which did not include attractions, should not be naively compared to computer simulation results with general physical interactions, including attractions. Lack of general appreciation of this point has lead to mistaken comparisons throughout the history of this subject. The rdfs permit evaluation of osmotic second virial coefficients B2. Those B 2 are consistent with the conclusion that incorporation of attractive interactions leads to more positive (repulsive) values. In all cases here, B2 becomes more attractive with increasing temperature below T = 360K, the so-call inverse temperature behavior. In 2010, the Gulf of Mexico Macondo well (Deepwater Horizon) oil spill focused the attention of the world on water-oil phase equilibrium. In response to the disaster, chemical dispersants were applied to break oil slicks into droplets and thus to avoid large-scale fouling of beaches and to speed up biodegradation. Eventually the dispersant COREXIT 9500 was used predominantly in responding to this accident. The formulation of COREXIT dispersants is somewhat complicated and the various constituents (and their interactions) deserve exhaustive study. Here we focus on sorbitan monooleate (SPAN80), one important component of COREXIT 9500, and we investigate its behavior in oil-water-surfactant systems. Extensive all-atom molecular dynamics calculations on the water-squalane interface for nine different loadings with SPAN80, at T = 300K, are analyzed for the surface tension equation of state, desorption free energy profiles as they depend on loading, and to evaluate escape times for absorbed SPAN80 into the bulk phases. These results suggest that loading only weakly affects accommodation of a SPAN80 molecule by this squalane-water interface. Specifically, the surface tension equation of state is simple from conditions of low loading (high tension) to high loading (lower tension) studied, and the desorption free energy profiles are weakly dependent on loading here. The perpendicular motion of the centroid of the SPAN80 head-group ring is well-described by a diffusional model near the minimum of the desorption free energy profile. Lateral diffusional motion is weakly dependent on loading. Escape times evaluated on the basis of a diffusional model and the desorption free energies are 0.07 s (into the squalane) and 300 h (into the water). The latter value is consistent with irreversible absorption observed by related experimental work.

Aggregation in charged nanoparticles solutions induced by different interactions

NASA Astrophysics Data System (ADS)

Abbas, S.; Kumar, Sugam; Aswal, V. K.; Kohlbrecher, J.

2016-05-01

Small-angle neutron scattering (SANS) has been used to study the aggregation of anionic silica nanoparticles as induced through different interactions. The nanoparticle aggregation is induced by addition of salt (NaCl), cationic protein (lysozyme) and non-ionic surfactant (C12E10) employing different kind of interactions. The results show that the interaction in presence of salt can be explained using DLVO theory whereas non-DLVO forces play important role for interaction of nanoparticles with protein and surfactant. The presence of salt screens the repulsion between charged nanoparticles giving rise to a net attraction in the DLVO potential. On the other hand, strong electrostatic attraction between nanoparticle and oppositely charged protein leads to protein-mediated nanoparticle aggregation. In case of non-ionic surfactant, the relatively long-range attractive depletion interaction is found to be responsible for the particle aggregation. Interestingly, the completely different interactions lead to similar kind of aggregate morphology. The nanoparticle aggregates formed are found to have mass fractal nature having a fractal dimension (~2.5) consistent with diffusion limited type of fractal morphology in all three cases.

Aggregation in charged nanoparticles solutions induced by different interactions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Abbas, S.; Kumar, Sugam; Aswal, V. K., E-mail: vkaswal@barc.gov.in

2016-05-23

Small-angle neutron scattering (SANS) has been used to study the aggregation of anionic silica nanoparticles as induced through different interactions. The nanoparticle aggregation is induced by addition of salt (NaCl), cationic protein (lysozyme) and non-ionic surfactant (C12E10) employing different kind of interactions. The results show that the interaction in presence of salt can be explained using DLVO theory whereas non-DLVO forces play important role for interaction of nanoparticles with protein and surfactant. The presence of salt screens the repulsion between charged nanoparticles giving rise to a net attraction in the DLVO potential. On the other hand, strong electrostatic attraction betweenmore » nanoparticle and oppositely charged protein leads to protein-mediated nanoparticle aggregation. In case of non-ionic surfactant, the relatively long-range attractive depletion interaction is found to be responsible for the particle aggregation. Interestingly, the completely different interactions lead to similar kind of aggregate morphology. The nanoparticle aggregates formed are found to have mass fractal nature having a fractal dimension (~2.5) consistent with diffusion limited type of fractal morphology in all three cases.« less

Spontaneous symmetry breaking in vortex systems with two repulsive lengthscales.

PubMed

Curran, P J; Desoky, W M; Milosević, M V; Chaves, A; Laloë, J-B; Moodera, J S; Bending, S J

2015-10-23

Scanning Hall probe microscopy (SHPM) has been used to study vortex structures in thin epitaxial films of the superconductor MgB2. Unusual vortex patterns observed in MgB2 single crystals have previously been attributed to a competition between short-range repulsive and long-range attractive vortex-vortex interactions in this two band superconductor; the type 1.5 superconductivity scenario. Our films have much higher levels of disorder than bulk single crystals and therefore both superconducting condensates are expected to be pushed deep into the type 2 regime with purely repulsive vortex interactions. We observe broken symmetry vortex patterns at low fields in all samples after field-cooling from above Tc. These are consistent with those seen in systems with competing repulsions on disparate length scales, and remarkably similar structures are reproduced in dirty two band Ginzburg-Landau calculations, where the simulation parameters have been defined by experimental observations. This suggests that in our dirty MgB2 films, the symmetry of the vortex structures is broken by the presence of vortex repulsions with two different lengthscales, originating from the two distinct superconducting condensates. This represents an entirely new mechanism for spontaneous symmetry breaking in systems of superconducting vortices, with important implications for pinning phenomena and high current density applications.

Synchronisation Induced by Repulsive Interactions in a System of van der Pol Oscillators

NASA Astrophysics Data System (ADS)

Martins, T. V.; Toral, R.

2011-09-01

We consider a system of identical van der Pol oscillators, globally coupled through their velocities, and study how the presence of competitive interactions affects its synchronisation properties. We will address the question from two points of view. Firstly, we will investigate the role of competitive interactions on the synchronisation among identical oscillators. Then, we will show that the presence of a fraction of repulsive links results in the appearance of macroscopic oscillations at that signal's rhythm, in regions where the individual oscillator is unable to synchronise with a weak external signal.

Data-driven modelling of social forces and collective behaviour in zebrafish.

PubMed

Zienkiewicz, Adam K; Ladu, Fabrizio; Barton, David A W; Porfiri, Maurizio; Bernardo, Mario Di

2018-04-14

Zebrafish are rapidly emerging as a powerful model organism in hypothesis-driven studies targeting a number of functional and dysfunctional processes. Mathematical models of zebrafish behaviour can inform the design of experiments, through the unprecedented ability to perform pilot trials on a computer. At the same time, in-silico experiments could help refining the analysis of real data, by enabling the systematic investigation of key neurobehavioural factors. Here, we establish a data-driven model of zebrafish social interaction. Specifically, we derive a set of interaction rules to capture the primary response mechanisms which have been observed experimentally. Contrary to previous studies, we include dynamic speed regulation in addition to turning responses, which together provide attractive, repulsive and alignment interactions between individuals. The resulting multi-agent model provides a novel, bottom-up framework to describe both the spontaneous motion and individual-level interaction dynamics of zebrafish, inferred directly from experimental observations. Copyright © 2018 Elsevier Ltd. All rights reserved.

Catalytically powered dynamic assembly of rod-shaped nanomotors and passive tracer particles

PubMed Central

Wang, Wei; Duan, Wentao; Sen, Ayusman; Mallouk, Thomas E.

2013-01-01

Nano- and microscale motors powered by catalytic reactions exhibit collective behavior such as swarming, predator–prey interactions, and chemotaxis that resemble those of biological microorganisms. A quantitative understanding of the catalytically generated forces between particles that lead to these behaviors has so far been lacking. Observations and numerical simulations of pairwise interactions between gold-platinum nanorods in hydrogen peroxide solutions show that attractive and repulsive interactions arise from the catalytically generated electric field. Electrokinetic effects drive the assembly of staggered doublets and triplets of nanorods that are moving in the same direction. None of these behaviors are observed with nanorods composed of a single metal. The motors also collect tracer microparticles at their head or tail, depending on the charge of the particles, actively assembling them into close-packed rafts and aggregates of rafts. These motor–tracer particle interactions can also be understood in terms of the catalytically generated electric field around the ends of the nanorod motors. PMID:24127603

Catalytically powered dynamic assembly of rod-shaped nanomotors and passive tracer particles.

PubMed

Wang, Wei; Duan, Wentao; Sen, Ayusman; Mallouk, Thomas E

2013-10-29

Nano- and microscale motors powered by catalytic reactions exhibit collective behavior such as swarming, predator-prey interactions, and chemotaxis that resemble those of biological microorganisms. A quantitative understanding of the catalytically generated forces between particles that lead to these behaviors has so far been lacking. Observations and numerical simulations of pairwise interactions between gold-platinum nanorods in hydrogen peroxide solutions show that attractive and repulsive interactions arise from the catalytically generated electric field. Electrokinetic effects drive the assembly of staggered doublets and triplets of nanorods that are moving in the same direction. None of these behaviors are observed with nanorods composed of a single metal. The motors also collect tracer microparticles at their head or tail, depending on the charge of the particles, actively assembling them into close-packed rafts and aggregates of rafts. These motor-tracer particle interactions can also be understood in terms of the catalytically generated electric field around the ends of the nanorod motors.

Dependence of Interaction Free Energy between Solutes on an External Electrostatic Field

PubMed Central

Yang, Pei-Kun

2013-01-01

To explore the athermal effect of an external electrostatic field on the stabilities of protein conformations and the binding affinities of protein-protein/ligand interactions, the dependences of the polar and hydrophobic interactions on the external electrostatic field, −Eext, were studied using molecular dynamics (MD) simulations. By decomposing Eext into, along, and perpendicular to the direction formed by the two solutes, the effect of Eext on the interactions between these two solutes can be estimated based on the effects from these two components. Eext was applied along the direction of the electric dipole formed by two solutes with opposite charges. The attractive interaction free energy between these two solutes decreased for solutes treated as point charges. In contrast, the attractive interaction free energy between these two solutes increased, as observed by MD simulations, for Eext = 40 or 60 MV/cm. Eext was applied perpendicular to the direction of the electric dipole formed by these two solutes. The attractive interaction free energy was increased for Eext = 100 MV/cm as a result of dielectric saturation. The force on the solutes along the direction of Eext computed from MD simulations was greater than that estimated from a continuum solvent in which the solutes were treated as point charges. To explore the hydrophobic interactions, Eext was applied to a water cluster containing two neutral solutes. The repulsive force between these solutes was decreased/increased for Eext along/perpendicular to the direction of the electric dipole formed by these two solutes. PMID:23852018

Aggregation of heteropolyanions in aqueous solutions exhibiting short-range attractions and long-range repulsions

DOE PAGES

Bera, Mrinal K.; Qiao, Baofu; Seifert, Soenke; ...

2015-12-15

Charged colloids and proteins in aqueous solutions interact via short-range attractions and long-range repulsions (SALR) and exhibit complex structural phases. These include homogeneously dispersed monomers, percolated monomers, clusters, and percolated clusters. We report the structural architectures of simple charged systems in the form of spherical, Keggin-type heteropolyanions (HPAs) by small-angle X-ray scattering (SAXS) and molecular dynamics (MD) simulations. Structure factors obtained from the SAXS measurements show that the HPAs interact via SALR. Concentration and temperature dependences of the structure factors for HPAs with –3e (e is the charge of an electron) charge are consistent with a mixture of nonassociated monomersmore » and associated randomly percolated monomers, whereas those for HPAs with –4e and –5e charges exhibit only nonassociated monomers in aqueous solutions. Our experiments show that the increase in magnitude of the charge of the HPAs increases their repulsive interactions and inhibits their aggregation in aqueous solutions. MD simulations were done to reveal the atomistic scale origins of SALR between HPAs. As a result, the short-range attractions result from water or proton-mediated hydrogen bonds between neighboring HPAs, whereas the long-range repulsions are due to the distributions of ions surrounding the HPAs.« less

Ion-mediated interactions in suspensions of oppositely charged nanoparticles

NASA Astrophysics Data System (ADS)

Dahirel, Vincent; Hansen, Jean Pierre

2009-08-01

The structure of oppositely charged spherical nanoparticles (polyions), dispersed in ionic solutions with continuous solvent (primitive model), is investigated by Monte Carlo (MC) simulations, within explicit and implicit microion representations, over a range of polyion valences and densities, and microion concentrations. Systems with explicit microions are explored by semigrand canonical MC simulations, and allow density-dependent effective polyion pair potentials vαβeff(r ) to be extracted from measured partial pair distribution functions. Implicit microion MC simulations are based on pair potentials of mean force vαβ(2)(r ) computed by explicit microion simulations of two charged polyions, in the low density limit. In the vicinity of the liquid-gas separation expected for oppositely charged polyions, the implicit microion representation leads to an instability against density fluctuations for polyion valences |Z| significantly below those at which the instability sets in within the exact explicit microion representation. Far from this instability region, the vαβ(2)(r ) are found to be fairly close to but consistently more repulsive than the effective pair potentials vαβeff(r ). This is corroborated by additional calculations of three-body forces between polyion triplets, which are repulsive when one polyion is of opposite charge to the other two. The explicit microion MC data were exploited to determine the ratio of salt concentrations c and co within the dispersion and the reservoir (Donnan effect). c /co is found to first increase before finally decreasing as a function of the polyion packing fraction.

Role of Anisotropic Interactions for Proteins and Patchy Nanoparticles

PubMed Central

2015-01-01

Protein–protein interactions are inherently anisotropic to some degree, with orientation-dependent interactions between repulsive and attractive or complementary regions or “patches” on adjacent proteins. In some cases it has been suggested that such patch–patch interactions dominate the thermodynamics of dilute protein solutions, as captured by the osmotic second virial coefficient (B22), but delineating when this will or will not be the case remains an open question. A series of simplified but exactly solvable models are first used to illustrate that a delicate balance exists between the strength of attractive patch–patch interactions and the patch size, and that repulsive patch–patch interactions contribute significantly to B22 for only those conditions where the repulsions are long-ranged. Finally, B22 is reformulated, without approximations, in terms of the density of states for a given interaction energy and particle–particle distance. Doing so illustrates the inherent balance of entropic and energetic contributions to B22. It highlights that simply having strong patch–patch interactions will only cause anisotropic interactions to dominate B22 solution properties if the unavoidable entropic penalties are overcome, which cannot occur if patches are too small. The results also indicate that the temperature dependence of B22 may be a simple experimental means to assess whether a small number of strongly attractive configurations dominate the dilute solution behavior. PMID:25302767

Mesoscale Particle-Based Model of Electrophoretic Deposition

DOE PAGES

Giera, Brian; Zepeda-Ruiz, Luis A.; Pascall, Andrew J.; ...

2016-12-20

In this paper, we present and evaluate a semiempirical particle-based model of electrophoretic deposition using extensive mesoscale simulations. We analyze particle configurations in order to observe how colloids accumulate at the electrode and arrange into deposits. In agreement with existing continuum models, the thickness of the deposit increases linearly in time during deposition. Resulting colloidal deposits exhibit a transition between highly ordered and bulk disordered regions that can give rise to an appreciable density gradient under certain simulated conditions. The overall volume fraction increases and falls within a narrow range as the driving force due to the electric field increasesmore » and repulsive intercolloidal interactions decrease. We postulate ordering and stacking within the initial layer(s) dramatically impacts the microstructure of the deposits. Finally, we find a combination of parameters, i.e., electric field and suspension properties, whose interplay enhances colloidal ordering beyond the commonly known approach of only reducing the driving force.« less

Growth of Bacterial Colonies

NASA Astrophysics Data System (ADS)

Warren, Mya; Hwa, Terence

2013-03-01

On hard agar gel, there is insufficient surface hydration for bacteria to swim or swarm. Instead, growth occurs in colonies of close-packed cells, which expand purely due to repulsive interactions: individual bacteria push each other out of the way through the force of their growth. In this way, bacterial colonies represent a new type of ``active'' granular matter. In this study, we investigate the physical, biochemical, and genetic elements that determine the static and dynamic aspects of this mode of bacterial growth for E. coli. We characterize the process of colony expansion empirically, and use discrete and continuum models to examine the extent to which our observations can be explained by the growth characteristics of non-communicating cells, coupled together by physical forces, nutrients, and waste products. Our results challenge the commonly accepted modes of bacterial colony growth and provide insight into sources of growth limitation in crowded bacterial communities.

Evanescent-wave bonding between optical waveguides.

PubMed

Povinelli, Michelle L; Loncar, Marko; Ibanescu, Mihai; Smythe, Elizabeth J; Johnson, Steven G; Capasso, Federico; Joannopoulos, John D

2005-11-15

Forces arising from overlap between the guided waves of parallel, microphotonic waveguides are calculated. Both attractive and repulsive forces, determined by the choice of relative input phase, are found. Using realistic parameters for a silicon-on-insulator material system, we estimate that the forces are large enough to cause observable displacements. Our results illustrate the potential for a broader class of optically tunable microphotonic devices and microstructured artificial materials.

Investigation of head group behaviour of lamellar liquid crystals

NASA Astrophysics Data System (ADS)

Delikatny, E. J.; Burnell, E. E.

A mean field equilibrium statistical mechanical model, based on the Samulski inertial frame model, was developed to simulate experimental dipolar and quadrupolar nmr couplings of isotopically substituted potassium palmitates. An isolated four spin system was synthesized (2,2,3,3,-H4-palmitic acid-d27) and in conjunction with data presented in a previous paper on perdeuterated and carbon 13 labelled soaps, the head group behaviour of the molecule was investigated. Two interactions were considered in the modelling procedure: a mean field steric interaction characterized by a constraining cylinder, and a head group interaction characterized by a mass on the end of a rod of variable length. The rod lies along the first C-C bond direction and accounts for the interaction between polar head group and water via its effect on the moment of inertia of the molecule. In potassium palmitate mean field steric repulsive forces remain constant over the entire temperature range studied. In contrast, electrostatic interactions between polar head group and water, approximately constant at higher temperatures, increase dramatically as the phase transition is approached. This evidence supports a previously proposed model of lipidwater interaction.

Fidelity study of superconductivity in extended Hubbard models

NASA Astrophysics Data System (ADS)

Plonka, N.; Jia, C. J.; Wang, Y.; Moritz, B.; Devereaux, T. P.

2015-07-01

The Hubbard model with local on-site repulsion is generally thought to possess a superconducting ground state for appropriate parameters, but the effects of more realistic long-range Coulomb interactions have not been studied extensively. We study the influence of these interactions on superconductivity by including nearest- and next-nearest-neighbor extended Hubbard interactions in addition to the usual on-site terms. Utilizing numerical exact diagonalization, we analyze the signatures of superconductivity in the ground states through the fidelity metric of quantum information theory. We find that nearest and next-nearest neighbor interactions have thresholds above which they destabilize superconductivity regardless of whether they are attractive or repulsive, seemingly due to competing charge fluctuations.

Attractive and Repulsive Forces on Particles in Oscillatory Flow

NASA Astrophysics Data System (ADS)

Agarwal, Siddhansh; Rallabandi, Bhargav; Raju, David; Thameem, Raqeeb; Hilgenfeldt, Sascha

2016-11-01

A large class of oscillating flows gives rise to rectified streaming motion of the fluid. It has recently been shown that particle transport in such flows, excited by bubbles oscillating at ultrasound frequencies, leads to differential displacement and efficient sorting of microparticles by size. We derive a general expression for the instantaneous radial force experienced by a small spherical particle in the vicinity of an oscillating interface, and generalize the radial projection of the Maxey-Riley equation to include this effect. Varying relevant system parameters, we show that the net effect on the particle can be either an attraction to or a repulsion from the bubble surface, depending in particular on the particle size and the particle/fluid density contrast. We demonstrate that these predictions are in agreement with a variety of experiments.

A single of MR sponge tactile sensor design for medical applications

NASA Astrophysics Data System (ADS)

Cha, Seung-Woo; Kang, Seok-Rae; Hwang, Yong-Hoon; Choi, Seung-Bok

2017-04-01

Recently, it is very popular in medical field to adopt robot surgery such as robot-assisted minimally invasive surgery (RMIS). However, there are some problems in the robot surgery. It is very hard to get the touch feeling of the organs during the surgical operation because the surgeons cannot touch and feel repulsive force from the organs directly. So, this work proposes a squeeze mode of single magneto-rheological (MR) sponge to realize viscoelastic property of human organs or skins and undertake a theoretical analysis of MR sponge. In addition, its effectiveness is verified through experimental tests. The similarity between MR sponge and real organs is identified and desired repulsive force of each organs can be achieved by proper selection of MR sponge cell associated with controlled input current.

Determination of the Contact Angle Based on the Casimir Effect

NASA Technical Reports Server (NTRS)

Mazuruk, K.; Volz, M. P.

2015-01-01

In several crystal growth processed based on capillarity, a melt comes into contact with a crucible wall at an angle defined as the contact angle. For molten metals and semiconductors, this contact angle is dependent upon both the crucible and melt material and typical values fall in the range 80-170deg. However, on a microscopic scale, there does not exist a precise and sharp contact angle but rather the melt and solid surfaces merge smoothly and continuously over a distance of up to several micrometers. Accurate modeling requires a more advanced treatment of this interaction. The interaction between the melt and solid surfaces can be calculated by considering two forces: a short-range repulsive force and a longer range (up to a few micrometers) Casimir force. The Casimir force between the two bodies of complex geometry is calculated using a retarded temperature Green's function (Matsubara type) for the photon in the medium. The governing equations are cast in the form of a set of boundary integral equations which are then solved numerically for the case of molten Ge on SiO2. The shape of the molten surface approaching the flat solid body is determined, and the contact angle is defined as the angle between the two surfaces at the microscopically asymptotic distance of 1-2 micrometers. The formulation of this model and the results of the numerical calculations will be presented and discussed.

Influence of a repulsive vector coupling in magnetized quark matter

NASA Astrophysics Data System (ADS)

Denke, Robson Z.; Pinto, Marcus Benghi

2013-09-01

We consider two flavor magnetized quark matter in the presence of a repulsive vector coupling (GV) devoting special attention to the low temperature region of the phase diagram to show how this type of interaction counterbalances the effects produced by a strong magnetic field. The most important effects occur at intermediate and low temperatures affecting the location of the critical end point as well as the region of first order chiral transitions. When GV=0 the presence of high magnetic fields (eB≥10mπ2) increases the density coexistence region with respect to the case when B and GV are absent while a decrease of this region is observed at high GV values and vanishing magnetic fields. Another interesting aspect observed at the low temperature region is that the usual decrease of the coexistence chemical value (inverse magnetic catalysis) at GV=0 is highly affected by the presence of the vector interaction which acts in the opposite way. Our investigation also shows that the presence of a repulsive vector interaction enhances the de Haas-van Alphen oscillations which, for very low temperatures, take place at eB≲6mπ2. We observe that the presence of a magnetic field, together with a repulsive vector interaction, gives rise to a complex transition pattern since B favors the appearance of multiple solutions to the gap equation whereas GV turns some metastable solutions into stable ones allowing for a cascade of transitions to occur.

Stresses in curved nematic membranes.

PubMed

Santiago, J A

2018-05-01

Ordering configurations of a director field on a curved membrane induces stress. In this work, we present a theoretical framework to calculate the stress tensor and the torque as a consequence of the nematic ordering; we use the variational principle and invariance of the energy under Euclidean motions. Euler-Lagrange equations of the membrane as well as the corresponding boundary conditions also appear as natural results. The stress tensor found includes attraction-repulsion forces between defects; likewise, defects are attracted to patches with the same sign in Gaussian curvature. These forces are mediated by the Green function of the Laplace-Beltrami operator of the surface. In addition, we find nonisotropic forces that involve derivatives of the Green function and the Gaussian curvature, even in the normal direction to the membrane. We examine the case of axial membranes to analyze the spherical one. For spherical vesicles we find the modified Young-Laplace law as a consequence of the nematic texture. In the case of spherical cap with defect at the north pole, we find that the force is repulsive with respect to the north pole, indicating that it is an unstable equilibrium point.

Multiscale treatment of theoretical mechanisms for the protection of hydrogel surfaces from adhesive forces

NASA Astrophysics Data System (ADS)

Sokoloff, J. B.

2014-09-01

One role of a lubricant is to prevent wear of two surfaces in contact, which is likely to be the result of adhesive forces that cause a pair of asperities belonging to two surfaces in contact to stick together. Such adhesive sticking of asperities can occur both for sliding surfaces and for surfaces which are pressed together and then pulled apart. The latter situation, for example, is important for contact lenses, as prevention of sticking reduces possible damage to the cornea as the lenses are inserted and removed from the eye. Contact lenses are made from both neutral and polyelectrolyte hydrogels. It is demonstrated here that sticking of neutral hydrogels can be prevented by repulsive forces between asperities in contact, resulting from polymers attached to the gel surface but not linked with each other. For polyelectrolyte hydrogels, it is shown that osmotic pressure due to counterions, held at the interface between asperities in contact by the electrostatic attraction between the ions and the fixed charges in the gel, can provide a sufficiently strong repulsive force to prevent adhesive sticking of small-length-scale asperities.

Stresses in curved nematic membranes

NASA Astrophysics Data System (ADS)

Santiago, J. A.

2018-05-01

Ordering configurations of a director field on a curved membrane induces stress. In this work, we present a theoretical framework to calculate the stress tensor and the torque as a consequence of the nematic ordering; we use the variational principle and invariance of the energy under Euclidean motions. Euler-Lagrange equations of the membrane as well as the corresponding boundary conditions also appear as natural results. The stress tensor found includes attraction-repulsion forces between defects; likewise, defects are attracted to patches with the same sign in Gaussian curvature. These forces are mediated by the Green function of the Laplace-Beltrami operator of the surface. In addition, we find nonisotropic forces that involve derivatives of the Green function and the Gaussian curvature, even in the normal direction to the membrane. We examine the case of axial membranes to analyze the spherical one. For spherical vesicles we find the modified Young-Laplace law as a consequence of the nematic texture. In the case of spherical cap with defect at the north pole, we find that the force is repulsive with respect to the north pole, indicating that it is an unstable equilibrium point.

Balancing the intermolecular forces in peptide amphiphiles for controlling self-assembly transitions.

PubMed

Buettner, C J; Wallace, A J; Ok, S; Manos, A A; Nicholl, M J; Ghosh, A; Tweedle, M F; Goldberger, J E

2017-06-21

While the influence of alkyl chain length and headgroup size on self-assembly behaviour has been well-established for simple surfactants, the rational control over the pH- and concentration-dependent self-assembly behaviour in stimuli responsive peptides remains an elusive goal. Here, we show that different amphiphilic peptides can have similar self-assembly phase diagrams, providing the relative strengths of the attractive and repulsive forces are balanced. Using palmitoyl-YYAAEEEEK(DO3A:Gd)-NH 2 and palmitoyl-YAAEEEEK(DO3A:Gd)-NH 2 as controls, we show that reducing hydrophobic attractive forces through fewer methylene groups in the alkyl chain will lead to a similar self-assembly phase diagram as increasing the electrostatic repulsive forces via the addition of a glutamic acid residue. These changes allow creation of self-assembled MRI vehicles with slightly different micelle and nanofiber diameters but with minimal changes in the spin-lattice T 1 relaxivity. These findings reveal a powerful strategy to design self-assembled vehicles with different sizes but with similar self-assembly profiles.

Effects of interactions on the generalized Hong–Ou–Mandel effect

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gertjerenken, B.; Kevrekidis, P. G.

2015-04-11

We numerically investigate the influence of interactions on the generalized Hong–Ou–Mandel (HOM) effect for bosonic particles in a (quasi-)one-dimensional set-up with weak harmonic confinement and show results for the cases of N = 2, N = 3 and N = 4 bosons interacting with a beam splitter, whose role is played by a δ-barrier. In particular, we focus on the effect of attractive interactions and compare the results with the repulsive case, as well as with the analytically available results for the non-interacting case (that we use as a benchmark). In addition, we observe a fermionization effect both for growingmore » repulsive and attractive interactions, i.e., the dip in the HOM coincidence count is progressively smeared out, for increasing interaction strengths. The role of input asymmetries is also explored.« less

Sodium triflate decreases interaggregate repulsion and induces phase separation in cationic micelles.

PubMed

Lima, Filipe S; Cuccovia, Iolanda M; Buchner, Richard; Antunes, Filipe E; Lindman, Björn; Miguel, Maria G; Horinek, Dominik; Chaimovich, Hernan

2015-03-10

Dodecyltrimethylammonium triflate (DTATf) micelles possess lower degree of counterion dissociation (α), lower hydration, and higher packing of monomers than other micelles of similar structure. Addition of sodium triflate ([NaTf] > 0.05 M) to DTATf solutions promotes phase separation. This phenomenon is commonly observed in oppositely charged surfactant mixtures, but it is rare for ionic surfactants and relatively simple counterions. While the properties of DTATf have already been reported, the driving forces for the observed phase separation with added salt remain unclear. Thus, we propose an interpretation for the observed phase separation in cationic surfactant solutions. Addition of up to 0.03 M NaTf to micellar DTATf solutions led to a limited increase of the aggregation number, to interface dehydration, and to a progressive decrease in α. The viscosity of DTATf solutions of higher concentration ([DTATf] ≥ 0.06 M) reached a maximum with increasing [NaTf], though the aggregation number slightly increased, and no shape change occurred. We hypothesize that this maximum results from a decrease in interaggregate repulsion, as a consequence of increased ion binding. This reduction in micellar repulsion without simultaneous infinite micellar growth is, probably, the major driving force for phase separation at higher [NaTf].

A new parameter-free soft-core potential for silica and its application to simulation of silica anomalies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Izvekov, Sergei, E-mail: sergiy.izvyekov.civ@mail.mil; Rice, Betsy M.

2015-12-28

A core-softening of the effective interaction between oxygen atoms in water and silica systems and its role in developing anomalous thermodynamic, transport, and structural properties have been extensively debated. For silica, the progress with addressing these issues has been hampered by a lack of effective interaction models with explicit core-softening. In this work, we present an extension of a two-body soft-core interatomic force field for silica recently reported by us [S. Izvekov and B. M. Rice, J. Chem. Phys. 136(13), 134508 (2012)] to include three-body forces. Similar to two-body interaction terms, the three-body terms are derived using parameter-free force-matching ofmore » the interactions from ab initio MD simulations of liquid silica. The derived shape of the O–Si–O three-body potential term affirms the existence of repulsion softening between oxygen atoms at short separations. The new model shows a good performance in simulating liquid, amorphous, and crystalline silica. By comparing the soft-core model and a similar model with the soft-core suppressed, we demonstrate that the topology reorganization within the local tetrahedral network and the O–O core-softening are two competitive mechanisms responsible for anomalous thermodynamic and kinetic behaviors observed in liquid and amorphous silica. The studied anomalies include the temperature of density maximum locus and anomalous diffusivity in liquid silica, and irreversible densification of amorphous silica. We show that the O–O core-softened interaction enhances the observed anomalies primarily through two mechanisms: facilitating the defect driven structural rearrangements of the silica tetrahedral network and modifying the tetrahedral ordering induced interactions toward multiple characteristic scales, the feature which underlies the thermodynamic anomalies.« less

Symmetry breaking and singularity structure in Bose-Einstein condensates

NASA Astrophysics Data System (ADS)

Commeford, K. A.; Garcia-March, M. A.; Ferrando, A.; Carr, Lincoln D.

2012-08-01

We determine the trajectories of vortex singularities that arise after a single vortex is broken by a discretely symmetric impulse in the context of Bose-Einstein condensates in a harmonic trap. The dynamics of these singularities are analyzed to determine the form of the imprinted motion. We find that the symmetry-breaking process introduces two effective forces: a repulsive harmonic force that causes the daughter trajectories to be ejected from the parent singularity and a Magnus force that introduces a torque about the axis of symmetry. For the analytical noninteracting case we find that the parent singularity is reconstructed from the daughter singularities after one period of the trapping frequency. The interactions between singularities in the weakly interacting system do not allow the parent vortex to be reconstructed. Analytic trajectories were compared to the actual minima of the wave function, showing less than 0.5% error for an impulse strength of v=0.00005. We show that these solutions are valid within the impulse regime for various impulse strengths using numerical integration of the Gross-Pitaevskii equation. We also show that the actual duration of the symmetry-breaking potential does not significantly change the dynamics of the system as long as the strength is below v=0.0005.

Numerical Solution of Dyson Brownian Motion and a Sampling Scheme for Invariant Matrix Ensembles

NASA Astrophysics Data System (ADS)

Li, Xingjie Helen; Menon, Govind

2013-12-01

The Dyson Brownian Motion (DBM) describes the stochastic evolution of N points on the line driven by an applied potential, a Coulombic repulsion and identical, independent Brownian forcing at each point. We use an explicit tamed Euler scheme to numerically solve the Dyson Brownian motion and sample the equilibrium measure for non-quadratic potentials. The Coulomb repulsion is too singular for the SDE to satisfy the hypotheses of rigorous convergence proofs for tamed Euler schemes (Hutzenthaler et al. in Ann. Appl. Probab. 22(4):1611-1641, 2012). Nevertheless, in practice the scheme is observed to be stable for time steps of O(1/ N 2) and to relax exponentially fast to the equilibrium measure with a rate constant of O(1) independent of N. Further, this convergence rate appears to improve with N in accordance with O(1/ N) relaxation of local statistics of the Dyson Brownian motion. This allows us to use the Dyson Brownian motion to sample N× N Hermitian matrices from the invariant ensembles. The computational cost of generating M independent samples is O( MN 4) with a naive scheme, and O( MN 3log N) when a fast multipole method is used to evaluate the Coulomb interaction.

Probing the Repulsive Core of the Nucleon-Nucleon Interaction via the 4He(e,e`pN) Triple-Coincidence Reaction

DOE PAGES

Korover, Igor; Muangma, Navaphon; Hen, Or; ...

2014-07-01

We studied simultaneously the 4He(e,e'p), 4He(e,e'pp), and 4He(e,e'pn) reactions at Q 2=2 [GeV/c] 2 and x B >1, for a (e,e'p) missing-momentum range of 400 to 830 MeV/c. The knocked-out proton was detected in coincidence with a proton or neutron recoiling almost back to back to the missing momentum, leaving the residual A=2 system at low excitation energy. These data were used to identify two-nucleon short-range correlated pairs and to deduce their isospin structure as a function of missing momentum in a region where the nucleon-nucleon force is expected to change from predominantly tensor to repulsive. Neutron-proton pairs dominate themore » high-momentum tail of the nucleon momentum distributions, but their abundance is reduced as the nucleon momentum increases beyond ~500 MeV/c. The extracted fraction of proton-proton pairs is small and almost independent of the missing momentum in the range we studied. Our data are compared with ab-initio calculations of two-nucleon momentum distributions in 4He.« less

On the theoretical description of weakly charged surfaces.

PubMed

Wang, Rui; Wang, Zhen-Gang

2015-03-14

It is widely accepted that the Poisson-Boltzmann (PB) theory provides a valid description for charged surfaces in the so-called weak coupling limit. Here, we show that the image charge repulsion creates a depletion boundary layer that cannot be captured by a regular perturbation approach. The correct weak-coupling theory must include the self-energy of the ion due to the image charge interaction. The image force qualitatively alters the double layer structure and properties, and gives rise to many non-PB effects, such as nonmonotonic dependence of the surface energy on concentration and charge inversion. In the presence of dielectric discontinuity, there is no limiting condition for which the PB theory is valid.

Electrostatic energy of transfer and macrobond analyses of intermolecular interactions and hydration effects in protein crystals in a low ionic environment

NASA Astrophysics Data System (ADS)

Sugawara, Yoko; Hirano, Yuji; Yamamura, Shigefumi; Endo, Shigeru; Ootaki, Masanori; Matsumoto, Naoki; Takahashi, Takuya

2017-06-01

We developed an electrostatic energy of transfer (EET) analysis applicable to periodic boundary condition, including a nonrectangular unit cell. It was applied to monoclinic ribonuclease A crystallized with ethanol as a precipitant. Macrobond analysis was also carried out. Owing to the low ionic strength of the solvent region, atomic EET values were non-negligible even at long-distance points. Most of the molecular EET values-defined as the individual contribution of each surrounding molecule-were positive. The inclusion of the molecular EET values of hydration water molecules reduced the repulsive force, and the evaluation of hydration effects in protein crystals was found to be imperative.

Simulation of shear thickening in attractive colloidal suspensions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pednekar, Sidhant; Chun, Jaehun; Morris, Jeffrey F.

2017-01-01

The influence of attractive forces between particles under conditions of large particle volume fraction is addressed using numerical simulations which account for hydrodynamic, Brownian, conservative and frictional contact forces. The focus is on conditions for which a significant increase in the apparent viscosity at small shear rates, and possibly the development of a yield stress, is observed. The high shear rate behavior for Brownian suspensions has been shown in recent work [R. Mari, R. Seto, J. F. Morris & M. M. Denn, PNAS, 2015, 112, 15326-15330] to be captured by the inclusion of pairwise forces of two forms, one amore » contact frictional interaction and the second a repulsive force common in stabilized colloidal dispersions. Under such conditions, shear thickening is observed when shear stress is comparable to the sum of the Brownian stress and a characteristic stress based on the combination of interparticle force with kT the thermal energy. At sufficiently large volume fraction, this shear thickening can be very abrupt. Here it is shown that when attractive interactions are present with the noted forces, the shear thickening is obscured, as the viscosity shear thins with increasing shear rate, eventually descending from an infinite value (yield stress conditions) to a plateau at large stress; this plateau is at the same level as the large-shear rate viscosity found in the shear thickened state without attractive forces. It is shown that this behavior is consistent with prior observations in shear thickening suspensions modified to be attractive through depletion flocculation [V. Gopalakrishnan & C. F. Zukoski J. Rheol., 2004, 48, 1321-1344]. The contributions of the contact, attractive, and hydrodynamics forces to the bulk stress are presented, as are the contact networks found at different attractive strengths.« less

Demonstrating Forces between Parallel Wires.

ERIC Educational Resources Information Center

Baker, Blane

2000-01-01

Describes a physics demonstration that dramatically illustrates the mutual repulsion (attraction) between parallel conductors using insulated copper wire, wooden dowels, a high direct current power supply, electrical tape, and an overhead projector. (WRM)

Nucleation theory with delayed interactions: An application to the early stages of the receptor-mediated adhesion/fusion kinetics of lipid vesicles

NASA Astrophysics Data System (ADS)

Raudino, Antonio; Pannuzzo, Martina

2010-01-01

A semiquantitative theory aimed to describe the adhesion kinetics between soft objects, such as living cells or vesicles, has been developed. When rigid bodies are considered, the adhesion kinetics is successfully described by the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) picture, where the energy profile of two approaching bodies is given by a two asymmetrical potential wells separated by a barrier. The transition probability from the long-distance to the short-distance minimum defines the adhesion rate. Conversely, soft bodies might follow a different pathway to reach the short-distance minimum: thermally excited fluctuations give rise to local protrusions connecting the approaching bodies. These transient adhesion sites are stabilized by short-range adhesion forces (e.g., ligand-receptor interactions between membranes brought at contact distance), while they are destabilized both by repulsive forces and by the elastic deformation energy. Above a critical area of the contact site, the adhesion forces prevail: the contact site grows in size until the complete adhesion of the two bodies inside a short-distance minimum is attained. This nucleation mechanism has been developed in the framework of a nonequilibrium Fokker-Planck picture by considering both the adhesive patch growth and dissolution processes. In addition, we also investigated the effect of the ligand-receptor pairing kinetics at the adhesion site in the time course of the patch expansion. The ratio between the ligand-receptor pairing kinetics and the expansion rate of the adhesion site is of paramount relevance in determining the overall nucleation rate. The theory enables one to self-consistently include both thermodynamics (energy barrier height) and dynamic (viscosity) parameters, giving rise in some limiting cases to simple analytical formulas. The model could be employed to rationalize fusion kinetics between vesicles, provided the short-range adhesion transition is the rate-limiting step to the whole adhesion process. Approximate relationships between the experimental fusion rates reported in the literature and parameters such as membrane elastic bending modulus, repulsion strength, temperature, osmotic forces, ligand-receptor binding energy, solvent and membrane viscosities are satisfactory explained by our model. The present results hint a possible role of the initial long-distance→short-distance transition in determining the whole fusion kinetics.

Nucleation theory with delayed interactions: an application to the early stages of the receptor-mediated adhesion/fusion kinetics of lipid vesicles.

PubMed

Raudino, Antonio; Pannuzzo, Martina

2010-01-28

A semiquantitative theory aimed to describe the adhesion kinetics between soft objects, such as living cells or vesicles, has been developed. When rigid bodies are considered, the adhesion kinetics is successfully described by the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) picture, where the energy profile of two approaching bodies is given by a two asymmetrical potential wells separated by a barrier. The transition probability from the long-distance to the short-distance minimum defines the adhesion rate. Conversely, soft bodies might follow a different pathway to reach the short-distance minimum: thermally excited fluctuations give rise to local protrusions connecting the approaching bodies. These transient adhesion sites are stabilized by short-range adhesion forces (e.g., ligand-receptor interactions between membranes brought at contact distance), while they are destabilized both by repulsive forces and by the elastic deformation energy. Above a critical area of the contact site, the adhesion forces prevail: the contact site grows in size until the complete adhesion of the two bodies inside a short-distance minimum is attained. This nucleation mechanism has been developed in the framework of a nonequilibrium Fokker-Planck picture by considering both the adhesive patch growth and dissolution processes. In addition, we also investigated the effect of the ligand-receptor pairing kinetics at the adhesion site in the time course of the patch expansion. The ratio between the ligand-receptor pairing kinetics and the expansion rate of the adhesion site is of paramount relevance in determining the overall nucleation rate. The theory enables one to self-consistently include both thermodynamics (energy barrier height) and dynamic (viscosity) parameters, giving rise in some limiting cases to simple analytical formulas. The model could be employed to rationalize fusion kinetics between vesicles, provided the short-range adhesion transition is the rate-limiting step to the whole adhesion process. Approximate relationships between the experimental fusion rates reported in the literature and parameters such as membrane elastic bending modulus, repulsion strength, temperature, osmotic forces, ligand-receptor binding energy, solvent and membrane viscosities are satisfactory explained by our model. The present results hint a possible role of the initial long-distance-->short-distance transition in determining the whole fusion kinetics.

Dark Energy and Dark Matter from Emergent Gravity Picture

NASA Astrophysics Data System (ADS)

Seok Yang, Hyun

2018-01-01

We suggest that dark energy and dark matter may be a cosmic uroboros of quantum gravity due to the coherent vacuum structure of spacetime. We apply the emergent gravity to a large N matrix model by considering the vacuum in the noncommutative (NC) Coulomb branch satisfying the Heisenberg algebra. We observe that UV fluctuations in the NC Coulomb branch are always paired with IR fluctuations and these UV/IR fluctuations can be extended to macroscopic scales. We show that space-like fluctuations give rise to the repulsive gravitational force while time-like fluctuations generate the attractive gravitational force. When considering the fact that the fluctuations are random in nature and we are living in the (3+1)-dimensional spacetime, the ratio of the repulsive and attractive components will end in ¾ : ¼= 75 : 25 and this ratio curiously coincides with the dark composition of our current Universe. If one includes ordinary matters which act as the attractive gravitational force, the emergent gravity may explain the dark sector of our Universe more precisely.

Shear thinning in non-Brownian suspensions.

PubMed

Chatté, Guillaume; Comtet, Jean; Niguès, Antoine; Bocquet, Lydéric; Siria, Alessandro; Ducouret, Guylaine; Lequeux, François; Lenoir, Nicolas; Ovarlez, Guillaume; Colin, Annie

2018-02-14

We study the flow of suspensions of non-Brownian particles dispersed into a Newtonian solvent. Combining capillary rheometry and conventional rheometry, we evidence a succession of two shear thinning regimes separated by a shear thickening one. Through X-ray radiography measurements, we show that during each of those regimes, the flow remains homogeneous and does not involve particle migration. Using a quartz-tuning fork based atomic force microscope, we measure the repulsive force profile and the microscopic friction coefficient μ between two particles immersed into the solvent, as a function of normal load. Coupling measurements from those three techniques, we propose that (1) the first shear-thinning regime at low shear rates occurs for a lubricated rheology and can be interpreted as a decrease of the effective volume fraction under increasing particle pressures, due to short-ranged repulsive forces and (2) the second shear thinning regime after the shear-thickening transition occurs for a frictional rheology and can be interpreted as stemming from a decrease of the microscopic friction coefficient at large normal load.

Molecular Structure and Equilibrium Forces of Bovine Submaxillary Mucin Adsorbed at a Solid-Liquid Interface.

PubMed

Zappone, Bruno; Patil, Navinkumar J; Madsen, Jan B; Pakkanen, Kirsi I; Lee, Seunghwan

2015-04-21

By combining dynamic light scattering, circular dichroism spectroscopy, atomic force microscopy, and surface force apparatus, the conformation of bovine submaxillary mucin in dilute solution and nanomechanical properties of mucin layers adsorbed on mica have been investigated. The samples were prepared by additional chromatographic purification of commercially available products. The mucin molecule was found to have a z-average hydrodynamic diameter of ca. 35 nm in phosphate buffered solution, without any particular secondary or tertiary structure. The contour length of the mucin is larger than, yet of the same order of magnitude as the diameter, indicating that the molecule can be modeled as a relatively rigid polymeric chain due to the large persistence length of the central glycosylated domain. Mucin molecules adsorbed abundantly onto mica from saline buffer, generating polymer-like, long-ranged, repulsive, and nonhysteretic forces upon compression of the adsorbed layers. Detailed analysis of such forces suggests that adsorbed mucins had an elongated conformation favored by the stiffness of the central domain. Acidification of aqueous media was chosen as means to reduce mucin-mucin and mucin-substrate electrostatic interactions. The hydrodynamic diameter in solution did not significantly change when the pH was lowered, showing that the large persistence length of the mucin molecule is due to steric hindrance between sugar chains, rather than electrostatic interactions. Remarkably, the force generated by an adsorbed layer with a fixed surface coverage also remained unaltered upon acidification. This observation can be linked to the surface-protective, pH-resistant role of bovine submaxillary mucin in the variable environmental conditions of the oral cavity.

Electric levitation using ϵ-near-zero metamaterials.

PubMed

Rodríguez-Fortuño, Francisco J; Vakil, Ashkan; Engheta, Nader

2014-01-24

The ability to manufacture metamaterials with exotic electromagnetic properties has potential for surprising new applications. Here we report how a specific type of metamaterial--one whose permittivity is near zero--exerts a repulsive force on an electric dipole source, resulting in levitation of the dipole. The phenomenon relies on the expulsion of the time-varying electric field from the metamaterial interior, resembling the perfect diamagnetic expulsion of magnetostatic fields. Leveraging this concept, we study some realistic requirements for the levitation or repulsion of a polarized particle radiating at any frequency, from microwave to optics.

Competition between excluded-volume and electrostatic interactions for nanogel swelling: effects of the counterion valence and nanogel charge.

PubMed

Adroher-Benítez, Irene; Martín-Molina, Alberto; Ahualli, Silvia; Quesada-Pérez, Manuel; Odriozola, Gerardo; Moncho-Jordá, Arturo

2017-03-01

In this work the equilibrium distribution of ions around a thermo-responsive charged nanogel particle in an electrolyte aqueous suspension is explored using coarse-grained Monte Carlo computer simulations and the Ornstein-Zernike integral equation theory. We explicitly consider the ionic size in both methods and study the interplay between electrostatic and excluded-volume effects for swollen and shrunken nanogels, monovalent and trivalent counterions, and for two different nanogel charges. We find good quantitative agreement between the ionic density profiles obtained using both methods when the excluded repulsive force exerted by the cross-linked polymer network is taken into account. For the shrunken conformation, the electrostatic repulsion between the charged groups provokes a heterogeneous polymer density profile, leading to a nanogel structure with an internal low density hole surrounded by a dense corona. The results show that the excluded-volume repulsion strongly hinders the ion permeation for shrunken nanogels, where volume exclusion is able to significantly reduce the concentration of counterions in the more dense regions of the nanogel. In general, we demonstrate that the thermosensitive behaviour of nanogels, as well as their internal structure, is strongly influenced by the valence of the counterions and also by the charge of the particles. On the one hand, an increase of the counterion valence moves the swelling transition to lower temperatures, and induces a major structuring of the charged monomers into internal and external layers around the crown for shrunken nanogels. On the other hand, increasing the particle charge shifts the swelling curve to larger values of the effective radius of the nanogel.

On colloid retention in saturated porous media in the presence of energy barriers: The failure of α, and opportunities to predict η

NASA Astrophysics Data System (ADS)

Johnson, William P.; Tong, Meiping; Li, Xiqing

2007-12-01

This contribution reviews recent findings that illuminate the processes governing colloid retention in porous media under environmentally relevant conditions. In the environment, colloids act as conveyors of contaminants, or even as contaminants themselves; however, despite decades of research, we are unable to accurately predict the retention of colloids in granular aquifer media under environmental conditions, where repulsion exists between colloids and surfaces. This failure cannot be blamed solely on the complexities of the subsurface, since colloid filtration theory (CFT) works well in the absence of colloid-collector repulsion despite its idealization of porous media as consisting of spherical grains completely surrounded by fluid envelopes. Rather, the failure of CFT stems from failure to incorporate the correct mechanisms of retention when repulsion exists. Recent observations implicate wedging in grain-to-grain contacts and retention in secondary energy minima as dominant mechanisms of colloid retention in the presence of an energy barrier. Mechanistic simulations in unit cells containing grain-to-grain contacts corroborate these mechanisms of colloid retention. The resulting concept for colloid retention in the presence of an energy barrier involves translation of colloids across the collector surfaces until they become wedged within grain-to-grain contacts, or are retained via secondary energy minima (without attachment) in zones where the balance of fluid drag, diffusion, gravitational, and colloid-collector interaction forces allow retention. The above findings highlight the pore domain geometry as a dominant governor of colloid retention in so far as the geometry gives rise to grain-to-grain contacts and zones of relatively low fluid drag.

Classical force field for hydrofluorocarbon molecular simulations. Application to the study of gas solubility in poly(vinylidene fluoride).

PubMed

Lachet, V; Teuler, J-M; Rousseau, B

2015-01-08

A classical all-atoms force field for molecular simulations of hydrofluorocarbons (HFCs) has been developed. Lennard-Jones force centers plus point charges are used to represent dispersion-repulsion and electrostatic interactions. Parametrization of this force field has been performed iteratively using three target properties of pentafluorobutane: the quantum energy of an isolated molecule, the dielectric constant in the liquid phase, and the compressed liquid density. The accuracy and transferability of this new force field has been demonstrated through the simulation of different thermophysical properties of several fluorinated compounds, showing significant improvements compared to existing models. This new force field has been applied to study solubilities of several gases in poly(vinylidene fluoride) (PVDF) above the melting temperature of this polymer. The solubility of CH4, CO2, H2S, H2, N2, O2, and H2O at infinite dilution has been computed using test particle insertions in the course of a NpT hybrid Monte Carlo simulation. For CH4, CO2, and their mixtures, some calculations beyond the Henry regime have also been performed using hybrid Monte Carlo simulations in the osmotic ensemble, allowing both swelling and solubility determination. An ideal mixing behavior is observed, with identical solubility coefficients in the mixtures and in pure gas systems.

Switching between attractive and repulsive Coulomb-interaction-mediated drag in an ambipolar GaAs/AlGaAs bilayer device

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zheng, B.; Croxall, A. F.; Waldie, J., E-mail: jw353@cam.ac.uk

2016-02-08

We present measurements of Coulomb drag in an ambipolar GaAs/AlGaAs double quantum well structure that can be configured as both an electron-hole bilayer and a hole-hole bilayer, with an insulating barrier of only 10 nm between the two quantum wells. Coulomb drag resistivity is a direct measure of the strength of interlayer particle-particle interactions. We explore the strongly interacting regime of low carrier densities (2D interaction parameter r{sub s} up to 14). Our ambipolar device design allows a comparison between the effects of the attractive electron-hole and repulsive hole-hole interactions and also shows the effects of the different effective masses ofmore » electrons and holes in GaAs.« less

Structures and interactions among globular proteins above the isoelectric point in the presence of divalent ions: A small angle neutron scattering and dynamic light scattering study

NASA Astrophysics Data System (ADS)

Kundu, Sarathi; Pandit, Subhankar; Abbas, Sohrab; Aswal, V. K.; Kohlbrecher, J.

2018-02-01

Small angle neutron scattering study reveals that at pD ≈ 7.0, above the isoelectric point of the globular protein Bovine Serum Albumin (BSA), in the presence of different divalent ions (Mg2+, Ca2+, Sr2+ and Ba2+), the short-range attractive interaction remains nearly constant and the intermediate-range repulsive interaction decreases with increasing salt concentration up to a certain concentration value but after that remains unchanged. However, for the monovalent ion (Na+), repulsive interaction decreases gradually up to 1 M salt concentration. Dynamic light scattering study shows that for all ions, diffusion coefficient of BSA decreases with increasing salt concentration and then nearly saturates.

Fidelity study of superconductivity in extended Hubbard models

DOE PAGES

Plonka, N.; Jia, C. J.; Wang, Y.; ...

2015-07-08

The Hubbard model with local on-site repulsion is generally thought to possess a superconducting ground state for appropriate parameters, but the effects of more realistic long-range Coulomb interactions have not been studied extensively. We study the influence of these interactions on superconductivity by including nearest- and next-nearest-neighbor extended Hubbard interactions in addition to the usual on-site terms. Utilizing numerical exact diagonalization, we analyze the signatures of superconductivity in the ground states through the fidelity metric of quantum information theory. Finally, we find that nearest and next-nearest neighbor interactions have thresholds above which they destabilize superconductivity regardless of whether they aremore » attractive or repulsive, seemingly due to competing charge fluctuations.« less

Instability of the sliding Luttinger liquid

NASA Astrophysics Data System (ADS)

Fleurov, V.; Kagalovsky, V.; Lerner, I. V.; Yurkevich, I. V.

2018-05-01

We revise a phase diagram for the sliding Luttinger liquid (SLL) of coupled one-dimensional quantum wires packed in two- or three-dimensional arrays in the absence of a magnetic field. We analyse whether physically justifiable (reasonable) inter-wire interactions, i.e. either the screened Coulomb or ‘Coulomb-blockade’ type interactions, stabilise the SLL phase. Calculating the scaling dimensions of the most relevant perturbations (the inter-wire single-particle hybridisation, charge-density wave, and superconducting inter-wire couplings), we find that their combination always destroys the SLL phase for the repulsive intra-wire interaction. However, suppressing the inter-wire tunnelling of repulsive fermions (when the charge-density wave is the only remaining perturbation), one can observe a stability region emerging due to the inter-wire forward scattering interaction.

Slit stimulation recruits Dock and Pak to the roundabout receptor and increases Rac activity to regulate axon repulsion at the CNS midline.

PubMed

Fan, Xueping; Labrador, Juan Pablo; Hing, Huey; Bashaw, Greg J

2003-09-25

Drosophila Roundabout (Robo) is the founding member of a conserved family of repulsive axon guidance receptors that respond to secreted Slit proteins. Here we present evidence that the SH3-SH2 adaptor protein Dreadlocks (Dock), the p21-activated serine-threonine kinase (Pak), and the Rac1/Rac2/Mtl small GTPases can function during Robo repulsion. Loss-of-function and genetic interaction experiments suggest that limiting the function of Dock, Pak, or Rac partially disrupts Robo repulsion. In addition, Dock can directly bind to Robo's cytoplasmic domain, and the association of Dock and Robo is enhanced by stimulation with Slit. Furthermore, Slit stimulation can recruit a complex of Dock and Pak to the Robo receptor and trigger an increase in Rac1 activity. These results provide a direct physical link between the Robo receptor and an important cytoskeletal regulatory protein complex and suggest that Rac can function in both attractive and repulsive axon guidance.

Analysis on Patterns of Globally Coupled Phase Oscillators with Attractive and Repulsive Interactions

NASA Astrophysics Data System (ADS)

Wang, Peng-Fei; Ruan, Xiao-Dong; Xu, Zhong-Bin; Fu, Xin

2015-11-01

The Hong-Strogatz (HS) model of globally coupled phase oscillators with attractive and repulsive interactions reflects the fact that each individual (oscillator) has its own attitude (attractive or repulsive) to the same environment (mean field). Previous studies on HS model focused mainly on the stable states on Ott-Antonsen (OA) manifold. In this paper, the eigenvalues of the Jacobi matrix of each fixed point in HS model are explicitly derived, with the aim to understand the local dynamics around each fixed point. Phase transitions are described according to relative population and coupling strength. Besides, the dynamics off OA manifold is studied. Supported by the National Basic Research Program of China under Grant No. 2015CB057301, the Applied Research Project of Public Welfare Technology of Zhejiang Province under Grant No. 201SC31109 and China Postdoctoral Science Foundation under Grant No. 2014M560483

Gravastars with higher dimensional spacetimes

NASA Astrophysics Data System (ADS)

Ghosh, Shounak; Ray, Saibal; Rahaman, Farook; Guha, B. K.

2018-07-01

We present a new model of gravastar in the higher dimensional Einsteinian spacetime including Einstein's cosmological constant Λ. Following Mazur and Mottola (2001, 2004) we design the star with three specific regions, as follows: (I) Interior region, (II) Intermediate thin spherical shell and (III) Exterior region. The pressure within the interior region is equal to the negative matter density which provides a repulsive force over the shell. This thin shell is formed by ultra relativistic plasma, where the pressure is directly proportional to the matter-energy density which does counter balance the repulsive force from the interior whereas the exterior region is completely vacuum assumed to be de Sitter spacetime which can be described by the generalized Schwarzschild solution. With this specification we find out a set of exact non-singular and stable solutions of the gravastar which seems physically very interesting and reasonable.

Casimir force in the Gödel space-time and its possible induced cosmological inhomogeneity

NASA Astrophysics Data System (ADS)

Khodabakhshi, Sh.; Shojai, A.

2017-07-01

The Casimir force between two parallel plates in the Gödel universe is computed for a scalar field at finite temperature. It is observed that when the plates' separation is comparable with the scale given by the rotation of the space-time, the force becomes repulsive and then approaches zero. Since it has been shown previously that the universe may experience a Gödel phase for a small period of time, the induced inhomogeneities from the Casimir force are also studied.

Compressible flow at high pressure with linear equation of state

NASA Astrophysics Data System (ADS)

Sirignano, William A.

2018-05-01

Compressible flow varies from ideal-gas behavior at high pressures where molecular interactions become important. Density is described through a cubic equation of state while enthalpy and sound speed are functions of both temperature and pressure, based on two parameters, A and B, related to intermolecular attraction and repulsion, respectively. Assuming small variations from ideal-gas behavior, a closed-form solution is obtained that is valid over a wide range of conditions. An expansion in these molecular-interaction parameters simplifies relations for flow variables, elucidating the role of molecular repulsion and attraction in variations from ideal-gas behavior. Real-gas modifications in density, enthalpy, and sound speed for a given pressure and temperature lead to variations in many basic compressible flow configurations. Sometimes, the variations can be substantial in quantitative or qualitative terms. The new approach is applied to choked-nozzle flow, isentropic flow, nonlinear-wave propagation, and flow across a shock wave, all for the real gas. Modifications are obtained for allowable mass-flow through a choked nozzle, nozzle thrust, sonic wave speed, Riemann invariants, Prandtl's shock relation, and the Rankine-Hugoniot relations. Forced acoustic oscillations can show substantial augmentation of pressure amplitudes when real-gas effects are taken into account. Shocks at higher temperatures and pressures can have larger pressure jumps with real-gas effects. Weak shocks decay to zero strength at sonic speed. The proposed framework can rely on any cubic equation of state and be applied to multicomponent flows or to more-complex flow configurations.

Variational Calculation of the Ground State of Closed-Shell Nuclei Up to $A$ = 40

DOE PAGES

Lonardoni, Diego; Lovato, Alessandro; Pieper, Steven C.; ...

2017-08-31

Variational calculations of ground-state properties of 4He, 16O and 40Ca are carried out employing realistic phenomenological two- and three-nucleon potentials. The trial wave function includes twoand three-body correlations acting on a product of single-particle determinants. Expectation values are evaluated with a cluster expansion for the spin-isospin dependent correlations considering up to five-body cluster terms. The optimal wave function is obtained by minimizing the energy expectation value over a set of up to 20 parameters by means of a nonlinear optimization library. We present results for the binding energy, charge radius, point density, single-nucleon momentum distribution, charge form factor, and Coulombmore » sum rule. We find that the employed three-nucleon interaction becomes repulsive for A ≥ 16. In 16O the inclusion of such a force provides a better description of the properties of the nucleus. In 40Ca instead, the repulsive behavior of the three-body interaction fails to reproduce experimental data for the charge radius and the charge form factor. We find that the high-momentum region of the momentum distributions, determined by the short-range terms of nuclear correlations, exhibit a universal behavior independent of the particular nucleus. The comparison of the Coulomb sum rules for 4He, 16O, and 40Ca reported in this work will help elucidate in-medium modifications of the nucleon form factors.« less

A repulsive magnetic force driven translation micromirror

NASA Astrophysics Data System (ADS)

Xue, Yuan; Zuo, Hui; He, Siyuan

2017-10-01

This paper presents a repulsive magnetic force driven micromirror with large displacement and high surface quality which well solves the limitation of the previous design, i.e. large variation in translation starting position and low repeatability, caused by the touching points between the moving film and substrate before and in operation. The new design utilizes a driving mechanism, i.e. permanent magnet ring above and electromagnet underneath the moving film, to lift the moving film from touching the substrate and generate a repulsive magnetic force (instead of attractive force in the previous design) to push the moving film up and away from the substrate for translation. Due to the touching, the previous design has to pre-oscillate for 20-30 min at 1 Hz before usage (after resting for a few hours) to reduce the starting position variation from ~15 µm to 3-4 µm. Even after the pre-oscillation, the repeatability is still low, which is 14.2% because of the touching in operation. In the design presented in this paper, the touching between the moving film and the substrate is completely eliminated before and in operation. As a result, the starting position of the translating mirror is constant each time and the repeatability is  <1%. In addition, this design does not need the residual stress gradient to curve up the moving film. The maximum displacement of 144 µm can be achieved when 140 mA current is applied on the electromagnet. As an application, the micromirror is used as the movable mirror in a Michelson interferometer to measure the wavelength of a laser beam. The result shows a measurement accuracy of 2.19% for a 532 nm laser beam.

Beth-Uhlenbeck approach for repulsive interactions between baryons in a hadron gas

NASA Astrophysics Data System (ADS)

Vovchenko, Volodymyr; Motornenko, Anton; Gorenstein, Mark I.; Stoecker, Horst

2018-03-01

The quantum mechanical Beth-Uhlenbeck (BU) approach for repulsive hard-core interactions between baryons is applied to the thermodynamics of a hadron gas. The second virial coefficient a2—the "excluded volume" parameter—calculated within the BU approach is found to be temperature dependent, and it differs dramatically from the classical excluded volume (EV) model result. At temperatures T =100 -200 MeV, the widely used classical EV model underestimates the EV parameter for nucleons at a given value of the nucleon hard-core radius by large factors of 3-4. Previous studies, which employed the hard-core radii of hadrons as an input into the classical EV model, have to be re-evaluated using the appropriately rescaled EV parameters. The BU approach is used to model the repulsive baryonic interactions in the hadron resonance gas (HRG) model. Lattice data for the second- and fourth-order net baryon susceptibilities are described fairly well when the temperature dependent BU baryonic excluded volume parameter corresponds to nucleon hard-core radii of rc=0.25 -0.3 fm. Role of the attractive baryonic interactions is also considered. It is argued that HRG model with a constant baryon-baryon EV parameter vN N≃1 fm3 provides a simple yet efficient description of baryon-baryon interaction in the crossover temperature region.

Bond lifetime and diffusion coefficient in colloids with short-range interactions.

PubMed

Ndong Mintsa, E; Germain, Ph; Amokrane, S

2015-03-01

We use molecular dynamics simulations to study the influence of short-range structures in the interaction potential between hard-sphere-like colloidal particles. Starting from model potentials and effective potentials in binary mixtures computed from the Ornstein-Zernike equations, we investigate the influence of the range and strength of a possible tail beyond the usual core repulsion or the presence of repulsive barriers. The diffusion coefficient and mean "bond" lifetimes are used as indicators of the effect of this structure on the dynamics. The existence of correlations between the variations of these quantities with the physical parameters is discussed to assess the interpretation of dynamics slowing down in terms of long-lived bonds. We also discuss the question of a universal behaviour determined by the second virial coefficient B ((2)) and the interplay of attraction and repulsion. While the diffusion coefficient follows the B ((2)) law for purely attractive tails, this is no longer true in the presence of repulsive barriers. Furthermore, the bond lifetime shows a dependence on the physical parameters that differs from that of the diffusion coefficient. This raises the question of the precise role of bonds on the dynamics slowing down in colloidal gels.

Induced Hyperon-Nucleon-Nucleon Interactions and the Hyperon Puzzle.

PubMed

Wirth, Roland; Roth, Robert

2016-10-28

We present the first ab initio calculations for p-shell hypernuclei including hyperon-nucleon-nucleon (YNN) contributions induced by a similarity renormalization group transformation of the initial hyperon-nucleon interaction. The transformation including the YNN terms conserves the spectrum of the Hamiltonian while drastically improving model-space convergence of the importance-truncated no-core model, allowing a precise extraction of binding and excitation energies. Results using a hyperon-nucleon interaction at leading order in chiral effective field theory for lower- to mid-p-shell hypernuclei show a good reproduction of experimental excitation energies while hyperon separation energies are typically overestimated. The induced YNN contributions are strongly repulsive and we show that they are related to a decoupling of the Σ hyperons from the hypernuclear system, i.e., a suppression of the Λ-Σ conversion terms in the Hamiltonian. This is linked to the so-called hyperon puzzle in neutron-star physics and provides a basic mechanism for the explanation of strong ΛNN three-baryon forces.

A quantitative analysis of weak intermolecular interactions & quantum chemical calculations (DFT) of novel chalcone derivatives

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chavda, Bhavin R., E-mail: chavdabhavin9@gmail.com; Dubey, Rahul P.; Patel, Urmila H.

The novel chalcone derivatives have widespread applications in material science and medicinal industries. The density functional theory (DFT) is used to optimized the molecular structure of the three chalcone derivatives (M-I, II, III). The observed discrepancies between the theoretical and experimental (X-ray data) results attributed to different environments of the molecules, the experimental values are of the molecule in solid state there by subjected to the intermolecular forces, like non-bonded hydrogen bond interactions, where as isolated state in gas phase for theoretical studies. The lattice energy of all the molecules have been calculated using PIXELC module in Coulomb –London –Paulimore » (CLP) package and is partitioned into corresponding coulombic, polarization, dispersion and repulsion contributions. Lattice energy data confirm and strengthen the finding of the X-ray results that the weak but significant intermolecular interactions like C-H…O, Π- Π and C-H… Π plays an important role in the stabilization of crystal packing.« less

Structure and stability of fluorine-substituted benzene-argon complexes: The decisive role of exchange-repulsion and dispersion interactions

NASA Astrophysics Data System (ADS)

Tarakeshwar, P.; Kim, Kwang S.; Kraka, Elfi; Cremer, Dieter

2001-10-01

The van der Waals complexes benzene-argon (BAr), fluorobenzene-argon (FAr), p-difluorobenzene-argon (DAr) are investigated at the second-order Møller-Plesset (MP2) level of theory using the 6-31+G(d), cc-pVDZ, aug-cc-pVTZ, and [7s4p2d1f/4s3p1d/3s1p] basis sets. Geometries, binding energies, harmonic vibrational frequencies, and density distribution are calculated where basis set superposition errors are corrected with the counterpoise method. Binding energies turn out to be almost identical (MP2/[7s4p2d1f/4s3p1d/3s1p]: 408, 409, 408 cm-1) for BAr, FAr, and DAr. Vibrationally corrected binding energies (357, 351, 364 cm-1) agree well with experimental values (340, 344, and 339 cm-1). Symmetry adapted perturbation theory (SAPT) is used to decompose binding energies and to examine the influence of attractive and repulsive components. Fluorine substituents lead to a contraction of the π density of the benzene ring, thus reducing the destabilizing exchange-repulsion and exchange-induction effects. At the same time, both the polarizing power and the polarizability of the π-density of the benzene derivative decreases thus reducing stabilizing induction and dispersion interactions. Stabilizing and destabilizing interactions largely cancel each other out to give comparable binding energies. The equilibrium geometry of the Ar complex is also a result of the decisive influence of exchange-repulsion and dispersive interactions.

Particle-wall tribology of slippery hydrogel particle suspensions.

PubMed

Shewan, Heather M; Stokes, Jason R; Cloitre, Michel

2017-03-08

Slip is an important phenomenon that occurs during the flow of yield stress fluids like soft materials and pastes. Densely packed suspensions of hydrogel microparticles are used to show that slip is governed by the tribological interactions occurring between the samples and shearing surfaces. Both attractive/repulsive interactions between the dispersed particles and surface, as well as the viscoelasticity of the suspension, are found to play key roles in slip occurring within rheometric flows. We specifically discover that for two completely different sets of microgels, the sliding stress at which slip occurs scales with both the modulus of the particles and the bulk suspension modulus. This suggests that hysteresis losses within the viscoelastic particles contribute to friction forces and thus slip at the particle-surface tribo-contact. It is also found that slip during large amplitude oscillatory shear and steady shear flows share the same generic features.

Elastic and inelastic collisions of swarms

NASA Astrophysics Data System (ADS)

Armbruster, Dieter; Martin, Stephan; Thatcher, Andrea

2017-04-01

Scattering interactions of swarms in potentials that are generated by an attraction-repulsion model are studied. In free space, swarms in this model form a well-defined steady state describing the translation of a stable formation of the particles whose shape depends on the interaction potential. Thus, the collision between a swarm and a boundary or between two swarms can be treated as (quasi)-particle scattering. Such scattering experiments result in internal excitations of the swarm or in bound states, respectively. In addition, varying a parameter linked to the relative importance of damping and potential forces drives transitions between elastic and inelastic scattering of the particles. By tracking the swarm's center of mass, a refraction rule is derived via simulations relating the incoming and outgoing directions of a swarm hitting the wall. Iterating the map derived from the refraction law allows us to predict and understand the dynamics and bifurcations of swarms in square boxes and in channels.

Lifetime of oil drops pressed by buoyancy against a planar interface: Large drops

NASA Astrophysics Data System (ADS)

Rojas, Clara; García-Sucre, Máximo; Urbina-Villalba, Germán

2010-11-01

In a previous report [C. Rojas, G. Urbina-Villalba, and M. García-Sucre, Phys. Rev. E 81, 016302 (2010)10.1103/PhysRevE.81.016302] it was shown that emulsion stability simulations are able to reproduce the lifetime of micrometer-size drops of hexadecane pressed by buoyancy against a planar water-hexadecane interface. It was confirmed that small drops (ri<10μm) stabilized with β -casein behave as nondeformable particles, moving with a combination of Stokes and Taylor tensors as they approach the interface. Here, a similar methodology is used to parametrize the potential of interaction of drops of soybean oil stabilized with bovine serum albumin. The potential obtained is then employed to study the lifetime of deformable drops in the range 10≤ri≤1000μm . It is established that the average lifetime of these drops can be adequately replicated using the model of truncated spheres. However, the results depend sensibly on the expressions of the initial distance of deformation and the maximum film radius used in the calculations. The set of equations adequate for large drops is not satisfactory for medium-size drops (10≤ri≤100μm) , and vice versa. In the case of large particles, the increase in the interfacial area as a consequence of the deformation of the drops generates a very large repulsive barrier which opposes coalescence. Nevertheless, the buoyancy force prevails. As a consequence, it is the hydrodynamic tensor of the drops which determine the characteristic behavior of the lifetime as a function of the particle size. While the average values of the coalescence time of the drops can be justified by the mechanism of film thinning, the scattering of the experimental data of large drops cannot be rationalized using the methodology previously described. A possible explanation of this phenomenon required elaborate simulations which combine deformable drops, capillary waves, repulsive interaction forces, and a time-dependent surfactant adsorption.

Discontinuous Shear Thickening and Dilatancy: Frictional Effects in Viscous Suspensions

NASA Astrophysics Data System (ADS)

Morris, Jeffrey

2015-03-01

Shear thickening in concentrated suspensions has been well-known for quite a long time, yet a firm consensus on the basis for very abrupt or ``discontinuous'' shear thickening (DST) seen in suspensions of large solid fraction, ϕ, has not been reached. This work addresses the DST phenomenon, and proposes a simulation method based in the Stokesian Dynamics algorithm to explore the role of various forces between the particles, including hydrodynamic, conservative potential, and frictional interactions. This work shows that allowance for friction between spherical particles suspended in a viscous liquid causes a significant reduction in the jamming solid fraction of the mixture, ϕmax, taken as the maximum fraction at which the suspension will flow. A consequence of this is a shifting of the singularity in the effective viscosity, η, to smaller ϕmax, and the frictional suspension has a larger viscosity than does the frictionless suspension of the same solid fraction, as is clear from the standard empirical modeling of η (ϕ) =(1 - ϕ /ϕmax) - α , α ~ 2 . When a counterbalancing repulsive force between the particles, representative for example of charge-induced repulsion, is incorporated in the dynamics, the mixture undergoes a transition from frictionless to frictional interactions, and from low to high effective viscosity, at a critical shear rate. Comparison with experimental data shows remarkable agreement in the features of DST captured by the method. The basic algorithm and results of both rate-controlled and stress-controlled simulations will be presented. Like the shear stress, the magnitude of the normal stress exerted by the suspended particles also increases abruptly at the critical shear rate, consistent with the long-standing notion that dilatancy and shear-thickening are synonymous. We will show that considering all shear thickening materials as dilatant is a misconception, but demonstrate the validity of the connection of dilatancy with DST in concentrated suspensions.

Perpendicular diffusion of a dilute beam of charged particles in the PK-4 dusty plasma

NASA Astrophysics Data System (ADS)

Liu, Bin; Goree, John

2015-09-01

We study the random walk of a dilute beam of projectile dust particles that drift through a target dusty plasma. This random walk is a diffusion that occurs mainly due to Coulomb collisions with target particles that have a different size. In the direction parallel to the drift, projectiles exhibit mobility-limited motion with a constant average velocity. We use a 3D molecular dynamics (MD) simulation of the dust particle motion to determine the diffusion and mobility coefficients for the dilute beam. The dust particles are assumed to interact with a shielded Coulomb repulsion. They also experience gas drag. The beam particles are driven by a prescribed net force that is not applied to the target particles; in the experiments this net force is due to an imbalance of the electric and ion drag forces. This simulation is motivated by microgravity experiments, with the expectation that the scattering of projectiles studied here will be observed in upcoming PK-4 experiments on the International Space Station. Supported by NASA and DOE.

SPH for impact force and ricochet behavior of water-entry bodies

NASA Astrophysics Data System (ADS)

Omidvar, Pourya; Farghadani, Omid; Nikeghbali, Pooyan

The numerical modeling of fluid interaction with a bouncing body has many applications in scientific and engineering application. In this paper, the problem of water impact of a body on free-surface is investigated, where the fixed ghost boundary condition is added to the open source code SPHysics2D1 to rectify the oscillations in pressure distributions with the repulsive boundary condition. First, after introducing the methodology of SPH and the option of boundary conditions, the still water problem is simulated using two types of boundary conditions. It is shown that the fixed ghost boundary condition gives a better result for a hydrostatics pressure. Then, the dam-break problem, which is a bench mark test case in SPH, is simulated and compared with available data. In order to show the behavior of the hydrostatics forces on bodies, a fix/floating cylinder is placed on free surface looking carefully at the force and heaving profile. Finally, the impact of a body on free-surface is successfully simulated for different impact angles and velocities.

Ensemble average theory of gravity

NASA Astrophysics Data System (ADS)

Khosravi, Nima

2016-12-01

We put forward the idea that all the theoretically consistent models of gravity have contributions to the observed gravity interaction. In this formulation, each model comes with its own Euclidean path-integral weight where general relativity (GR) has automatically the maximum weight in high-curvature regions. We employ this idea in the framework of Lovelock models and show that in four dimensions the result is a specific form of the f (R ,G ) model. This specific f (R ,G ) satisfies the stability conditions and possesses self-accelerating solutions. Our model is consistent with the local tests of gravity since its behavior is the same as in GR for the high-curvature regime. In the low-curvature regime the gravitational force is weaker than in GR, which can be interpreted as the existence of a repulsive fifth force for very large scales. Interestingly, there is an intermediate-curvature regime where the gravitational force is stronger in our model compared to GR. The different behavior of our model in comparison with GR in both low- and intermediate-curvature regimes makes it observationally distinguishable from Λ CDM .

Modeling and simulation of dielectrophoretic collective dynamics in a suspension of polarizable particles under the action of a gradient AC electric field.

PubMed

Tada, Shigeru; Shen, Yan; Qiu, Zhiyong

2017-06-01

When a suspension of polarizable particles is subjected to a gradient AC electric field, the particles exhibit collective motion due to an interaction between the dipole induced in the particles and the spatial gradient of the electric field; this is known as dielectrophoresis. In the present study, the collective dynamics of suspended particles in a parallel-plate electric chamber was investigated by simulating numerically the trajectories of individual particles under the action of combined dielectrophoretic and dipole-dipole interparticle forces. The particles were transported by the dielectrophoretic forces toward the grounded electrodes. Before long, when the particles approached the site of the minimum field strength, attractive/repulsive interparticle forces became dominant and acted among the particles attempting to form a column-like cluster, having the particles distribution in concentric circles in its cross-section, in line with the centerline of the grounded electrodes. Our results also well reproduced the transient particle aggregation that was observed experimentally. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Pattern formation, social forces, and diffusion instability in games with success-driven motion

NASA Astrophysics Data System (ADS)

Helbing, Dirk

2009-02-01

A local agglomeration of cooperators can support the survival or spreading of cooperation, even when cooperation is predicted to die out according to the replicator equation, which is often used in evolutionary game theory to study the spreading and disappearance of strategies. In this paper, it is shown that success-driven motion can trigger such local agglomeration and may, therefore, be used to supplement other mechanisms supporting cooperation, like reputation or punishment. Success-driven motion is formulated here as a function of the game-theoretical payoffs. It can change the outcome and dynamics of spatial games dramatically, in particular as it causes attractive or repulsive interaction forces. These forces act when the spatial distributions of strategies are inhomogeneous. However, even when starting with homogeneous initial conditions, small perturbations can trigger large inhomogeneities by a pattern-formation instability, when certain conditions are fulfilled. Here, these instability conditions are studied for the prisoner’s dilemma and the snowdrift game. Furthermore, it is demonstrated that asymmetrical diffusion can drive social, economic, and biological systems into the unstable regime, if these would be stable without diffusion.

Quaterrylene molecules on Ag(111): self-assembly behavior and voltage pulse induced trimer formation.

PubMed

He, Yangyong; Cai, Zeying; Shao, Jian; Xu, Li; She, Limin; Zheng, Yue; Zhong, Dingyong

2018-05-03

The self-assembly behavior of quaterrylene (QR) molecules on Ag(111) surfaces has been investigated by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. It is found that the QR molecules are highly mobile on the Ag(111) surface at 78 K. No ordered assembled structure is formed on the surface with a sub-monolayer coverage up to 0.8 monolayer due to the intermolecular repulsive interactions, whereas ordered molecular structures are observed at one monolayer coverage. According to our DFT calculations, charge transfer occurs between the substrate and the adsorbed QR molecule. As a result, out-of-plane dipoles appear at the interface, which are ascribed to the repulsive dipole-dipole interactions between the QR molecules. Furthermore, due to the planar geometry, the QR molecules exhibit relatively low diffusion barriers on Ag(111). By applying a voltage pulse between the tunneling gap, immobilization and aggregation of QR molecules take place, resulting in the formation of a triangle-shaped trimer. Our work demonstrates the ability of manipulating intermolecular repulsive and attractive interactions at the single molecular level.

NH3 adsorption on anatase-TiO2(101)

NASA Astrophysics Data System (ADS)

Koust, Stig; Adamsen, Kræn C.; Kolsbjerg, Esben Leonhard; Li, Zheshen; Hammer, Bjørk; Wendt, Stefan; Lauritsen, Jeppe V.

2018-03-01

The adsorption of ammonia on anatase TiO2 is of fundamental importance for several catalytic applications of TiO2 and for probing acid-base interactions. Utilizing high-resolution scanning tunneling microscopy (STM), synchrotron X-ray photoelectron spectroscopy, temperature-programmed desorption (TPD), and density functional theory (DFT), we identify the adsorption mode and quantify the adsorption strength on the anatase TiO2(101) surface. It was found that ammonia adsorbs non-dissociatively as NH3 on regular five-fold coordinated titanium surface sites (5f-Ti) with an estimated exothermic adsorption energy of 1.2 eV for an isolated ammonia molecule. For higher adsorbate coverages, the adsorption energy progressively shifts to smaller values, due to repulsive intermolecular interactions. The repulsive adsorbate-adsorbate interactions are quantified using DFT and autocorrelation analysis of STM images, which both showed a repulsive energy of ˜50 meV for nearest neighbor sites and a lowering in binding energy for an ammonia molecule in a full monolayer of 0.28 eV, which is in agreement with TPD spectra.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Filippov, A. V., E-mail: fav@triniti.ru

The interaction of two charged point macroparticles located in Wigner–Seitz cells of simple cubic (SC), body-centered cubic (BCC), or face-centered cubic (FCC) lattices in an equilibrium plasma has been studied within the Debye approximation or, more specifically, based on the linearized Poisson–Boltzmann model. The shape of the outer boundary is shown to exert a strong influence on the pattern of electrostatic interaction between the two macroparticles, which transforms from repulsion at small interparticle distances to attraction as the interparticle distance approaches half the length of the computational cell. The macroparticle pair interaction potential in an equilibrium plasma is shown tomore » be nevertheless the Debye one and purely repulsive for likely charged macroparticles.« less

Fluxoids configurations in finite superconducting networks

NASA Astrophysics Data System (ADS)

Sharon, Omri J.; Haham, Noam; Shaulov, Avner A.; Yeshurun, Yosef

2017-12-01

Analysis of superconducting ladders consisting of rectangular loops, yields an Ising like expression for the total energy of the ladders as a function of the loops vorticities and the applied magnetic field. This expression shows that fluxoids can be treated as repulsively interacting objects driven towards the ladder center by the applied field. Distinctive repulsive interactions between fluxoids are obtained depending on the ratio l between the loops length and the common width of adjacent loops. A 'short range' and a 'long range' interactions obtained for l ≳ 1 and l ≪ 1, respectively, give rise to remarkably different fluxoid configurations. The different configurations of fluxoids in different types of ladders are illustrated by simulations.

Analyzing the vibrational response of an AFM cantilever in liquid with the consideration of tip mass by comparing the hydrodynamic and contact repulsive force models in higher modes

NASA Astrophysics Data System (ADS)

Korayem, Moharam Habibnejad; Nahavandi, Amir

2017-04-01

This paper investigates the vibration of a tapping-mode Atomic Force Microscope (AFM) cantilever covered with two whole piezoelectric layers in a liquid medium. The authors of this article have already modeled the vibration of a cantilever immersed in liquid over rough surfaces. Five new ideas have been considered for improving the results of the previous work. Mass and damping of a cantilever probe tip have been considered. Since the probe tip of an AFM cantilever has a mass, which can itself affect the natural frequency of vibration, the significance of this mass has been explored. Also, two hydrodynamic force models for analyzing the mass and damping added to a cantilever in liquid medium have been evaluated. In modeling the vibration of a cantilever in liquid, simplifications are made to the theoretical equations used in the modeling, which may make the obtained results different from those in the real case. So, two hydrodynamic force models are introduced and compared with each other. In addition to the already introduced DMT model, the JKR model has been proposed. The forces acting on a probe tip have attractive and repulsive effects. The attractive Van der Waals force can vary depending on the surface smoothness or roughness, and the repulsive contact force, which is independent of the type of surface roughness and usually varies with the hardness or softness of a surface. When the first mode is used in the vibration of an AFM cantilever, the changes of the existing physical parameters in the simulation do not usually produce a significant difference in the response. Thus, three cantilever vibration modes have been investigated. Finally, an analytical approach for obtaining the response of equations is presented which solves the resulting motion equation by the Laplace method and, thus, a time function is obtained for cantilever deflection is determined. Also, using the COMSOL software to model a cantilever in a liquid medium, the computed natural frequencies have been compared.

Phenomenological Model of Hydrophobic and Hydrophilic Interactions

NASA Astrophysics Data System (ADS)

Menshikov, L. I.; Menshikov, P. L.; Fedichev, P. O.

2017-12-01

Hydration forces acting between macroscopic bodies at distances L ≤ 3 nm in pure water are calculated based on the phenomenological model of polar liquids. It is shown that depending on the properties of the bodies, the interacting surfaces polarize the liquid differently, and wetting properties of the surfaces are completely characterized by two parameters. If the surfaces are hydrophilic, liquid molecules are polarized at right angles to the surfaces, and the interaction is the short-range repulsion (the forces of interaction decrease exponentially over the characteristic length λ ≈ 0.2 nm). The interaction between the hydrophobic surfaces is more diversified and has been studied less. For L ≤ 3 nm, the interaction exhibits universal properties, while for L ≤ 3 nm, it considerably depends on the properties of the surfaces and on the distances between them, as well as on the composition of the polar liquid. In full agreement with the available experimental results we find that if the interfaces are mostly hydrophobic, then the interaction is attractive and long-range (the interaction forces diminish exponentially with decay length 1.2 nm). In this case, the resultant polarization of water molecules is parallel to the surface. It is shown that hydration forces are determined by nonlinear effects of polarization of the liquid in the bulk or by analogous nonlinearity of the interaction of water with a submerged body. This means that the forces of interaction cannot be calculated correctly in the linear response approximation. The forces acting between hydrophobic or hydrophilic surfaces are of the entropy type or electrostatic, respectively. It is shown that hydrophobic and hydrophilic surfaces for L ≤ 3 nm repel each other. The calculated intensity of their interaction is in agreement with experimental data. We predict the existence of an intermediate regime in which a body cannot order liquid molecules, which results in a much weaker attraction that decreases in inverse proportion to the squared distance between the surfaces of bodies. The difference between the microscopic structures of liquids confined in nanovolumes from liquids in large volumes is considered. The proposed model is applicable for a quantitative description of the properties of water at temperatures T satisfying the condition 0 < ( T-T c )/ T c ≪ 1, where T c ≈ 230 K is the temperature of the ferroelectric phase transition observed in supercooled water. Under standard conditions, the model can be used for obtaining qualitative estimates.

Optimizing pH response of affinity between protein G and IgG Fc: how electrostatic modulations affect protein-protein interactions.

PubMed

Watanabe, Hideki; Matsumaru, Hiroyuki; Ooishi, Ayako; Feng, Yanwen; Odahara, Takayuki; Suto, Kyoko; Honda, Shinya

2009-05-01

Protein-protein interaction in response to environmental conditions enables sophisticated biological and biotechnological processes. Aiming toward the rational design of a pH-sensitive protein-protein interaction, we engineered pH-sensitive mutants of streptococcal protein G B1, a binder to the IgG constant region. We systematically introduced histidine residues into the binding interface to cause electrostatic repulsion on the basis of a rigid body model. Exquisite pH sensitivity of this interaction was confirmed by surface plasmon resonance and affinity chromatography employing a clinically used human IgG. The pH-sensitive mechanism of the interaction was analyzed and evaluated from kinetic, thermodynamic, and structural viewpoints. Histidine-mediated electrostatic repulsion resulted in significant loss of exothermic heat of the binding that decreased the affinity only at acidic conditions, thereby improving the pH sensitivity. The reduced binding energy was partly recovered by "enthalpy-entropy compensation." Crystal structures of the designed mutants confirmed the validity of the rigid body model on which the effective electrostatic repulsion was based. Moreover, our data suggested that the entropy gain involved exclusion of water molecules solvated in a space formed by the introduced histidine and adjacent tryptophan residue. Our findings concerning the mechanism of histidine-introduced interactions will provide a guideline for the rational design of pH-sensitive protein-protein recognition.

Buckyplates and buckybowls: examining the effects of curvature on π-π interactions.

PubMed

Kennedy, Matthew R; Burns, Lori A; Sherrill, C David

2012-12-06

π-π interactions are integral to many areas of chemistry, biochemistry, and materials science. Here we use electronic structure theory to analyze how π-π interactions change as the π-systems are curved in model complexes based on coronene and corannulene dimers. Curvature redistributes electronic charge in the π-cloud and creates a dipole moment in these systems, leading to enhanced intermolecular electrostatic interactions in the concave-convex (nested) geometries that are the focus of this work. Curvature of both monomers also has a geometric effect on the interaction by decreasing the average C-C distance between monomers and by increasing the magnitude of both favorable London dispersion interactions and unfavorable exchange-repulsion interactions. Overall, increasing curvature in nested π-π interactions leads to more favorable interaction energies regardless of the native state of the monomers, except at short distances where the most highly curved systems are less favorable as exchange repulsion terms begin to dominate the interaction.

Cosmological evolution of a complex scalar field with repulsive or attractive self-interaction

NASA Astrophysics Data System (ADS)

Suárez, Abril; Chavanis, Pierre-Henri

2017-03-01

We study the cosmological evolution of a complex scalar field with a self-interaction potential V (|φ |2) , possibly describing self-gravitating Bose-Einstein condensates, using a fully general relativistic treatment. We generalize the hydrodynamic representation of the Klein-Gordon-Einstein equations in the weak field approximation developed in our previous paper [A. Suárez and P.-H. Chavanis, Phys. Rev. D 92, 023510 (2015), 10.1103/PhysRevD.92.023510]. We establish the general equations governing the evolution of a spatially homogeneous complex scalar field in an expanding background. We show how they can be simplified in the fast oscillation regime (equivalent to the Thomas-Fermi, or semiclassical, approximation) and derive the equation of state of the scalar field in parametric form for an arbitrary potential V (|φ |2) . We explicitly consider the case of a quartic potential with repulsive or attractive self-interaction. For repulsive self-interaction, the scalar field undergoes a stiff matter era followed by a pressureless dark matter era in the weakly self-interacting regime and a stiff matter era followed by a radiationlike era and a pressureless dark matter era in the strongly self-interacting regime. For attractive self-interaction, the scalar field undergoes an inflation era followed by a stiff matter era and a pressureless dark matter era in the weakly self-interacting regime and an inflation era followed by a cosmic stringlike era and a pressureless dark matter era in the strongly self-interacting regime (the inflation era is suggested, not demonstrated). We also find a peculiar branch on which the scalar field emerges suddenly at a nonzero scale factor with a finite energy density. At early times, it behaves as a gas of cosmic strings. At later times, it behaves as dark energy with an almost constant energy density giving rise to a de Sitter evolution. This is due to spintessence. We derive the effective cosmological constant produced by the scalar field. Throughout the paper, we analytically characterize the transition scales of the scalar field and establish the domain of validity of the fast oscillation regime. We analytically confirm and complement the important results of Li, Rindler-Daller, and Shapiro [Phys. Rev. D 89, 083536 (2014), 10.1103/PhysRevD.89.083536]. We determine the phase diagram of a scalar field with repulsive or attractive self-interaction. We show that the transition between the weakly self-interacting regime and the strongly self-interacting regime depends on how the scattering length of the bosons compares with their effective Schwarzschild radius. We also constrain the parameters of the scalar field from astrophysical and cosmological observations. Numerical applications are made for ultralight bosons without self-interaction (fuzzy dark matter), for bosons with repulsive self-interaction, and for bosons with attractive self-interaction (QCD axions and ultralight axions).

Enhancement of spin polarization induced by Coulomb on-site repulsion between localized pz electrons in graphene embedded with line defects.

PubMed

Ren, Ji-Chang; Wang, Zhigang; Zhang, Rui-Qin; Ding, Zejun; Van Hove, Michel A

2015-11-11

It is well known that the effect of Coulomb on-site repulsion can significantly alter the physical properties of the systems that contain localized d and/or f electrons. However, little attention has been paid to the Coulomb on-site repulsion between localized p electrons. In this study, we demonstrated that Coulomb on-site repulsion between localized pz electrons also plays an important role in graphene embedded with line defects. It is shown that the magnetism of the system largely depends on the choice of the effective Coulomb on-site parameter Ueff. Ueff at the edges of the defect enhances the exchange splitting, which increases the magnetic moment and stabilizes a ferromagnetic state of the system. In contrast, Ueff at the center of the defect weakens the spin polarization of the system. The behavior of the magnetism is explained with the Stoner criterion and the charge accumulation at the edges of the defect. Based on the linear response approach, we estimate reasonable values of Ueff to be 2.55 eV (2.3 eV) at the center (edges) of the defects. More importantly, using a DFT+U+J method, we find that exchange interactions between localized p electrons also play an important role in the spin polarization of the system. These results imply that Coulomb on-site repulsion is necessary to describe the strong interaction between localized pz electrons of carbon related materials.

Vacuum Fluctuation Force on a Rigid Casimir Cavity in de Sitter and Schwarzschild-De Sitter Space-Time

NASA Astrophysics Data System (ADS)

Chen, Xiang

2012-11-01

We investigate the net force on a rigid Casimir cavity generated by vacuum fluctuations of electromagnetic field in three cases: de Sitter space-time, de Sitter space-time with weak gravitational field and Schwarzschild-de Sitter space-time. In de Sitter space-time the resulting net force follows the square inverse law but unfortunately it is too weak to be measurable due to the large universe radius. By introducing a weak gravitational field into the de Sitter space-time, we find that the net force can now be split into two parts, one is the gravitational force due to the induced effective mass between the two plates and the other one is generated by the metric structure of de Sitter space-time. In order to investigate the vacuum fluctuation force on the rigid cavity under strong gravitational field, we perform a similar analysis in Schwarzschild-de Sitter space-time and results are obtained in three different limits. The most interesting one is when the cavity gets closer to the horizon of a blackhole, square inverse law is recovered and the repulsive force due to negative energy/mass of the cavity now has an observable strength. More importantly the force changes from being repulsive to attractive when the cavity crosses the event horizon, so that the energy/mass of the cavity switches the sign, which suggests the unusual time direction inside the event horizon.

Lattice QCD studies of s-wave meson-baryon interactions

NASA Astrophysics Data System (ADS)

Ikeda, Yoichi

2011-10-01

We study the s-wave KN interactions in the isospin I = 0, 1 channels and associated exotic state Θ+ from 2+1 flavor full lattice QCD simulation for relatively heavy quark mass corresponding to mπ = 871 MeV. The s-wave KN potentials are obtained from the Bethe-Salpeter amplitudes. Potentials in both channels reveal short range repulsions: Strength of the repulsion is stronger in the I = 1 potential. The I = 0 potential is found to have attractive well at mid range. The KN scattering phase shifts are calculated and compared with the experimental data.

Blow-up behavior of ground states for a nonlinear Schrödinger system with attractive and repulsive interactions

NASA Astrophysics Data System (ADS)

Guo, Yujin; Zeng, Xiaoyu; Zhou, Huan-Song

2018-01-01

We consider a nonlinear Schrödinger system arising in a two-component Bose-Einstein condensate (BEC) with attractive intraspecies interactions and repulsive interspecies interactions in R2. We get ground states of this system by solving a constrained minimization problem. For some kinds of trapping potentials, we prove that the minimization problem has a minimizer if and only if the attractive interaction strength ai (i = 1 , 2) of each component of the BEC system is strictly less than a threshold a*. Furthermore, as (a1 ,a2) ↗ (a* ,a*), the asymptotical behavior for the minimizers of the minimization problem is discussed. Our results show that each component of the BEC system concentrates at a global minimum of the associated trapping potential.

Dynamical Mass Generation.

NASA Astrophysics Data System (ADS)

Mendel Horwitz, Roberto Ruben

1982-03-01

In the framework of the Glashow-Weinberg-Salem model without elementary scalar particles, we show that masses for fermions and intermediate vector bosons can be generated dynamically. The mechanism is the formation of fermion-antifermion pseudoscalar bound states of zero total four momentum, which form a condensate in the physical vacuum. The force responsible for the binding is the short distance part of the net Coulomb force due to photon and Z exchange. Fermions and bosons acquire masses through their interaction with this condensate. The neutrinos remain massless because their righthanded components have no interactions. Also the charge -1/3 quarks remain massless because the repulsive force from the Z exchange dominates over the Coulomb force. To correct this, we propose two possible modifications to the theory. One is to cut off the Z exchange at very small distances, so that all fermions except the neutrinos acquire masses, which are then, purely electromagnetic in origin. The other is to introduce an additional gauge boson that couples to all quarks with a pure vector coupling. To make this vector boson unobservable at usual energies, at least two new fermions must couple to it. The vector boson squared masses receive additive contributions from all the fermion squared masses. The photon remains massless and the masses of the Z and W('(+OR -)) bosons are shown to be related through the Weinberg angle in the conventional way. Assuming only three families of fermions, we obtain estimates for the top quark mass.

Effective particle size from molecular dynamics simulations in fluids

NASA Astrophysics Data System (ADS)

Ju, Jianwei; Welch, Paul M.; Rasmussen, Kim Ø.; Redondo, Antonio; Vorobieff, Peter; Kober, Edward M.

2018-04-01

We report molecular dynamics simulations designed to investigate the effective size of colloidal particles suspended in a fluid in the vicinity of a rigid wall where all interactions are defined by smooth atomic potential functions. These simulations are used to assess how the behavior of this system at the atomistic length scale compares to continuum mechanics models. In order to determine the effective size of the particles, we calculate the solvent forces on spherical particles of different radii as a function of different positions near and overlapping with the atomistically defined wall and compare them to continuum models. This procedure also then determines the effective position of the wall. Our analysis is based solely on forces that the particles sense, ensuring self-consistency of the method. The simulations were carried out using both Weeks-Chandler-Andersen and modified Lennard-Jones (LJ) potentials to identify the different contributions of simple repulsion and van der Waals attractive forces. Upon correction for behavior arising the discreteness of the atomic system, the underlying continuum physics analysis appeared to be correct down to much less than the particle radius. For both particle types, the effective radius was found to be ˜ 0.75σ , where σ defines the length scale of the force interaction (the LJ diameter). The effective "hydrodynamic" radii determined by this means are distinct from commonly assumed values of 0.5σ and 1.0σ , but agree with a value developed from the atomistic analysis of the viscosity of such systems.

Order from Force; A natural history of the vacuum

NASA Astrophysics Data System (ADS)

Williams, Jeffrey H.

2015-11-01

The laws of physics govern our lives, and the fundamental constants of Nature (for example, the mass and charge of the electron) define our very morphology. If a human body were totally dehydrated there would only remain about thirty kilograms of crystals and powder, after having removed about fifty litres of water. Yet the amazing machine that is our body functions because of the forces of interaction (attraction and repulsion) that exist between the molecules in that powder when fully hydrated. These forces of interaction are mediated and directed by the vast amount of water that is present. It is the precise orientation of one hydrated molecule with respect to another hydrated molecule, at a well-defined separation, in our central nervous systems that allows a nerve impulse to tunnel quantum-mechanically through the intermediate space between two nerve cells at a synapse. Thereby allowing us to observe our environment, and contemplate our existence. It is the arrangement of the water molecules along molecules of muscle proteins that allows one protein molecule to slide over neighbouring protein molecules, thereby allowing us to do exercise and work, or to hunt and to gather. The precise distances and orientations between the molecules of which our bodies are composed are determined by subtle intermolecular electrostatic forces, whose magnitude is determined by the various constants of Nature, and whose operation is dictated by the laws of physics. We are merely living representations of these immutable physical laws.

Effective particle size from molecular dynamics simulations in fluids

NASA Astrophysics Data System (ADS)

Ju, Jianwei; Welch, Paul M.; Rasmussen, Kim Ø.; Redondo, Antonio; Vorobieff, Peter; Kober, Edward M.

2017-12-01

We report molecular dynamics simulations designed to investigate the effective size of colloidal particles suspended in a fluid in the vicinity of a rigid wall where all interactions are defined by smooth atomic potential functions. These simulations are used to assess how the behavior of this system at the atomistic length scale compares to continuum mechanics models. In order to determine the effective size of the particles, we calculate the solvent forces on spherical particles of different radii as a function of different positions near and overlapping with the atomistically defined wall and compare them to continuum models. This procedure also then determines the effective position of the wall. Our analysis is based solely on forces that the particles sense, ensuring self-consistency of the method. The simulations were carried out using both Weeks-Chandler-Andersen and modified Lennard-Jones (LJ) potentials to identify the different contributions of simple repulsion and van der Waals attractive forces. Upon correction for behavior arising the discreteness of the atomic system, the underlying continuum physics analysis appeared to be correct down to much less than the particle radius. For both particle types, the effective radius was found to be ˜ 0.75σ , where σ defines the length scale of the force interaction (the LJ diameter). The effective "hydrodynamic" radii determined by this means are distinct from commonly assumed values of 0.5σ and 1.0σ , but agree with a value developed from the atomistic analysis of the viscosity of such systems.

Interactions of aqueous NOM with nanoscale TiO2: implications for ceramic membrane filtration-ozonation hybrid process.

PubMed

Kim, Jeonghwan; Shan, Wenqian; Davies, Simon H R; Baumann, Melissa J; Masten, Susan J; Tarabara, Volodymyr V

2009-07-15

The combined effect of pH and calcium on the interactions of nonozonated and ozonated natural organic matter (NOM) with nanoscale TiO2 was investigated. The approach included characterization of TiO2 nanoparticles and NOM, extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) modeling of NOM-TiO2 and NOM-NOM interactions, batch study on the NOM adsorption onto TiO2 surface, and bench-scale study on the treatment of NOM-containing feed waters using a hybrid process that combines ozonation and ultrafiltration with a 5 kDa ceramic (TiO2 surface) membrane. It was demonstrated that depending on pH and TiO2 loading, the adsorption of NOM species is controlled by either the availability of divalent cations or by preozonation of NOM. XDLVO surface energy analysis predicts NOM adsorption onto TiO2 in the ozone-controlled regime but not in the calcium-controlled regime. In both regimes, short-range NOM-NOM and NOM-TiO2 interactions were governed by acid-base and van der Waals forces, whereas the role of electrostatic forces was relatively insignificant. Ozonation increased the surface energy of NOM, contributing to the hydrophilic repulsion component of the NOM-NOM and NOM-TiO2 interactions. In the calcium-controlled regime, neither NOM-TiO2 nor NOM-NOM interaction controlled adsorption. Non-XDLVO interactions such as intermolecular bridging by calcium were hypothesized to be responsible for the observed adsorption behavior. Adsorption data proved to be highly predictive of the permeate flux performance.

Intuitive experimentation in the physical world.

PubMed

Bramley, Neil R; Gerstenberg, Tobias; Tenenbaum, Joshua B; Gureckis, Todd M

2018-06-06

Many aspects of our physical environment are hidden. For example, it is hard to estimate how heavy an object is from visual observation alone. In this paper we examine how people actively "experiment" within the physical world to discover such latent properties. In the first part of the paper, we develop a novel framework for the quantitative analysis of the information produced by physical interactions. We then describe two experiments that present participants with moving objects in "microworlds" that operate according to continuous spatiotemporal dynamics similar to everyday physics (i.e., forces of gravity, friction, etc.). Participants were asked to interact with objects in the microworlds in order to identify their masses, or the forces of attraction/repulsion that governed their movement. Using our modeling framework, we find that learners who freely interacted with the physical system selectively produced evidence that revealed the physical property consistent with their inquiry goal. As a result, their inferences were more accurate than for passive observers and, in some contexts, for yoked participants who watched video replays of an active learner's interactions. We characterize active learners' actions into a range of micro-experiment strategies and discuss how these might be learned or generalized from past experience. The technical contribution of this work is the development of a novel analytic framework and methodology for the study of interactively learning about the physical world. Its empirical contribution is the demonstration of sophisticated goal directed human active learning in a naturalistic context. Copyright © 2018 Elsevier Inc. All rights reserved.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Seiler, M.; Seiß, M.; Hoffmann, H.

The reconstruction of the orbital evolution of the propeller structure Blériot orbiting in Saturn’s A ring from recurrent observations in Cassini ISS images yielded a considerable offset motion from the expected Keplerian orbit. This offset motion can be composed by three sinusoidal harmonics with amplitudes and periods of 1845, 152, 58 km and 11.1, 3.7, and 2.2 years, respectively. In this paper we present results from N -body simulations, where we integrated the orbital evolution of a moonlet, which is placed at the radial position of Blériot under the gravitational action of the Saturnian satellites. Our simulations yield that, especiallymore » the gravitational interactions with Prometheus, Pandora, and Mimas are forcing the moonlet to librate with the right frequencies, but the libration amplitudes are too small to explain the observations. Thus, further mechanisms are needed to explain the amplitudes of the forced librations—e.g., moonlet–ring interactions. Here, we develop a model, where the moonlet is allowed to be slightly displaced with respect to its created gaps breaking the point symmetry and causing a repulsive force in this way. As a result, the evolution of the moonlet’s longitude can be described by a harmonic oscillator. In the presence of external forcing by the outer moons, the libration amplitudes get the more amplified the closer the forcing frequency is to the eigenfrequency of the disturbed propeller oscillator. Applying our model to Blériot, it is possible to reproduce a libration period of 13 years with an amplitude of about 2000 km.« less

A Librational Model for the Propeller Blériot in the Saturnian Ring System

NASA Astrophysics Data System (ADS)

Seiler, M.; Sremčević, M.; Seiß, M.; Hoffmann, H.; Spahn, F.

2017-05-01

The reconstruction of the orbital evolution of the propeller structure Blériot orbiting in Saturn’s A ring from recurrent observations in Cassini ISS images yielded a considerable offset motion from the expected Keplerian orbit. This offset motion can be composed by three sinusoidal harmonics with amplitudes and periods of 1845, 152, 58 km and 11.1, 3.7, and 2.2 years, respectively. In this paper we present results from N-body simulations, where we integrated the orbital evolution of a moonlet, which is placed at the radial position of Blériot under the gravitational action of the Saturnian satellites. Our simulations yield that, especially the gravitational interactions with Prometheus, Pandora, and Mimas are forcing the moonlet to librate with the right frequencies, but the libration amplitudes are too small to explain the observations. Thus, further mechanisms are needed to explain the amplitudes of the forced librations—e.g., moonlet-ring interactions. Here, we develop a model, where the moonlet is allowed to be slightly displaced with respect to its created gaps breaking the point symmetry and causing a repulsive force in this way. As a result, the evolution of the moonlet’s longitude can be described by a harmonic oscillator. In the presence of external forcing by the outer moons, the libration amplitudes get the more amplified the closer the forcing frequency is to the eigenfrequency of the disturbed propeller oscillator. Applying our model to Blériot, it is possible to reproduce a libration period of 13 years with an amplitude of about 2000 km.

Dynamics of neural cryptography

NASA Astrophysics Data System (ADS)

Ruttor, Andreas; Kinzel, Wolfgang; Kanter, Ido

2007-05-01

Synchronization of neural networks has been used for public channel protocols in cryptography. In the case of tree parity machines the dynamics of both bidirectional synchronization and unidirectional learning is driven by attractive and repulsive stochastic forces. Thus it can be described well by a random walk model for the overlap between participating neural networks. For that purpose transition probabilities and scaling laws for the step sizes are derived analytically. Both these calculations as well as numerical simulations show that bidirectional interaction leads to full synchronization on average. In contrast, successful learning is only possible by means of fluctuations. Consequently, synchronization is much faster than learning, which is essential for the security of the neural key-exchange protocol. However, this qualitative difference between bidirectional and unidirectional interaction vanishes if tree parity machines with more than three hidden units are used, so that those neural networks are not suitable for neural cryptography. In addition, the effective number of keys which can be generated by the neural key-exchange protocol is calculated using the entropy of the weight distribution. As this quantity increases exponentially with the system size, brute-force attacks on neural cryptography can easily be made unfeasible.

Dynamics of neural cryptography.

PubMed

Ruttor, Andreas; Kinzel, Wolfgang; Kanter, Ido

2007-05-01

Synchronization of neural networks has been used for public channel protocols in cryptography. In the case of tree parity machines the dynamics of both bidirectional synchronization and unidirectional learning is driven by attractive and repulsive stochastic forces. Thus it can be described well by a random walk model for the overlap between participating neural networks. For that purpose transition probabilities and scaling laws for the step sizes are derived analytically. Both these calculations as well as numerical simulations show that bidirectional interaction leads to full synchronization on average. In contrast, successful learning is only possible by means of fluctuations. Consequently, synchronization is much faster than learning, which is essential for the security of the neural key-exchange protocol. However, this qualitative difference between bidirectional and unidirectional interaction vanishes if tree parity machines with more than three hidden units are used, so that those neural networks are not suitable for neural cryptography. In addition, the effective number of keys which can be generated by the neural key-exchange protocol is calculated using the entropy of the weight distribution. As this quantity increases exponentially with the system size, brute-force attacks on neural cryptography can easily be made unfeasible.

Tribology of steel/steel interaction in oil-in-water emulsion; a rationale for lubricity.

PubMed

Kumar, Deepak; Daniel, Jency; Biswas, S K

2010-05-15

Oil droplets are dispersed in water by an anionic surfactant to form an emulsion. The lubricity of this emulsion in steel/steel interaction is explored in a ball on flat nanotribometer. The droplet size and charge are measured using dynamic light scattering, while the substrate charge density is estimated using the pH titration method. These data are combined to calculate the DLVO forces for the droplets generated for a range of surfactant concentration and two oil to water volume ratios. The droplets have a clear bi-modal size distribution. The study shows that the smaller droplets which experience weak repulsion are situated (at the highest DLVO barrier) much closer to the substrate than the bigger droplets, which experience the same DLVO force, are. We suggest that the smaller droplets thus play a more important role in lubricity than what the bigger droplets do. The largest volume of such small droplets occurs in the 0.5 mM-1 mM range of surfactant concentration and 1% oil to water volume ratio, where the coefficient of friction is also observed to be the least. Copyright © 2010 Elsevier Inc. All rights reserved.

Comparison of multilayer formation between different cellulose nanofibrils and cationic polymers.

PubMed

Eronen, Paula; Laine, Janne; Ruokolainen, Janne; Osterberg, Monika

2012-05-01

The multilayer formation between polyelectrolytes of opposite charge offers possibility for creating new tailored materials. Exchanging one or both components for charged nanofibrillated cellulose (NFC) further increases the variety of achievable properties. We explored this by introducing unmodified, low charged NFC and high charged TEMPO-oxidized NFC. Systematic evaluation of the effect of both NFC charge and properties of cationic polyelectrolytes on the structure of the multilayers was performed. As the cationic component cationic NFC was compared with two different cationic polyelectrolytes, poly(dimethyldiallylammoniumchloride) and cationic starch. Quartz crystal microbalance with dissipation (QCM-D) was used to monitor the multilayer formation and AFM colloidal probe microscopy (CPM) was further applied to probe surface interactions in order to gain information about fundamental interactions and layer properties. Generally, the results verified the characteristic multilayer formation between NFC of different charge and how the properties of formed multilayers can be tuned. However, the strong nonelectrostatic affinity between cellulosic fibrils was observed. CPM measurements revealed monotonically repulsive forces, which were in good correspondence with the QCM-D observations. Significant increase in adhesive forces was detected between the swollen high charged NFC. Copyright © 2011 Elsevier Inc. All rights reserved.

Dynamics of neural cryptography

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ruttor, Andreas; Kinzel, Wolfgang; Kanter, Ido

2007-05-15

Synchronization of neural networks has been used for public channel protocols in cryptography. In the case of tree parity machines the dynamics of both bidirectional synchronization and unidirectional learning is driven by attractive and repulsive stochastic forces. Thus it can be described well by a random walk model for the overlap between participating neural networks. For that purpose transition probabilities and scaling laws for the step sizes are derived analytically. Both these calculations as well as numerical simulations show that bidirectional interaction leads to full synchronization on average. In contrast, successful learning is only possible by means of fluctuations. Consequently,more » synchronization is much faster than learning, which is essential for the security of the neural key-exchange protocol. However, this qualitative difference between bidirectional and unidirectional interaction vanishes if tree parity machines with more than three hidden units are used, so that those neural networks are not suitable for neural cryptography. In addition, the effective number of keys which can be generated by the neural key-exchange protocol is calculated using the entropy of the weight distribution. As this quantity increases exponentially with the system size, brute-force attacks on neural cryptography can easily be made unfeasible.« less

Analysis and Design of a Double-Divert Spiral Groove Seal

NASA Technical Reports Server (NTRS)

Zheng, Xiaoqing; Berard, Gerald

2007-01-01

This viewgraph presentation describes the design and analysis of a double spiral groove seal. The contents include: 1) Double Spiral Design Features; 2) Double Spiral Operational Features; 3) Mating Ring/Rotor Assembly; 4) Seal Ring Assembly; 5) Insert Segment Joints; 6) Rotor Assembly Completed Prototype Parts; 7) Seal Assembly Completed Prototype Parts; 8) Finite Element Analysis; 9) Computational Fluid Dynamics (CFD) Analysis; 10) Restrictive Orifice Design; 11) Orifice CFD Model; 12) Orifice Results; 13) Restrictive Orifice; 14) Seal Face Coning; 15) Permanent Magnet Analysis; 16) Magnetic Repulsive Force; 17) Magnetic Repulsive Test Results; 18) Spin Testing; and 19) Testing and Validation.

Four-body interaction energy for compressed solid krypton from quantum theory.

PubMed

Tian, Chunling; Wu, Na; Liu, Fusheng; Saxena, Surendra K; Zheng, Xingrong

2012-07-28

The importance of the four-body contribution in compressed solid krypton was first evaluated using the many-body expansion method and the coupled cluster theory with full single and double excitations plus perturbative treatment of triples. All different four-atom clusters existing in the first- and second-nearest neighbor shells of face-centered cubic krypton were considered, and both self-consistent-field Hartree-Fock and correlation parts of the four-body interaction were accurately determined from the ambient conditions up to eightfold volume compression. We find that the four-body interaction energy is negative at compression ratio lower than 2, where the dispersive forces play a dominant role. With increasing the compression, the four-body contribution becomes repulsive and significantly cancels the over-softening effects of the three-body potential. The obtained equation of state (EOS) was compared with the experiments and the density-functional theory calculations. It shows that combination of the four-body effects with two- and three-body interactions leads to an excellent agreement with EOS measurements throughout the whole experimental range 0-130 GPa, and extends the prediction to 300 GPa.

Illuminating the Reaction Pathways of Viromimetic Assembly.

PubMed

Cingil, Hande E; Boz, Emre B; Biondaro, Giovanni; de Vries, Renko; Cohen Stuart, Martien A; Kraft, Daniela J; van der Schoot, Paul; Sprakel, Joris

2017-04-05

The coassembly of well-defined biological nanostructures relies on a delicate balance between attractive and repulsive interactions between biomolecular building blocks. Viral capsids are a prototypical example, where coat proteins exhibit not only self-interactions but also interact with the cargo they encapsulate. In nature, the balance between antagonistic and synergistic interactions has evolved to avoid kinetic trapping and polymorphism. To date, it has remained a major challenge to experimentally disentangle the complex kinetic reaction pathways that underlie successful coassembly of biomolecular building blocks in a noninvasive approach with high temporal resolution. Here we show how macromolecular force sensors, acting as a genome proxy, allow us to probe the pathways through which a viromimetic protein forms capsids. We uncover the complex multistage process of capsid assembly, which involves recruitment and complexation, followed by allosteric growth of the proteinaceous coat. Under certain conditions, the single-genome particles condense into capsids containing multiple copies of the template. Finally, we derive a theoretical model that quantitatively describes the kinetics of recruitment and growth. These results shed new light on the origins of the pathway complexity in biomolecular coassembly.

An in vivo study of electrical charge distribution on the bacterial cell wall by atomic force microscopy in vibrating force mode

NASA Astrophysics Data System (ADS)

Marlière, Christian; Dhahri, Samia

2015-05-01

We report an in vivo electromechanical atomic force microscopy (AFM) study of charge distribution on the cell wall of Gram+ Rhodococcus wratislaviensis bacteria, naturally adherent to a glass substrate, under physiological conditions. The method presented in this paper relies on a detailed study of AFM approach/retract curves giving the variation of the interaction force versus distance between the tip and the sample. In addition to classical height and mechanical (as stiffness) data, mapping of local electrical properties, such as bacterial surface charge, was proved to be feasible at a spatial resolution better than a few tens of nanometers. This innovative method relies on the measurement of the cantilever's surface stress through its deflection far from (>10 nm) the repulsive contact zone: the variations of surface stress come from the modification of electrical surface charge of the cantilever (as in classical electrocapillary measurements) likely stemming from its charging during contact of both the tip and the sample electrical double layers. This method offers an important improvement in local electrical and electrochemical measurements at the solid/liquid interface, particularly in high-molarity electrolytes when compared to techniques focused on the direct use of electrostatic force. It thus opens a new way to directly investigate in situ biological electrical surface processes involved in numerous practical applications and fundamental problems such as bacterial adhesion, biofilm formation, microbial fuel cells, etc.We report an in vivo electromechanical atomic force microscopy (AFM) study of charge distribution on the cell wall of Gram+ Rhodococcus wratislaviensis bacteria, naturally adherent to a glass substrate, under physiological conditions. The method presented in this paper relies on a detailed study of AFM approach/retract curves giving the variation of the interaction force versus distance between the tip and the sample. In addition to classical height and mechanical (as stiffness) data, mapping of local electrical properties, such as bacterial surface charge, was proved to be feasible at a spatial resolution better than a few tens of nanometers. This innovative method relies on the measurement of the cantilever's surface stress through its deflection far from (>10 nm) the repulsive contact zone: the variations of surface stress come from the modification of electrical surface charge of the cantilever (as in classical electrocapillary measurements) likely stemming from its charging during contact of both the tip and the sample electrical double layers. This method offers an important improvement in local electrical and electrochemical measurements at the solid/liquid interface, particularly in high-molarity electrolytes when compared to techniques focused on the direct use of electrostatic force. It thus opens a new way to directly investigate in situ biological electrical surface processes involved in numerous practical applications and fundamental problems such as bacterial adhesion, biofilm formation, microbial fuel cells, etc. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr00968e

Drosophila Neurexin IV Interacts with Roundabout and is Required for Repulsive Midline Axon Guidance

PubMed Central

Banerjee, Swati; Blauth, Kevin; Peters, Kimberly; Rogers, Stephen L.; Fanning, Alan S.; Bhat, Manzoor A.

2010-01-01

Slit/Roundabout (Robo) signaling controls midline repulsive axon guidance. However, proteins that interact with Slit/Robo at the cell surface remain largely uncharacterized. Here, we report that the Drosophila transmembrane septate junction-specific protein, Neurexin IV (Nrx IV), functions in midline repulsive axon guidance. Nrx IV is expressed in the neurons of the developing ventral nerve cord and nrx IV mutants show crossing and circling of ipsilateral axons and fused commissures. Interestingly, the axon guidance defects observed in nrx IV mutants seem independent of its other binding partners such as Contactin and Neuroglian and the midline glia protein Wrapper that interacts in trans with Nrx IV. nrx IV mutants show diffuse Robo localization and dose-dependent genetic interactions between nrx IV/robo and nrx IV/slit indicate that they function in a common pathway. In vivo biochemical studies reveal that Nrx IV associates with Robo, Slit and Syndecan, and interactions between Robo and Slit, or Nrx IV and Slit, are affected in nrx IV and robo mutants, respectively. Coexpression of Nrx IV and Robo in mammalian cells confirms that these proteins retain the ability to interact in a heterologous system. Furthermore, we demonstrate that the extracellular region of Nrx IV is sufficient to rescue Robo localization and axon guidance phenotypes in nrx IV mutants. Together our studies establish that Nrx IV is essential for proper Robo localization, and identify Nrx IV as a novel interacting partner of the Slit/Robo signaling pathway. PMID:20410118

Drosophila neurexin IV interacts with Roundabout and is required for repulsive midline axon guidance.

PubMed

Banerjee, Swati; Blauth, Kevin; Peters, Kimberly; Rogers, Stephen L; Fanning, Alan S; Bhat, Manzoor A

2010-04-21

Slit/Roundabout (Robo) signaling controls midline repulsive axon guidance. However, proteins that interact with Slit/Robo at the cell surface remain largely uncharacterized. Here, we report that the Drosophila transmembrane septate junction-specific protein Neurexin IV (Nrx IV) functions in midline repulsive axon guidance. Nrx IV is expressed in the neurons of the developing ventral nerve cord, and nrx IV mutants show crossing and circling of ipsilateral axons and fused commissures. Interestingly, the axon guidance defects observed in nrx IV mutants seem independent of its other binding partners, such as Contactin and Neuroglian and the midline glia protein Wrapper, which interacts in trans with Nrx IV. nrx IV mutants show diffuse Robo localization, and dose-dependent genetic interactions between nrx IV/robo and nrx IV/slit indicate that they function in a common pathway. In vivo biochemical studies reveal that Nrx IV associates with Robo, Slit, and Syndecan, and interactions between Robo and Slit, or Nrx IV and Slit, are affected in nrx IV and robo mutants, respectively. Coexpression of Nrx IV and Robo in mammalian cells confirms that these proteins retain the ability to interact in a heterologous system. Furthermore, we demonstrate that the extracellular region of Nrx IV is sufficient to rescue Robo localization and axon guidance phenotypes in nrx IV mutants. Together, our studies establish that Nrx IV is essential for proper Robo localization and identify Nrx IV as a novel interacting partner of the Slit/Robo signaling pathway.

The Resistive-Wall Instability in Multipulse Linear Induction Accelerators

DOE PAGES

Ekdahl, Carl

2017-05-01

The resistive-wall instability results from the Lorentz force on the beam due to the beam image charge and current. If the beam pipe is perfectly conducting, the electric force due to the image charge attracts the beam to the pipe wall, and the magnetic force due to the image current repels the beam from the wall. For a relativistic beam, these forces almost cancel, leaving a slight attractive force, which is easily overcome by external magnetic focusing. However, if the beam pipe is not perfectly conducting, the magnetic field due to the image current decays on a magnetic-diffusion time scale.more » If the beam pulse is longer than the magnetic diffusion time, the repulsion of the beam tail will be weaker than the repulsion of the beam head. In the absence of an external focusing force, this causes a head-to-tail sweep of the beam toward the wall. This instability is usually thought to be a concern only for long-pulse relativistic electron beams. However, with the advent of multipulse, high current linear induction accelerators, the possibility of pulse-to-pulse coupling of this instability should be investigated. Lastly, we have explored pulse-to-pulse coupling using the linear accelerator model for Dual Axis Radiography for Hydrodynamic Testing beam dynamics code, and we present the results of this paper.« less

The Resistive-Wall Instability in Multipulse Linear Induction Accelerators

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ekdahl, Carl

The resistive-wall instability results from the Lorentz force on the beam due to the beam image charge and current. If the beam pipe is perfectly conducting, the electric force due to the image charge attracts the beam to the pipe wall, and the magnetic force due to the image current repels the beam from the wall. For a relativistic beam, these forces almost cancel, leaving a slight attractive force, which is easily overcome by external magnetic focusing. However, if the beam pipe is not perfectly conducting, the magnetic field due to the image current decays on a magnetic-diffusion time scale.more » If the beam pulse is longer than the magnetic diffusion time, the repulsion of the beam tail will be weaker than the repulsion of the beam head. In the absence of an external focusing force, this causes a head-to-tail sweep of the beam toward the wall. This instability is usually thought to be a concern only for long-pulse relativistic electron beams. However, with the advent of multipulse, high current linear induction accelerators, the possibility of pulse-to-pulse coupling of this instability should be investigated. Lastly, we have explored pulse-to-pulse coupling using the linear accelerator model for Dual Axis Radiography for Hydrodynamic Testing beam dynamics code, and we present the results of this paper.« less

Meissner motor using high-Tc ceramic superconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Takeoka, A.; Ishikawa, A.; Suzuki, M.

1989-03-01

The authors developed a brand new superconducting motor using high-Tc ceramic superconductors for the first time. This motor utilizes the repulsive force caused by the Meissner effect, which appears below Tc and disappears above that, and is therefore referred to as the Meissner Motor. The motor rotated at a maximum speed of 40 rpm. Though the repulsive force to drive the motor increased with the decrease of temperature or the increase of the gradient magnetic field, it was only about 1.1 gf/g at 77 K in 3500 G/cm. The motor has a maximum torque of 5.0 gf-cm theoretically, but actuallymore » had a torque below 0.66 gf-cm, because it took some time to be cooled below Tc. The rotating speed of the motor was limited by heating ability and its torque was limited by cooling ability.« less

Intrinsic anharmonic effects on the phonon frequencies and effective spin-spin interactions in a quantum simulator made from trapped ions in a linear Paul trap

NASA Astrophysics Data System (ADS)

McAneny, M.; Freericks, J. K.

2014-11-01

The Coulomb repulsion between ions in a linear Paul trap gives rise to anharmonic terms in the potential energy when expanded about the equilibrium positions. We examine the effect of these anharmonic terms on the accuracy of a quantum simulator made from trapped ions. To be concrete, we consider a linear chain of Yb171+ ions stabilized close to the zigzag transition. We find that for typical experimental temperatures, frequencies change by no more than a factor of 0.01 % due to the anharmonic couplings. Furthermore, shifts in the effective spin-spin interactions (driven by a spin-dependent optical dipole force) are also, in general, less than 0.01 % for detunings to the blue of the transverse center-of-mass frequency. However, detuning the spin interactions near other frequencies can lead to non-negligible anharmonic contributions to the effective spin-spin interactions. We also examine an odd behavior exhibited by the harmonic spin-spin interactions for a range of intermediate detunings, where nearest-neighbor spins with a larger spatial separation on the ion chain interact more strongly than nearest neighbors with a smaller spatial separation.

Experimental detection of long-distance interactions between biomolecules through their diffusion behavior: numerical study.

PubMed

Nardecchia, Ilaria; Spinelli, Lionel; Preto, Jordane; Gori, Matteo; Floriani, Elena; Jaeger, Sebastien; Ferrier, Pierre; Pettini, Marco

2014-08-01

The dynamical properties and diffusive behavior of a collection of mutually interacting particles are numerically investigated for two types of long-range interparticle interactions: Coulomb-electrostatic and dipole-electrodynamic. It is shown that when the particles are uniformly distributed throughout the accessible space, the self-diffusion coefficient is always lowered by the considered interparticle interactions, irrespective of their attractive or repulsive character. This fact is also confirmed by a simple model to compute the correction to the Brownian diffusion coefficient due to the interactions among the particles. These interactions are also responsible for the onset of dynamical chaos and an associated chaotic diffusion which still follows an Einstein-Fick-like law for the mean-square displacement as a function of time. Transitional phenomena are observed for Coulomb-electrostatic (repulsive) and dipole-electrodynamic (attractive) interactions considered both separately and in competition. The outcomes reported in this paper clearly indicate a feasible experimental method to probe the activation of resonant electrodynamic interactions among biomolecules.

Quasiparticle-continuum level repulsion in a quantum magnet

DOE Office of Scientific and Technical Information (OSTI.GOV)

Plumb, K. W.; Hwang, Kyusung; Qiu, Y.

2015-11-30

When the energy eigenvalues of two coupled quantum states approach each other in a certain parameter space, their energy levels repel each other and level crossing is avoided. Such level repulsion, or avoided level crossing, is commonly used to describe the dispersion relation of quasiparticles in solids. But, little is known about the level repulsion when more than two quasiparticles are present; for example, in a strongly interacting quantum system where a quasiparticle can spontaneously decay into a many-particle continuum. Here we show that even in this case level repulsion exists between a long-lived quasiparticle state and a continuum. Here,more » we observe a renormalization of the quasiparticle dispersion relation due to the presence of the continuum of multi-quasiparticle states, in our fine-resolution neutron spectroscopy study of magnetic quasiparticles in the frustrated quantum magnet BiCu 2PO 6.« less

Emergence of amplitude death scenario in a network of oscillators under repulsive delay interaction

NASA Astrophysics Data System (ADS)

Bera, Bidesh K.; Hens, Chittaranjan; Ghosh, Dibakar

2016-07-01

We report the existence of amplitude death in a network of identical oscillators under repulsive mean coupling. Amplitude death appears in a globally coupled network of identical oscillators with instantaneous repulsive mean coupling only when the number of oscillators is more than two. We further investigate that, amplitude death may emerge even in two coupled oscillators as well as network of oscillators if we introduce delay time in the repulsive mean coupling. We have analytically derived the region of amplitude death island and find out how strength of delay controls the death regime in two coupled or a large network of coupled oscillators. We have verified our results on network of delayed Mackey-Glass systems where parameters are set in hyperchaotic regime. We have also tested our coupling approach in two paradigmatic limit cycle oscillators: Stuart-Landau and Van der Pol oscillators.

Crossover from attractive to repulsive Casimir forces and vice versa.

PubMed

Schmidt, Felix M; Diehl, H W

2008-09-05

Systems described by an O(n) symmetrical varphi;{4} Hamiltonian are considered in a d-dimensional film geometry at their bulk critical points. The critical Casimir forces between the film's boundary planes B_{j}, j=1,2, are investigated as functions of film thickness L for generic symmetry-preserving boundary conditions partial differential_{n}phi=c[over composite function]_{j}phi. The L-dependent part of the reduced excess free energy per cross-sectional area takes the scaling form f_{res} approximately D(c_{1}L;{Phi/nu},c_{2}L;{Phi/nu})/L;{d-1} when d<4, where c_{i} are scaling fields associated with the variables c[over composite function]_{i} and Phi is a surface crossover exponent. Explicit two-loop renormalization group results for the function D(c_{1},c_{2}) at d=4- dimensions are presented. These show that (i) the Casimir force can have either sign, depending on c_{1} and c_{2}, and (ii) for appropriate choices of the enhancements c[over composite function]_{j}, crossovers from attraction to repulsion and vice versa occur as L increases.

Raman Spectroscopy of Single Light-Absorbing Carbonaceous Particles Levitated in Air Using an Annular Laser Beam.

PubMed

Uraoka, Masaru; Maegawa, Keisuke; Ishizaka, Shoji

2017-12-05

A laser trapping technique is a powerful means to investigate the physical and chemical properties of single aerosol particles in a noncontact manner. However, optical trapping of strongly light-absorbing particles such as black carbon or soot is quite difficult because the repulsive force caused by heat is orders of magnitude larger than the attractive force of radiation pressure. In this study, a laser trapping and Raman microspectroscopy system using an annular laser beam was constructed to achieve noncontact levitation of single light-absorbing particles in air. Single acetylene carbon black or candle soot particles were arbitrarily selected with a glass capillary connected to a three-axis oil hydraulic micromanipulator and introduced into a minute space surrounded by a repulsive force at the focal point of an objective lens. Using the developed system, we achieved optical levitation of micrometer-sized carbonaceous particles and observation of their Raman spectra in air. Furthermore, we demonstrated in situ observations of changes in the morphology and chemical composition of optically trapped carbonaceous particles in air, which were induced by heterogeneous oxidation reactions with ozone and hydroxyl radicals.

A new electromagnetic NDI-technique based on the measurement of source-sample reaction forces

NASA Astrophysics Data System (ADS)

Fitzpatrick, G. L.; Skaugset, R. L.; Shih, W. C. L.

2001-04-01

Faraday's law of induction, Lenz's law, the Lorentz force law and Newton's third law, taken together, insure that sources (e.g., coil sources) of time-dependent electromagnetic fields, and nearby "nonmagnetic" electrical conductors (e.g., aluminum), always experience mutually repulsive (source-conductor) forces. This fact forms the basis for a new method for detecting cracks and corrosion in (aging) multi-layer airframes. The presence of cracks or corrosion (e.g., material thinning) in these structures is observed to reduce (second-harmonic) source-conductor reaction forces.

Shear-induced aggregation or disaggregation in edible oils: Models, computer simulation, and USAXS measurements

NASA Astrophysics Data System (ADS)

Townsend, B.; Peyronel, F.; Callaghan-Patrachar, N.; Quinn, B.; Marangoni, A. G.; Pink, D. A.

2017-12-01

The effects of shear upon the aggregation of solid objects formed from solid triacylglycerols (TAGs) immersed in liquid TAG oils were modeled using Dissipative Particle Dynamics (DPD) and the predictions compared to experimental data using Ultra-Small Angle X-ray Scattering (USAXS). The solid components were represented by spheres interacting via attractive van der Waals forces and short range repulsive forces. A velocity was applied to the liquid particles nearest to the boundary, and Lees-Edwards boundary conditions were used to transmit this motion to non-boundary layers via dissipative interactions. The shear was created through the dissipative forces acting between liquid particles. Translational diffusion was simulated, and the Stokes-Einstein equation was used to relate DPD length and time scales to SI units for comparison with USAXS results. The SI values depended on how large the spherical particles were (250 nm vs. 25 nm). Aggregation was studied by (a) computing the Structure Function and (b) quantifying the number of pairs of solid spheres formed. Solid aggregation was found to be enhanced by low shear rates. As the shear rate was increased, a transition shear region was manifested in which aggregation was inhibited and shear banding was observed. Aggregation was inhibited, and eventually eliminated, by further increases in the shear rate. The magnitude of the transition region shear, γ˙ t, depended on the size of the solid particles, which was confirmed experimentally.

Evidences of Changes in Surface Electrostatic Charge Distribution during Stabilization of HPV16 Virus-Like Particles

PubMed Central

Vega, Juan F.; Vicente-Alique, Ernesto; Núñez-Ramírez, Rafael; Wang, Yang; Martínez-Salazar, Javier

2016-01-01

The stabilization of human papillomavirus type 16 virus-like particles has been examined by means of different techniques including dynamic and static light scattering, transmission electron microscopy and electrophoretic mobility. All these techniques provide different and often complementary perspectives about the aggregation process and generation of stabilized virus-like particles after a period of time of 48 hours at a temperature of 298 K. Interestingly, static light scattering results point towards a clear colloidal instability in the initial systems, as suggested by a negative value of the second virial coefficient. This is likely related to small repulsive electrostatic interactions among the particles, and in agreement with relatively small absolute values of the electrophoretic mobility and, hence, of the net surface charges. At this initial stage the small repulsive interactions are not able to compensate binding interactions, which tend to aggregate the particles. As time proceeds, an increase of the size of the particles is accompanied by strong increases, in absolute values, of the electrophoretic mobility and net surface charge, suggesting enhanced repulsive electrostatic interactions and, consequently, a stabilized colloidal system. These results show that electrophoretic mobility is a useful methodology that can be applied to screen the stabilization factors for virus-like particles during vaccine development. PMID:26885635

Intermolecular interactions and the thermodynamic properties of supercritical fluids.

PubMed

Yigzawe, Tesfaye M; Sadus, Richard J

2013-05-21

The role of different contributions to intermolecular interactions on the thermodynamic properties of supercritical fluids is investigated. Molecular dynamics simulation results are reported for the energy, pressure, thermal pressure coefficient, thermal expansion coefficient, isothermal and adiabatic compressibilities, isobaric and isochoric heat capacities, Joule-Thomson coefficient, and speed of sound of fluids interacting via both the Lennard-Jones and Weeks-Chandler-Andersen potentials. These properties were obtained for a wide range of temperatures, pressures, and densities. For each thermodynamic property, an excess value is determined to distinguish between attraction and repulsion. It is found that the contributions of intermolecular interactions have varying effects depending on the thermodynamic property. The maxima exhibited by the isochoric and isobaric heat capacities, isothermal compressibilities, and thermal expansion coefficient are attributed to interactions in the Lennard-Jones well. Repulsion is required to obtain physically realistic speeds of sound and both repulsion and attraction are necessary to observe a Joule-Thomson inversion curve. Significantly, both maxima and minima are observed for the isobaric and isochoric heat capacities of the supercritical Lennard-Jones fluid. It is postulated that the loci of these maxima and minima converge to a common point via the same power law relationship as the phase coexistence curve with an exponent of β = 0.32. This provides an explanation for the terminal isobaric heat capacity maximum in supercritical fluids.

The contribution of phosphate–phosphate repulsions to the free energy of DNA bending

PubMed Central

Range, Kevin; Mayaan, Evelyn; Maher, L. J.; York, Darrin M.

2005-01-01

DNA bending is important for the packaging of genetic material, regulation of gene expression and interaction of nucleic acids with proteins. Consequently, it is of considerable interest to quantify the energetic factors that must be overcome to induce bending of DNA, such as base stacking and phosphate–phosphate repulsions. In the present work, the electrostatic contribution of phosphate–phosphate repulsions to the free energy of bending DNA is examined for 71 bp linear and bent-form model structures. The bent DNA model was based on the crystallographic structure of a full turn of DNA in a nucleosome core particle. A Green's function approach based on a linear-scaling smooth conductor-like screening model was applied to ascertain the contribution of individual phosphate–phosphate repulsions and overall electrostatic stabilization in aqueous solution. The effect of charge neutralization by site-bound ions was considered using Monte Carlo simulation to characterize the distribution of ion occupations and contribution of phosphate repulsions to the free energy of bending as a function of counterion load. The calculations predict that the phosphate–phosphate repulsions account for ∼30% of the total free energy required to bend DNA from canonical linear B-form into the conformation found in the nucleosome core particle. PMID:15741179

The path and surviving tail of a comet that fell into the sun

NASA Technical Reports Server (NTRS)

Sekanina, Z.

1982-01-01

A satisfactory orbital solution for Comet Howard-Koomen-Michels 1979 XI is found on the assumption that the comet's line of apsides coincided with that of the Kreutz sungrazing comet group. The derived perihelion distance then shows that this is the first known case of a comet falling into the sun. A dust tail that survived the comet is studied as a particle flow phenomenon controlled by no force other than solar gravity and solar radiation pressure. The tail's outline is interpreted in terms of an onset of dust production, a peak repulsive force on the particles, and a circumsolar dustfree zone due to particle sublimation. It is shown that the surviving debris consisted mostly of absorbing, submicron size particles in hyperbolic trajectories convex to the sun and curving toward the earth. The tail width may be a product of the interaction of charged dust in the tail with a complicated structure of the coronal magnetic field.

The physics of charge separation preceding lightning strokes in thunderclouds

NASA Technical Reports Server (NTRS)

Kyrala, Ali

1987-01-01

The physics of charge separation preceding lightning strokes in thunderclouds is presented by three types of arguments: An explanation is given for the aggregation of electrical charges of like sign overcoming Coulomb repulsion by attraction due to exchange interaction. The latter is well known in quantum mechanics from the theories of the nuclear bond and the covalent bond. A classical electrostatic model of charge balls of segregated positive and negative charges in the thundercloud is presented. These charge balls can only be maintained in temporarily stable locations by a containing vortex. Because they will be of different sizes and masses, they will stabilize at different altitudes when drag forces are included with the given electrostatic force. The question of how the charges become concentrated again after lightning discharges is approached by means of the collisional Boltzmann transport equation to explain quasi-periodic recharging. It is shown that solutions cannot be separable in both position and time if they are to represent aggregation.

Sequential vortex hopping in an array of artificial pinning centers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Keay, J. C.

2010-02-24

We use low-temperature magnetic force microscopy (MFM) to study the hopping motion of vortices in an array of artificial pinning centers (APCs). The array consists of nanoscale holes etched in a niobium thin film by Ar-ion sputtering through an anodic aluminum-oxide template. Variable-temperature magnetometry shows a transition temperature of 7.1 K and an enhancement of the magnetization up to the third matching field at 5 K. Using MFM with attractive and repulsive tip-vortex interaction, we measure the vortex-pinning strength and investigate the motion of individual vortices in the APC array. The depinning force for individual vortices at low field rangedmore » from 0.7 to 1.2 pN. The motion of individual vortices was found to be reproducible and consistent with movement between adjacent holes in the film. The movements are repeatable but the sequence of hops depends on the scan direction. This asymmetry in the motion indicates nonuniform local pinning, a consequence of array disorder and hole-size variation.« less

Experimental and numerical analysis on aluminum/steel pipe using magnetic pulse welding

NASA Astrophysics Data System (ADS)

Shim, J. Y.; Kim, I. S.; Lee, K. J.; Kang, B. Y.

2011-12-01

Recently, there has been a trend in the automotive industry to focus on the improvement of lightweight materials, such as aluminum and magnesium because the welding of dissimilar metals causes many welding defects. Magnetic pulse welding (MPW), one of the solid state welding technologies, uses electromagnetic force from current discharged through a working coil which develops a repulsive force between the induced currents flowing parallel and in the opposite direction in the tube to be welded. The objective of this paper is to develop a numerical model for analysis of the interaction between the outer pipe and the working coil using a finite element method (FEM) in the MPW process. Four Maxwell equations are solved using a general electromagnetic mechanics computer program, ANSYS/EMAG code. Experiments were also carried out with a W-MPW60 machine manufactured by WELMATE CO., LTD. with the Al1070 and SM45C for Al pipe and steel bar respectively. The calculated and measured results were compared to verify the proposed model.

Pairwise frictional profile between particles determines discontinuous shear thickening transition in non-colloidal suspensions

PubMed Central

Comtet, Jean; Chatté, Guillaume; Niguès, Antoine; Bocquet, Lydéric; Siria, Alessandro; Colin, Annie

2017-01-01

The process by which sheared suspensions go through a dramatic change in viscosity is known as discontinuous shear thickening. Although well-characterized on the macroscale, the microscopic mechanisms at play in this transition are still poorly understood. Here, by developing new experimental procedures based on quartz-tuning fork atomic force microscopy, we measure the pairwise frictional profile between approaching pairs of polyvinyl chloride and cornstarch particles in solvent. We report a clear transition from a low-friction regime, where pairs of particles support a finite normal load, while interacting purely hydrodynamically, to a high-friction regime characterized by hard repulsive contact between the particles and sliding friction. Critically, we show that the normal stress needed to enter the frictional regime at nanoscale matches the critical stress at which shear thickening occurs for macroscopic suspensions. Our experiments bridge nano and macroscales and provide long needed demonstration of the role of frictional forces in discontinuous shear thickening. PMID:28561032

Pairwise frictional profile between particles determines discontinuous shear thickening transition in non-colloidal suspensions.

PubMed

Comtet, Jean; Chatté, Guillaume; Niguès, Antoine; Bocquet, Lydéric; Siria, Alessandro; Colin, Annie

2017-05-31

The process by which sheared suspensions go through a dramatic change in viscosity is known as discontinuous shear thickening. Although well-characterized on the macroscale, the microscopic mechanisms at play in this transition are still poorly understood. Here, by developing new experimental procedures based on quartz-tuning fork atomic force microscopy, we measure the pairwise frictional profile between approaching pairs of polyvinyl chloride and cornstarch particles in solvent. We report a clear transition from a low-friction regime, where pairs of particles support a finite normal load, while interacting purely hydrodynamically, to a high-friction regime characterized by hard repulsive contact between the particles and sliding friction. Critically, we show that the normal stress needed to enter the frictional regime at nanoscale matches the critical stress at which shear thickening occurs for macroscopic suspensions. Our experiments bridge nano and macroscales and provide long needed demonstration of the role of frictional forces in discontinuous shear thickening.

Hyperfine structure of the hydroxyl free radical (OH) in electric and magnetic fields

NASA Astrophysics Data System (ADS)

Maeda, Kenji; Wall, Michael L.; Carr, Lincoln D.

2015-05-01

We investigate single-particle energy spectra of the hydroxyl free radical (OH) in the lowest electronic and rovibrational level under combined static electric and magnetic fields, as an example of heteronuclear polar diatomic molecules. In addition to the fine-structure interactions, the hyperfine interactions and centrifugal distortion effects are taken into account to yield the zero-field spectrum of the lowest 2Π3 / 2 manifold to an accuracy of less than 2kHz. We also examine level crossings and repulsions in the hyperfine structure induced by applied electric and magnetic fields. Compared to previous work, we found more than 10 percent reduction of the magnetic fields at level repulsions in the Zeeman spectrum subjected to a perpendicular electric field. In addition, we find new level repulsions, which we call Stark-induced hyperfine level repulsions, that require both an electric field and hyperfine structure. It is important to take into account hyperfine structure when we investigate physics of OH molecules at micro-Kelvin temperatures and below. This research was supported in part by AFOSR Grant No.FA9550-11-1-0224 and by the NSF under Grants PHY-1207881 and NSF PHY-1125915. We appreciate the Aspen Center for Physics, supported in part by the NSF Grant No.1066293, for hospitality.

A Taoist Paradigm of EAP Consultation.

ERIC Educational Resources Information Center

Gerstein, Lawrence H.; Sturmer, Paul

1993-01-01

Describes new Taoist model as alternative approach to conceptualizing consultation process and to formulating successful, isomorphic interventions constructed to facilitate four change processes. Presents model stressing importance of interrelationships between individuals and groups; integrating repulsion and assimilation forces; balancing human…

[Evaluation of three dimensional orthodontic force produced by magnet of fix appliance].

PubMed

Dai, Xin; Hou, Zhi-ming; Yao, Ge; Wen, Jing-long

2008-12-01

To analyze the feature and magnitude of three dimensional orthodontic force produced by the magnet of fix appliance. Forces detected by universal fatigue test system included the attractive and repulsive,the inclined and rotated orthodontic forces of two magnets in different air gaps, and the integrated inclined and rotated orthodontic forces of two magnets and NiTi wire. The attractive and repulsive forces of two magnets were 4.68 to 0.45 N and 3.00 to 0.40 N respectively in the air gaps of 0 to 5 mm. The inclined orthodontic forces were 1.54 to 1.67 N, 0.63 to 0.69 N, 0.47 to 0.54 N when the magnets were vertically inclined 10 degrees to 40 degrees in the air gaps of 0, 1, 2mm. The rotated orthodontic forces were 0.97 to 1.32 N, 0.53 to 0.59 N, 0.39 to 0.48 N when the magnets were horizontally rotated 10 degrees to 40 degrees in the air gaps of 0, 1, 2mm. The integrated orthodontic force of two magnets and 0.014-inch NiTi wire was 0.32 to 0.5 N when the magnets was vertically inclined 10 degrees to 40 degrees in the air gap of 4 mm. The integrated orthodontic force of two magnets and 0.012-inch NiTi wire was 0.32 to 0.39 N when the magnets were horizontally rotated 10 degrees to 40 degrees in the air gap of 3 mm. Magnets made into orthodontic brackets to some extent could replace the mechanical orthodontic force produced by orthodontic wires and elastics.

Acrosome reaction inducers impose alterations in repulsive strain and hydration barrier in human sperm membranes.

PubMed

Purohit, S B; Laloraya, M; Kumar, G P

1998-06-01

Spin labeling studies of the lipophilic domains of human spermatozoa during capacitation and during acrosome reaction (AR) under the influence of selected AR-inducers were performed. Significantly enhanced rotational function of molecules was obvious during capacitation, with no significant changes in membrane packaging or the lateral diffusion of molecules. The AR inducers appeared to restrict the rotational freedom of molecules, dramatically enhancing the lateral diffusion and ordering coefficients. A significant decrease in superoxide anion generation was observed in the acrosome reacted groups when compared to the non-acrosome reacted groups. A high level of superoxide anion radical (O2.-) level maintained in capacitated spermatozoa would add to the Van der Waal's repulsive forces at the polar head of phospholipids, holding the membrane in strain where the molecular enjoy little freedom for lateral motion. A sudden drop in the levels of O2.- in spermatozoa upon addition of AR inducers could abruptly release the local hydrophobic repulsive strain within the membrane. This loss of hydration barrier explains the observed enhancement in lateral diffusion profiles of lipids and the packaging of molecules. It is reasonable to assume that these phenomena could be amplified further by interplay of Ca2+ by modifying the local charge aggregation. Thus, we would conclude that AR inducers release the oxyradical load in capacitated spermatozoa, which would modify the repulsive strain and hydration barrier forces in the lipophilic domains permitting vesiculation of the membranes. It appears that various acrosome reaction inducers act as effectors of grossly similar physical alterations in sperm membranes and that the resulting signal cascades proceed through intercalating biochemical sequences.

Quantum mechanics study of repulsive π-π interaction and flexibility of phenyl moiety in the iron azodioxide complex

NASA Astrophysics Data System (ADS)

Liu, Yuemin; Liu, Yucheng; Murru, Siva; Tzeng, Nianfeng; Srivastava, Radhey S.

2015-10-01

In this study, repulsive π-π interactions within iron azodioxide complex Fe[Ph(O)NN(O)Ph]3 were quantum mechanically characterized using DFT, MP2 and CCSD(T) methods. Flexibility of six phenyl moieties in this complex structure was also investigated by structural optimization approach using the DFT methods. Our MP2 and CCSD(T) calculations of the closest pair provided interaction energy of 6.62 and 8.29 kcal/mol respectively, which indicate a strongest repulsion among these intra-molecular π-π interactions. Interaction energy of the particular π-π pair calculated from 24 hybrid DFT methods ranges from 4.56 kcal/mol from BHandH method to 15.15 kcal/mol from O3LYP method. Cares should be exercised when interpreting interaction energy and geometry optimization from DFT simulation of systems containing π-π interaction. Comparison between the DFT results and the benchmark CCSD(T) results shows that the DFT calculations of π-π interaction are reasonable but still need to be interpreted with caution. Furthermore, MP2 interaction energy of -44.69 kcal/mol between two substituted π systems/phenyl rings Ph(O)N-moieties suggested that above energetically unfavorable π-π interaction can be compensated by the covalent bond N-N in a single ligand Ph(O)NN(O)Ph, which allows for a reasonable stability across the complex molecules. Optimizations of the entire complex molecule using B3LYP and M06HF methods produced a large variation of π-π distances and orientations, which implied that the complex molecule may perform catalysis at room temperature.

Cavity-photon contribution to the effective interaction of electrons in parallel quantum dots

NASA Astrophysics Data System (ADS)

Gudmundsson, Vidar; Sitek, Anna; Abdullah, Nzar Rauf; Tang, Chi-Shung; Manolescu, Andrei

2016-05-01

A single cavity photon mode is expected to modify the Coulomb interaction of an electron system in the cavity. Here we investigate this phenomena in a parallel double quantum dot system. We explore properties of the closed system and the system after it has been opened up for electron transport. We show how results for both cases support the idea that the effective electron-electron interaction becomes more repulsive in the presence of a cavity photon field. This can be understood in terms of the cavity photons dressing the polarization terms in the effective mutual electron interaction leading to nontrivial delocalization or polarization of the charge in the double parallel dot potential. In addition, we find that the effective repulsion of the electrons can be reduced by quadrupolar collective oscillations excited by an external classical dipole electric field.

On the physical origin of blue-shifted hydrogen bonds.

PubMed

Li, Xiaosong; Liu, Lei; Schlegel, H Bernhard

2002-08-14

For blue-shifted hydrogen-bonded systems, the hydrogen stretching frequency increases rather than decreases on complexation. In computations at various levels of theory, the blue-shift in the archetypical system, F(3)C-H.FH, is reproduced at the Hartree-Fock level, indicating that electron correlation is not the primary cause. Calculations also demonstrate that a blue-shift does not require either a carbon center or the absence of a lone pair on the proton donor, because F(3)Si-H.OH(2), F(2)NH.FH, F(2)PH.NH(3), and F(2)PH.OH(2) have substantial blue-shifts. Orbital interactions are shown to lengthen the X-H bond and lower its vibrational frequency, and thus cannot be the source of the blue-shift. In the F(3)CH.FH system, the charge redistribution in F(3)CH can be reproduced very well by replacing the FH with a simple dipole, which suggests that the interactions are predominantly electrostatic. When modeled with a point charge for the proton acceptor, attractive electrostatic interactions elongate the F(3)C-H, while repulsive interactions shorten it. At the equilibrium geometry of a hydrogen-bonded complex, the electrostatic attraction between the dipole moments of the proton donor and proton acceptor must be balanced by the Pauli repulsion between the two fragments. In the absence of orbital interactions that cause bond elongation, this repulsive interaction leads to compression of the X-H bond and a blue-shift in its vibrational frequency.

Antiswarming: Structure and dynamics of repulsive chemically active particles

NASA Astrophysics Data System (ADS)

Yan, Wen; Brady, John F.

2017-12-01

Chemically active Brownian particles with surface catalytic reactions may repel each other due to diffusiophoretic interactions in the reaction and product concentration fields. The system behavior can be described by a "chemical" coupling parameter Γc that compares the strength of diffusiophoretic repulsion to Brownian motion, and by a mapping to the classical electrostatic one component plasma (OCP) system. When confined to a constant-volume domain, body-centered cubic (bcc) crystals spontaneously form from random initial configurations when the repulsion is strong enough to overcome Brownian motion. Face-centered cubic (fcc) crystals may also be stable. The "melting point" of the "liquid-to-crystal transition" occurs at Γc≈140 for both bcc and fcc lattices.

Ab initio calculation of atomic interactions on Al(110): implications for epitaxial growth

NASA Astrophysics Data System (ADS)

Fichthorn, Kristen; Tiwary, Yogesh

2007-03-01

Using first-principles calculations based on density-functional theory, we resolved atomic interactions between adsorbed Al atoms on Al(110). Relevant pair and trio interactions were quantified. We find that pair interactions extend to the third in-channel and second cross-channel neighbor on the anisotropic (110) surface. Beyond these distances, pair interactions are negligible. The nearest-neighbor interaction in the in-channel direction is attractive, but nearest-neighbor cross-channel interaction is repulsive. While nearest-neighbor, cross-channel repulsion does not support the experimental observation of 3D hut formation in Al/Al(110) homoepitaxial growth [1], we find that trio interactions can be significant and attractive and they support cross-channel bonding. The pair and trio interactions have direct and indirect components. We have quantified the electronic and elastic components of the indirect, substrate-mediated interactions. We also probe the influence of these interactions on the energy barriers for adatom hopping. [1] F. Buatier de Mongeot, W. Zhu, A. Molle, R. Buzio, C. Boragno, U. Valbusa, E. Wang, and Z. Zhang, Phys. Rev. Lett. 91, 016102 (2003).

Assessment of Simple Models for Molecular Simulation of Ethylene Carbonate and Propylene Carbonate as Solvents for Electrolyte Solutions.

PubMed

Chaudhari, Mangesh I; Muralidharan, Ajay; Pratt, Lawrence R; Rempe, Susan B

2018-02-12

Progress in understanding liquid ethylene carbonate (EC) and propylene carbonate (PC) on the basis of molecular simulation, emphasizing simple models of interatomic forces, is reviewed. Results on the bulk liquids are examined from the perspective of anticipated applications to materials for electrical energy storage devices. Preliminary results on electrochemical double-layer capacitors based on carbon nanotube forests and on model solid-electrolyte interphase (SEI) layers of lithium ion batteries are considered as examples. The basic results discussed suggest that an empirically parameterized, non-polarizable force field can reproduce experimental structural, thermodynamic, and dielectric properties of EC and PC liquids with acceptable accuracy. More sophisticated force fields might include molecular polarizability and Buckingham-model description of inter-atomic overlap repulsions as extensions to Lennard-Jones models of van der Waals interactions. Simple approaches should be similarly successful also for applications to organic molecular ions in EC/PC solutions, but the important case of Li[Formula: see text] deserves special attention because of the particularly strong interactions of that small ion with neighboring solvent molecules. To treat the Li[Formula: see text] ions in liquid EC/PC solutions, we identify interaction models defined by empirically scaled partial charges for ion-solvent interactions. The empirical adjustments use more basic inputs, electronic structure calculations and ab initio molecular dynamics simulations, and also experimental results on Li[Formula: see text] thermodynamics and transport in EC/PC solutions. Application of such models to the mechanism of Li[Formula: see text] transport in glassy SEI models emphasizes the advantage of long time-scale molecular dynamics studies of these non-equilibrium materials.

Predicting the influence of long-range molecular interactions on macroscopic-scale diffusion by homogenization of the Smoluchowski equation

NASA Astrophysics Data System (ADS)

Kekenes-Huskey, P. M.; Gillette, A. K.; McCammon, J. A.

2014-05-01

The macroscopic diffusion constant for a charged diffuser is in part dependent on (1) the volume excluded by solute "obstacles" and (2) long-range interactions between those obstacles and the diffuser. Increasing excluded volume reduces transport of the diffuser, while long-range interactions can either increase or decrease diffusivity, depending on the nature of the potential. We previously demonstrated [P. M. Kekenes-Huskey et al., Biophys. J. 105, 2130 (2013)] using homogenization theory that the configuration of molecular-scale obstacles can both hinder diffusion and induce diffusional anisotropy for small ions. As the density of molecular obstacles increases, van der Waals (vdW) and electrostatic interactions between obstacle and a diffuser become significant and can strongly influence the latter's diffusivity, which was neglected in our original model. Here, we extend this methodology to include a fixed (time-independent) potential of mean force, through homogenization of the Smoluchowski equation. We consider the diffusion of ions in crowded, hydrophilic environments at physiological ionic strengths and find that electrostatic and vdW interactions can enhance or depress effective diffusion rates for attractive or repulsive forces, respectively. Additionally, we show that the observed diffusion rate may be reduced independent of non-specific electrostatic and vdW interactions by treating obstacles that exhibit specific binding interactions as "buffers" that absorb free diffusers. Finally, we demonstrate that effective diffusion rates are sensitive to distribution of surface charge on a globular protein, Troponin C, suggesting that the use of molecular structures with atomistic-scale resolution can account for electrostatic influences on substrate transport. This approach offers new insight into the influence of molecular-scale, long-range interactions on transport of charged species, particularly for diffusion-influenced signaling events occurring in crowded cellular environments.

A simulation study of sperm motility hydrodynamics near fish eggs and spheres.

PubMed

Ishimoto, Kenta; Cosson, Jacky; Gaffney, Eamonn A

2016-01-21

For teleost fish fertilisation, sperm must proceed through a small opening on the egg surface, referred to as the micropyle. In this paper, we have used boundary element simulations to explore whether the hydrodynamic attraction between sperm and a fish egg can be a sperm guidance cue. Hydrodynamical egg-sperm interactions alone do not increase the chances of an egg encounter, nor do they induce surface swimming for virtual turbot fish sperm across smooth spheres with a diameter of 1mm, which is representative of a turbot fish egg. When a repulsive surface force between the virtual turbot sperm and the egg is introduced, as motivated by surface charge and van-der-Waals interactions for instance, we find that extended surface swimming of the virtual sperm across a model turbot egg occurs, but ultimately the sperm escapes from the egg. This is due to the small exit angle of the scattering associated with the initial sperm-egg interaction at the egg surface, leading to a weak drift away from the egg, in combination with a weak hydrodynamical attraction between both gametes, though the latter is not sufficient to prevent eventual escape. The resulting transience is not observed experimentally but is a detailed quantitative difference between theory and observation in that stable surface swimming is predicted for eggs with radii larger than about 1.8mm. Regardless, the extended sperm swimming trajectory across the egg constitutes a two-dimensional search for the micropyle and thus the egg is consistently predicted to provide a guidance cue for sperm once they are sufficiently close. In addition, the observation that the virtual turbot sperm swims stably next to a flat plane given repulsive surface interactions, but does not swim stably adjacent to a turbot-sized egg, which is extremely large by sperm-lengthscales, also highlights that the stability of sperm swimming near a boundary is very sensitive to geometry. Copyright © 2015 Elsevier Ltd. All rights reserved.

Delayed response and biosonar perception explain movement coordination in trawling bats.

PubMed

Giuggioli, Luca; McKetterick, Thomas J; Holderied, Marc

2015-03-01

Animal coordinated movement interactions are commonly explained by assuming unspecified social forces of attraction, repulsion and alignment with parameters drawn from observed movement data. Here we propose and test a biologically realistic and quantifiable biosonar movement interaction mechanism for echolocating bats based on spatial perceptual bias, i.e. actual sound field, a reaction delay, and observed motor constraints in speed and acceleration. We found that foraging pairs of bats flying over a water surface swapped leader-follower roles and performed chases or coordinated manoeuvres by copying the heading a nearby individual has had up to 500 ms earlier. Our proposed mechanism based on the interplay between sensory-motor constraints and delayed alignment was able to recreate the observed spatial actor-reactor patterns. Remarkably, when we varied model parameters (response delay, hearing threshold and echolocation directionality) beyond those observed in nature, the spatio-temporal interaction patterns created by the model only recreated the observed interactions, i.e. chases, and best matched the observed spatial patterns for just those response delays, hearing thresholds and echolocation directionalities found to be used by bats. This supports the validity of our sensory ecology approach of movement coordination, where interacting bats localise each other by active echolocation rather than eavesdropping.

Behavior of the Enthalpy of Adsorption in Nanoporous Materials Close to Saturation Conditions

PubMed Central

2017-01-01

Many important industrial separation processes based on adsorption operate close to saturation. In this regime, the underlying adsorption processes are mostly driven by entropic forces. At equilibrium, the entropy of adsorption is closely related to the enthalpy of adsorption. Thus, studying the behavior of the enthalpy of adsorption as a function of loading is fundamental to understanding separation processes. Unfortunately, close to saturation, the enthalpy of adsorption is hard to measure experimentally and hard to compute in simulations. In simulations, the enthalpy of adsorption is usually obtained from energy/particle fluctuations in the grand-canonical ensemble, but this methodology is hampered by vanishing insertions/deletions at high loading. To investigate the fundamental behavior of the enthalpy and entropy of adsorption at high loading, we develop a simplistic model of adsorption in a channel and show that at saturation the enthalpy of adsorption diverges to large positive values due to repulsive intermolecular interactions. However, there are many systems that can avoid repulsive intermolecular interactions and hence do not show this drastic increase in enthalpy of adsorption close to saturation. We find that the conventional grand-canonical Monte Carlo method is incapable of determining the enthalpy of adsorption from energy/particle fluctuations at high loading. Here, we show that by using the continuous fractional component Monte Carlo, the enthalpy of adsorption close to saturation conditions can be reliably obtained from the energy/particle fluctuations in the grand-canonical ensemble. The best method to study properties at saturation is the NVT energy (local-) slope methodology. PMID:28521093

Entropic depletion in colloidal suspensions and polymer liquids: Role of nanoparticle surface topography

DOE Office of Scientific and Technical Information (OSTI.GOV)

Banerjee, Debapriya; Yang, Jian; Schweizer, Kenneth S.

2015-01-01

Here, we employ a hybrid Monte Carlo plus integral equation theory approach to study how dense fluids of small nanoparticles or polymer chains mediate entropic depletion interactions between topographically rough particles where all interaction potentials are hard core repulsion. The corrugated particle surfaces are composed of densely packed beads which present variable degrees of controlled topographic roughness and free volume associated with their geometric crevices. This pure entropy problem is characterized by competing ideal translational and (favorable and unfavorable) excess entropic contributions. Surface roughness generically reduces particle depletion aggregation relative to the smooth hard sphere case. However, the competition betweenmore » ideal and excess packing entropy effects in the bulk, near the particle surface and in the crevices, results in a non-monotonic variation of the particle-monomer packing correlation function as a function of the two dimensionless length scale ratios that quantify the effective surface roughness. As a result, the inter-particle potential of mean force (PMF), second virial coefficient, and spinodal miscibility volume fraction vary non-monotonically with the surface bead to monomer diameter and particle core to surface bead diameter ratios. A miscibility window is predicted corresponding to an optimum degree of surface roughness that completely destroys depletion attraction resulting in a repulsive PMF. Variation of the (dense) matrix packing fraction can enhance or suppress particle miscibility depending upon the amount of surface roughness. Connecting the monomers into polymer chains destabilizes the system via enhanced contact depletion attraction, but the non-monotonic variations with surface roughness metrics persist.« less

Pair interactions in polyelectrolyte-nanoparticle systems: Influence of dielectric inhomogeneities and the partial dissociation of polymers and nanoparticles.

PubMed

Pryamitsyn, Victor; Ganesan, Venkat

2015-10-28

We study the effective pair interactions between two charged spherical particles in polyelectrolyte solutions using polymer self-consistent field theory. In a recent study [V. Pryamitsyn and V. Ganesan, Macromolecules 47, 6095 (2015)], we considered a model in which the particles possess fixed charge density, the polymers contain a prespecified amount of dissociated charges and, the dielectric constant of the solution was assumed to be homogeneous in space and independent of the polymer concentration. In this article, we present results extending our earlier model to study situations in which either or both the particle and the polymers possess partially dissociable groups. Additionally, we also consider the case when the dielectric constant of the solution depends on the local concentration of the polymers and when the particle's dielectric constant is lower than that of the solvent. For each case, we quantify the polymer-mediated interactions between the particles as a function of the polymer concentrations and the degree of dissociation of the polymer and particles. Consistent with the results of our previous study, we observe that the polymer-mediated interparticle interactions consist of a short-range attraction and a long-range repulsion. The partial dissociablity of the polymer and particles was seen to have a strong influence on the strength of the repulsive portion of the interactions. Rendering the dielectric permittivity to be inhomogeneous has an even stronger effect on the repulsive interactions and results in changes to the qualitative nature of interactions in some parametric ranges.

Rheology of attractive emulsions

NASA Astrophysics Data System (ADS)

Datta, Sujit S.; Gerrard, Dustin D.; Rhodes, Travers S.; Mason, Thomas G.; Weitz, David A.

2011-10-01

We show how attractive interactions dramatically influence emulsion rheology. Unlike the repulsive case, attractive emulsions below random close packing, φRCP, can form soft gel-like elastic solids. However, above φRCP, attractive and repulsive emulsions have similar elasticities. Such compressed attractive emulsions undergo an additional shear-driven relaxation process during yielding. Our results suggest that attractive emulsions begin to yield at weak points through the breakage of bonds, and, above φRCP, also undergo droplet configurational rearrangements.

Rheology of attractive emulsions.

PubMed

Datta, Sujit S; Gerrard, Dustin D; Rhodes, Travers S; Mason, Thomas G; Weitz, David A

2011-10-01

We show how attractive interactions dramatically influence emulsion rheology. Unlike the repulsive case, attractive emulsions below random close packing, φ(RCP), can form soft gel-like elastic solids. However, above φ(RCP), attractive and repulsive emulsions have similar elasticities. Such compressed attractive emulsions undergo an additional shear-driven relaxation process during yielding. Our results suggest that attractive emulsions begin to yield at weak points through the breakage of bonds, and, above φ(RCP), also undergo droplet configurational rearrangements.

Fluid-sensitive nanoscale switching with quantum levitation controlled by α -Sn/β -Sn phase transition

NASA Astrophysics Data System (ADS)

Boström, Mathias; Dou, Maofeng; Malyi, Oleksandr I.; Parashar, Prachi; Parsons, Drew F.; Brevik, Iver; Persson, Clas

2018-03-01

We analyze the Lifshitz pressure between silica and tin separated by a liquid mixture of bromobenzene and chlorobenzene. We show that the phase transition from semimetallic α -Sn to metallic β -Sn can switch Lifshitz forces from repulsive to attractive. This effect is caused by the difference in dielectric functions of α -Sn and β -Sn , giving both attractive and repulsive contributions to the total Lifshitz pressure in different frequency regions controlled by the composition of the intervening liquid mixture. In this way, one may be able to produce phase-transition-controlled quantum levitation in a liquid medium.

Reversible Adhesion with Polyelectrolyte Brushes Tailored via the Uptake and Release of Trivalent Lanthanum Ions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Farina, Robert; Laugel, Nicolas; Yu, Jing

Applications of end-tethered polyelectrolyte “brushes” to modify solid surfaces have been developed and studied for their colloidal stabilization and high lubrication properties. Current efforts have expanded into biological realms and stimuli-responsive materials. Our work explores responsive and reversible aspects of polyelectrolyte brush behavior when polyelectrolyte chains interact with oppositely charged multivalent ions and complexes, which act as counterions. There is a significant void in the polyelectrolyte literature regarding interactions with multivalent species. This paper demonstrates that interactions between solid surfaces bearing negatively charged polyelectrolyte brushes are highly sensitive to the presence of trivalent lanthanum, La3+. Lanthanum cations have unique interactionsmore » with polyelectrolyte chains, in part due to their small size and hydration radius which results in a high local charge density. Using La3+ in conjunction with the surface forces apparatus (SFA), adhesion has been observed to reversibly appear and disappear upon the uptake and release, respectively, of these multivalent cations acting as counterions. In media of fixed ionic strength set by monovalent sodium salt, at I0 = 0.003 M and I0 = 0.3 M, the sign of the interaction forces between overlapping brushes changes from repulsive to attractive when La3+ concentrations reach 0.1 mol % of the total ion concentration. These results are also shown to be generally consistent with, but subtlety different from, previous polyelectrolyte brush experiments using trivalent ruthenium hexamine in the role of the multivalent counterion.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Solares, Santiago D.

This study introduces a quasi-3-dimensional (Q3D) viscoelastic model and software tool for use in atomic force microscopy (AFM) simulations. The model is based on a 2-dimensional array of standard linear solid (SLS) model elements. The well-known 1-dimensional SLS model is a textbook example in viscoelastic theory but is relatively new in AFM simulation. It is the simplest model that offers a qualitatively correct description of the most fundamental viscoelastic behaviors, namely stress relaxation and creep. However, this simple model does not reflect the correct curvature in the repulsive portion of the force curve, so its application in the quantitative interpretationmore » of AFM experiments is relatively limited. In the proposed Q3D model the use of an array of SLS elements leads to force curves that have the typical upward curvature in the repulsive region, while still offering a very low computational cost. Furthermore, the use of a multidimensional model allows for the study of AFM tips having non-ideal geometries, which can be extremely useful in practice. Examples of typical force curves are provided for single- and multifrequency tappingmode imaging, for both of which the force curves exhibit the expected features. Lastly, a software tool to simulate amplitude and phase spectroscopy curves is provided, which can be easily modified to implement other controls schemes in order to aid in the interpretation of AFM experiments.« less

Effect of Pendant Side-Chain Sterics and Dipole Forces on Short Range Ordering in Random Polyelectrolytes

NASA Astrophysics Data System (ADS)

Nwosu, Chinomso; Pandey, Tara; Herring, Andrew; Coughlin, Edward; University of Massachusetts, Amherst Collaboration; Colorado School of Mines Collaboration

Backbone-to-backbone spacing in polymers is known to be dictated by the length of the pendant side-chains. Dipole forces in random polyelectrolytes lead to ionic clusters with a characteristic spacing that can be observed by SAXS. Repulsion due to side-chain sterics will compete with dipole forces driving cluster formation in random polyelectrolytes. A model study on short range order in anion exchange membranes (AEMs) of quaternized P4VP-ran-PI is presented. Quaternization of P4VP with alkyl bromides having different numbers of carbons, CnBr, introduces pendant side-chains as well as charges. X-ray scattering performed on PQ4VP-ran-PI(CnBr) show that when n <5 the dipole forces dominate leading to the formation of ionic clusters. However, when n >4, the chains remain separated due to sterics, forming a distinct backbone-to-backbone spacing morphology. For n=3, both dipole clustering and backbone spacing can coexist. Crosslinking of the isoprene units increased the coexistence window from n=3 to n=6. Impedance measurements show that a maximum conductivity of 110mS/cm was obtained for PQ4VP-ran-PI(C3Br). A discussion on short range order due to competition, or counter balancing, of steric repulsion and dipole forces will be presented. US Army MURI project (W911NF1010520).

Controlling the rheological behavior of ceramic slurries and consolidated bodies: Interpenetrating networks and ion size effects

NASA Astrophysics Data System (ADS)

Fisher, Matthew Lyle

Colloidal processing has been demonstrated as an effective technique for increasing the reliability of ceramic components by reducing the flaw populations in sintered bodies. The formation of long-range repulsive potentials produces a dispersed slurry which can be filtered to remove heterogeneities and truncate the flaw size distribution. When the pair potentials are changed from repulsive to weakly attractive, a short-range repulsive potential can be developed in the slurry state which prevents mass segregation, allows particles to consolidate to high volume fractions, and produces plastic consolidated bodies. Plastic behavior in saturated ceramic compacts would allow plastic shape forming technologies to be implemented on advanced ceramic powders. Two networks of different interparticle potential have been mixed to control the rheological properties of slurries and develop clay-like plasticity in consolidated bodies. The elastic modulus and yield stress of slurries were found to increase with volume fraction in a power law fashion. Consolidated bodies containing mixtures of alkylated and non-alkylated powder pack to high volume fraction and exhibit similar flow properties to clay. The mixing of aqueous networks of different pair potential can also be effective in tailoring the flow properties. The flow stress of saturated compacts has been adjusted by the addition of a second network of uncoated particles which is stabilized electrostatically. The influence of the addition of silica of various sizes on the viscosity and zeta potentials of alumina suspensions has been investigated. The adsorption of nano-silica to the surface of alumina shifts the iep. The amount of silica at which the maximum shift in zeta potential occurs is consistent with the silica required to produce the minimum viscosity. This level of silica on the surface is consistent with calculations of the amount necessary for dense random parking of silica spheres around alumina. The influence of counterion size on short range repulsive forces at high salt concentrations was investigated with alumina and silica slurries coagulated with the chlorides of Li+, Na+, K+, Cs+ and TMA+ (tetramethylammonium+). The results clearly show that the range of the repulsive forces correlated with the size of the unhydrated ion, namely stronger particle networks are achieved with smaller counterions. The findings are contradictory to the widely accepted hydration force model. Silica and alumina slurries were also studied at and below the iep where the indifferent electrolyte cations would not be expected to adsorb. It appears that a lyotropic sequence for excluded ions exists and is correlated to the hydration of ions and surfaces.

EXTRACTING PLANET MASS AND ECCENTRICITY FROM TTV DATA

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lithwick, Yoram; Xie Jiwei; Wu Yanqin

2012-12-20

Most planet pairs in the Kepler data that have measured transit time variations (TTVs) are near first-order mean-motion resonances. We derive analytical formulae for their TTV signals. We separate planet eccentricity into free and forced parts, where the forced part is purely due to the planets' proximity to resonance. This separation yields simple analytical formulae. The phase of the TTV depends sensitively on the presence of free eccentricity: if the free eccentricity vanishes, the TTV will be in phase with the longitude of conjunctions. This effect is easily detectable in current TTV data. The amplitude of the TTV depends onmore » planet mass and free eccentricity, and it determines planet mass uniquely only when the free eccentricity is sufficiently small. We analyze the TTV signals of six short-period Kepler pairs. We find that three of these pairs (Kepler 18, 24, 25) have a TTV phase consistent with zero. The other three (Kepler 23, 28, 32) have small TTV phases, but ones that are distinctly non-zero. We deduce that the free eccentricities of the planets are small, {approx}< 0.01, but not always vanishing. Furthermore, as a consequence of this, we deduce that the true masses of the planets are fairly accurately determined by the TTV amplitudes, within a factor of {approx}< 2. The smallness of the free eccentricities suggests that the planets have experienced substantial dissipation. This is consistent with the hypothesis that the observed pile-up of Kepler pairs near mean-motion resonances is caused by resonant repulsion. But the fact that some of the planets have non-vanishing free eccentricity suggests that after resonant repulsion occurred there was a subsequent phase in the planets' evolution when their eccentricities were modestly excited, perhaps by interplanetary interactions.« less

What do a foam film and a real gas have in common?

PubMed

Stubenrauch, Cosima

2005-01-01

The stability of well-drained quasistatic foam films (thickness <100 nm) is usually discussed in terms of surface forces, which create an excess pressure normal to the film interfaces, called the disjoining pressure pi The disjoining pressure is the sum of repulsive electrostatic (pi(elec)), attractive van der Waals (pi(vdW)), and repulsive steric (pi(sr)) forces on the assumption that structural forces can be neglected. On the basis of these forces two different types of thin foam films are distinguished, namely common black films (CBF), which are mainly stabilized by pi(elec), and Newton black films (NBF), the stability of which is determined by pi(sr),With a thin-film pressure balance (TFPB) the thickness h of a foam film can be measured as a function of the applied pressure from which the disjoining pressure pi can be calculated. A thorough analysis of the results published so far reveals that the pi-h curves of nonionic surfactants measured at different surfactant concentrations resemble p-V(m) isotherms of a real gas measured at different temperatures. On the basis of these observations the van der Waals description of a real gas can be applied to foam films and a phase diagram for a foam film was constructed using the Maxwell construction.

Role of electrostatic interactions in the assembly of empty spherical viral capsids

NASA Astrophysics Data System (ADS)

Šiber, Antonio; Podgornik, Rudolf

2007-12-01

We examine the role of electrostatic interactions in the assembly of empty spherical viral capsids. The charges on the protein subunits that make the viral capsid mutually interact and are expected to yield electrostatic repulsion acting against the assembly of capsids. Thus, attractive protein-protein interactions of nonelectrostatic origin must act to enable the capsid formation. We investigate whether the interplay of repulsive electrostatic and attractive interactions between the protein subunits can result in the formation of spherical viral capsids of a preferred radius. For this to be the case, we find that the attractive interactions must depend on the angle between the neighboring protein subunits (i.e., on the mean curvature of the viral capsid) so that a particular angle(s) is (are) preferred energywise. Our results for the electrostatic contributions to energetics of viral capsids nicely correlate with recent experimental determinations of the energetics of protein-protein contacts in the hepatitis B virus [P. Ceres A. Zlotnick, Biochemistry 41, 11525 (2002)].

Thermodynamic analysis of membrane fouling in a submerged membrane bioreactor and its implications.

PubMed

Hong, Huachang; Peng, Wei; Zhang, Meijia; Chen, Jianrong; He, Yiming; Wang, Fangyuan; Weng, Xuexiang; Yu, Haiying; Lin, Hongjun

2013-10-01

The thermodynamic interactions between membrane and sludge flocs in a submerged membrane bioreactor (MBR) were investigated. It was found that Lewis acid-base (AB) interaction predominated in the total interactions. The interaction energy composition of membrane-sludge flocs combination was quite similar to that of membrane-bovine serum albumin (BSA) combination, indicating the critical role of proteins in adhesion process. Detailed analysis revealed the existence of a repulsive energy barrier in membrane-foulants interaction. Calculation results demonstrated that small flocs possessed higher attractive interaction energy per unit mass, and therefore adhered to membrane surface more easily as compared to large flocs. Meanwhile, initial sludge adhesion would facilitate the following adhesion due to the reduced repulsive energy barrier. Membrane with high electron donor surface tension component was a favor option for membrane fouling abatement. These findings offered new insights into membrane fouling, and also provided significant implications for fouling control in MBRs. Copyright © 2013 Elsevier Ltd. All rights reserved.