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Hydrodynamics of binary-fluid mixtures -An augmented Multiparticle Collison dynamics approach

EPL 2018年第2期121卷

作者： Eisenstecken, Thomas Hornung, Raphael Winkler, Roland G. Gompper, Gerhard Forschungszentrum Julich Inst Adv Simulat Theoret Soft Matter & Biophys D-52425 Julich Germany Forschungszentrum Julich Inst Complex Syst D-52425 Julich Germany

The Multiparticle Collision dynamics technique (MPC) for hydrodynamics simulations is generalized to binary-fluid mixtures and multiphase flows, by coupling the particle-based fluid dynamics to a Ginzburg-Landau free-energy functional for phase-separating binary fluids. To describe fluids with a non-ideal equation of state, an additional density-dependent term is introduced. The new approach is verified by applying it to thermodynamics near the critical demixing point, and interface fluctuations of droplets. The interfacial tension obtained from the analysis of the capillary-wave spectrum agrees well with the results based on the Laplace-Young equation. Phase separation dynamics follows the Lifshitz-Slyozov law. Copyright (C) EPLA, 2018

关键词： Computational methods in fluid dynamics computer simulation of molecular and particle dynamics Phase transitions: general studies

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Bond-breaking mechanism of vitreous silica densification by IR femtosecond laser pulses

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EPL 2016年第2期114卷

作者： Shcheblanov, Nikita S. Povarnitsyn, Mikhail E. Univ Missouri Dept Mech & Aerosp Engn E 2412 Lafferre Hall Columbia MO 65211 USA Univ Lyon 1 CNRS Inst Lumiere Mat UMR 5306 F-69622 Villeurbanne France RAS Joint Inst High Temp 13 Bld 2 Izhorskaya Str Moscow 125412 Russia

The densification of the vitreous silica (v-SiO2) due to laser irradiation appears reasonable to cause the change in refractive index. In this letter, the v-SiO2 densification under IR femtosecond laser irradiation is studied within molecular-dynamics simulation. The single-and multi-pulse interactions are explored numerically with an account of the bond-breaking mechanism. By analyzing the network at nanoscale, the nature of v-SiO2 densification is assigned to the reduction of major ring fractions of six-and seven-membered rings to minor fractions of three- and four-membered rings (related to D-2 and D-1 Raman signatures, respectively). The athermal behavior of v-SiO2 densification is disclosed at different degrees of ionization for both the single- and multi-pulse cases at sub-threshold regimes. The good agreement between calculated and measured D-2 defect line and Si-O-Si angle changes argues in favor of the found mechanism. Copyright (C) EPLA, 2016

关键词： Ultraviolet, visible, and infrared radiation effects (including laser radiation) Amorphous materials, glasses and other disordered solids computer simulation of molecular and particle dynamics

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Rectification and phase locking of graphite

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Frontiers of physics 2015年第4期10卷 111-118页

作者： Zhen-Bin Zhang Ru-Juan Jia Jasmina Tekic Yang Yang Cang-Long Wang Jia-Wei Li Xiao-Yun Wang Wen-Shan Duan Lei Yang College of Physics and Electronic Engineering Northwest Norvnal University Lanzhou 730070 China Theoretical Physics Department 020 "Vinca" Institute of Nuclear Sciences University of Belgrade P.O. Box 522 11001 Belgrade Serbia Institute of Modern Physics Chinese Academy of Sciences Lanzhou 730000 China Department of Chemistry Northwest Normal University Lanzhou 730070 China Department of Mathematics and Physics Lanzhou Jiao Tong University Lanzhou 730070 China

Rectification phenomena and the phase locking in a two-dimensional overdamped Frenkel-Kontorova model with a graphite periodic substrate were studied. The presence of dc and ac forces in the longitudinal direction causes the appearance of dynamicalmode locking and the steps in the response function of the system. On the other hand, the presence of an ac force in the transverse direction causes the appearance of rectification,even though there is no net dc force in the transverse direction. It is found that whereas the longitudinal velocity increases in a series of steps, rectification in the transverse direction can occur only between two neighbor steps. The amplitude and phase of the external ac driving force affect the depinning force,rectification of the system and particles trajectories.

关键词： classical transport friction and lubrication computer simulation of molecular and particle dynamics

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Accurate modelling of flow induced stresses in rigid colloidal aggregates

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computer PHYSICS COMMUNICATIONS 2015年 192卷 70-90页

作者： Vanni, Marco Politecn Torino Dip Sci Applicata & Tecnol I-10129 Turin Italy

A method has been developed to estimate the motion and the internal stresses induced by a fluid flow on a rigid aggregate. The approach couples Stokesian dynamics and structural mechanics in order to take into account accurately the effect of the complex geometry of the aggregates on hydrodynamic forces and the internal redistribution of stresses. The intrinsic error of the method, due to the low-order truncation of the multipole expansion of the Stokes solution, has been assessed by comparison with the analytical solution for the case of a doublet in a shear flow. In addition, it has been shown that the error becomes smaller as the number of primary particles in the aggregate increases and hence it is expected to be negligible for realistic reproductions of large aggregates. The evaluation of internal forces is performed by an adaptation of the matrix methods of structural mechanics to the geometric features of the aggregates and to the particular stress-strain relationship that occurs at intermonomer contacts. A preliminary investigation on the stress distribution in rigid aggregates and their mode of breakup has been performed by studying the response to an elongational flow of both realistic reproductions of colloidal aggregates (made of several hundreds monomers) and highly simplified structures. A very different behaviour has been evidenced between low-density aggregates with isostatic or weakly hyperstatic structures and compact aggregates with highly hyperstatic configuration. In low-density clusters breakup is caused directly by the failure of the most stressed intermonomer contact, which is typically located in the inner region of the aggregate and hence originates the birth of fragments of similar size. On the contrary, breakup of compact and highly cross-linked clusters is seldom caused by the failure of a single bond. When this happens, it proceeds through the removal of a tiny fragment from the external part of the structure. More commonly, however, br

关键词： Colloidal aggregates Stress distribution Breakup Colloids Emulsions and suspensions computer simulation of molecular and particle dynamics

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Relaxation of curvature-induced elastic stress by the Asaro-Tiller-Grinfeld instability

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EPL 2015年第4期111卷

作者： Koehler, C. Backofen, R. Voigt, A. Tech Univ Dresden Inst Comp Sci D-01062 Dresden Germany Tech Univ Dresden CfAED D-01062 Dresden Germany

A two-dimensional crystal on the surface of a sphere experiences elastic stress due to the incompatibility of the crystal axes and the curvature. A common mechanism to relax elastic stress is the Asaro-Tiller-Grinfeld (ATG) instability. With a combined numerical and analytical approach, we demonstrate that also curvature-induced stress in surface crystals can be relaxed by the long-wavelength ATG instability. The numerical results are obtained using a surface phasefield crystal (PFC) model, from which we determine the characteristic wave numbers of the ATG instability for various surface coverages corresponding to different curvature-induced compressions. The results are compared with an analytic expression for the characteristic wave number, obtained from a continuum approach which accounts for hexagonal crystals and intrinsic PFC symmetries. We find our numerical results in accordance with the analytical predictions. Copyright (C) EPLA, 2015

关键词： Theory and models of crystal growth, physics and chemistry of crystal growth, crystal morphology, and orientation computer simulation of molecular and particle dynamics Liquid-solid interface structure: measurements and simulations

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Crossovers in supercooled solvation water: Effects of hydrophilic and hydrophobic interactions

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EPL 2015年第3期110卷

作者： Titantah, John Tatini Karttunen, Mikko Univ Western Ontario Dept Appl Math London ON N6A 3K7 Canada Univ Waterloo Dept Chem Waterloo ON N2L 3G1 Canada Univ Waterloo Waterloo Inst Nanotechnol Waterloo ON N2L 3G1 Canada Eindhoven Univ Technol Dept Math & Comp Sci NL-5600 MB Eindhoven Netherlands Eindhoven Univ Technol Inst Complex Mol Syst NL-5600 MB Eindhoven Netherlands

Systematic 8 ns ab initio molecular dynamics (AIMD) were performed to study the structure and dynamics of water in bulk and close to hydrophobic (CH3) and hydrophilic (carbonyl) groups of tetramethylurea (TMU). Dynamical behaviour showed two crossovers: The first around the hydrophobic group at T-X = 256 +/- 4K, and the second at 265 +/- 5K related to the relative strengths of water-water and water-carbonyl hydrogen bonds (HBs). For bulk water, relaxation times appear to diverge at T-c = 213 +/- 10 K, rendering support to the liquid-liquid critical point hypothesis. To identify the effects due to the hydrophilic carbonyl group, systems of water with one methane molecule were used as references. Our findings are related to the structural and thermodynamic transitions reported for proteins in solution and may play a role in cold denaturation. Copyright (C) EPLA, 2015

关键词： Hydrogen bonding, hydrophilic effects computer simulation of molecular and particle dynamics

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Melt of polymer rings: The decorated loop model

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EPL 2014年第4期105卷

作者： Obukhov, S. Johner, A. Baschnagel, J. Meyer, H. Wittmer, J. P. CNRS UdS Inst Charles Sadron F-67034 Strasbourg 2 France Univ Florida Dept Phys Gainesville FL 32611 USA

Melts of unconcatenated and unknotted polymer rings are a paradigm for soft matter ruled by topological interactions. We propose a description of a system of rings of length N as a collection of smaller polydisperse Gaussian loops, ranging from the entanglement length to the skeleton ring length similar to N-2/3, assembled in random trees. Individual rings in the melt are predicted to be marginally compact with a mean square radius of gyration R-g(2) similar to N-2/3 (1-const center dot N-1/3). As a rule, simple power laws for asymptotically long rings come with sluggish crossovers. Experiments and computer simulations merely deal with crossover regimes typically extending to N similar to 10(3-4). The estimated crossover functions allow for a satisfactory fit of simulation data. Copyright (C) EPLA, 2014

关键词： Polymer melts Physical properties of polymers computer simulation of molecular and particle dynamics

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Osmosis with active solutes

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EPL 2014年第3期106卷

作者： Lion, Thomas W. Allen, Rosalind J. Univ Edinburgh Sch Phys & Astron SUPA Edinburgh EH9 3JZ Midlothian Scotland

Despite much current interest in active matter, little is known about osmosis in active systems. Using molecular dynamics simulations, we investigate how active solutes perturb osmotic steady states. We find that solute activity increases the osmotic pressure, and can also expel solvent from the solution i.e. cause reverse osmosis. The latter effect cannot be described by an effective temperature, but can be reproduced by mapping the active solution onto a passive one with the same degree of local structuring as the passive solvent component. Our results provide a basic framework for understanding active osmosis, and suggest that activity-induced structuring of the passive component may play a key role in the physics of active-passive mixtures. Copyright (C) EPLA, 2014

关键词： Colloidal systems computer simulation of molecular and particle dynamics Nonequilibrium and irreversible thermodynamics

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Polymorphic crystals selected in the nucleation stage

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EPL 2014年第4期107卷

作者： Zhang, Hui-Jun Peng, Shu-Ming Zhou, Xiao-Song Ju, Xin Univ Sci & Technol Beijing Sch Math & Phys Beijing 100083 Peoples R China China Acad Engn Phys Inst Nucl Phys & Chem Mianyang 621900 Peoples R China

molecular dynamics simulations are used to explore the atomic mechanism of formation of polymorphic crystals. Cooling the Lennard-Jones systems, we observe that the system almost always evolves into a polymorphic crystal with either fivefold-symmetric stacking faults or single-direction stacking faults. The detailed analysis reveals that such an evolution depends on the configuration of fee/hcp concomitance in the nucleation stage. A defect-induced model is then introduced to illustrate these two evolution routes. Through calculating the formation energies of the defective critical nuclei, we find that the polymorphic crystals seem to be determined by their critical nuclei, in which the relatively lower formation energy ensures the preponderance of the fivefold-symmetric cluster. Before the nucleation, we observe that thermal fluctuations prefer hcp-like particles over fee-like ones while in the nucleation and growth stage this preference reverses. Notably, an extended step rule of Ostwald is seemingly suitable to characterise the growth process because of the temporary hcp layers appearing among fee layers in the growth stage. Although the crystalline cluster with single-direction stacking faults has higher growth rate and structural order than its competitor, the component (fee and hcp) proportion of the final crystals is almost always constant regardless of the polymorphic type. Our finding renews the understanding of the polymorphism of crystals, and possibly draws more attention of people intending to control the polymorphic structures through nucleation. Copyright (C) EPLA, 2014

关键词： Solid-liquid transitions computer simulation of molecular and particle dynamics Point defects and defect clusters

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Alignment and segregation of bidisperse colloids in a shear-thinning viscoelastic fluid under shear flow

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EPL 2013年第2期101卷

作者： de Oliveira, I. S. Santos den Otter, W. K. Briels, W. J. Univ Twente Inst Nanotechnol Computat BioPhys & MESA NL-7500 AE Enschede Netherlands

computer simulations are presented of colloids, bidisperse in size, suspended in a shear-thinning viscoelastic fluid with the flow characteristics of a surfactant solution. The worm-like micelles are modeled in Responsive particle dynamics (RaPiD) as single soft particles obeying a generalized Brownian equation of motion including transient forces that effectively account for the entanglements of the polymeric chains. The colloids mix homogeneously in the quiescent fluid, but in a shear flow they string together and form colloidal trains. Besides alignment, we also observe simultaneous segregation of the colloids by size. Experimental studies have reported on separation by size occurring near the walls of the rheometer, while in the current study the colloids segregate in the bulk of the fluid. Copyright (C) EPLA, 2013

关键词： computer simulation of molecular and particle dynamics Suspensions, dispersions, pastes, slurries, colloids Shear rate-dependent structure (shear thinning and shear thickening)

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