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ABSTRACT: We study the nonlinear rheology of a glass-forming binary mixture under the reversal of shear flow using molecular dynamics simulations and a schematic model of the mode-coupling theory of the glass transition (MCT). Memory effects lead to a history-dependent response, as exemplified by the vanishing of a stress-overshoot phenomenon in the stress-strain curves of the sheared liquid, and a change in the apparent elastic coefficients around states with zero stress. We investigate the various retarded contributions to the stress response at a given time schematically within MCT. The connection of this macroscopic response to single-particle motion is demonstrated using molecular-dynamics simulation.
The Journal of chemical physics 03/2013; 138(12):12A513. · 3.09 Impact Factor
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ABSTRACT: We study the variation of electrophoretic mobility μ of highly charged spherical colloidal macroions for varying surface charge density σ on the colloid using computer simulations of the primitive model for charged colloids. Hydrodynamic interactions between ions are incorporated by coupling the primitive model of charged colloids to the lattice Boltzmann model (LB) of the fluid. In the highly charged regime, the mobility μ of the colloid is known to decrease with the increase of bare charge Q of the colloid; the aim of this paper is to investigate the cause of this. We have identified that the two main factors contributing to the decrease of μ are counterion charge condensation on the highly charged colloid and an increase in effective friction of the macroion-counterion complex due to the condensed counterions. Thus the established O'Brien and White theory, which identified the dipolar force originating from distortion of the electric double layer as the cause of decreasing μ, seems to break down for the case of highly charged colloids with σ in the range of 30-400 µC cm( - 2). To arrive at our conclusions, we counted the number of counterions q(0) moving along with the spherical macroion. We observe in our simulations that q(0) increases with the increase of bare charge Q, such that the effective charge Q(eff) = Q - q(0) remains approximately constant. Interestingly for our nanometer-sized charged colloid, we observe that, if surface charge density σ of the colloid is increased by decreasing the radius R(M) of the colloid but fixed bare charge Q, the effective charge Q - q(0) decreases with the increase of σ. This behavior is qualitatively different when σ is increased by increasing Q keeping R(M) fixed. Our observations address a controversy about the effective charge of a strongly charged macroion: some studies claim that effective charge is independent of the bare charge (Alexander et al 1984 J. Chem. Phys. 80 5776; Trizac et al 2003 Langmuir 19 4027) whereas others claim that Q(eff) decreases with increasing bare charge Q (dos Santos 2009 J. Chem. Phys. 130 124110; Diehl et al 2004 J. Chem. Phys. 121 12100; Groot et al 1991 J. Chem. Phys. 95 9191) at relatively high values of σ.
Journal of Physics Condensed Matter 11/2010; 22(49):494102. · 2.55 Impact Factor
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ABSTRACT: The mass transport in soft-sphere mixtures of small and big particles as well as in the disordered Lorentz gas (LG) model is studied using molecular dynamics (MD) computer simulations. The soft-sphere mixture shows anomalous small-particle diffusion signifying a localization transition separate from the big-particle glass transition. Switching off small-particle excluded volume constraints slows down the small-particle dynamics, as indicated by incoherent intermediate scattering functions. A comparison of logarithmic time derivatives of the mean-squared displacements reveals qualitative similarities between the localization transition in the soft-sphere mixture and its counterpart in the LG. Nevertheless, qualitative differences emphasize the need for further research elucidating the connection between both models.
The European Physical Journal Special Topics 06/2010; · 1.56 Impact Factor
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ABSTRACT: A coarse-grained model for colloid-polymer mixtures is investigated where both colloids and polymer coils are represented as point-like particles interacting with spherically symmetric effective potentials. Colloid-colloid and colloid-polymer interactions are described by Weeks-Chandler-Andersen potentials, while the polymer-polymer interaction is very soft, of strength k(B)T/2 for maximum polymer-polymer overlap. This model can be efficiently simulated both by Monte Carlo and molecular dynamics methods, and its phase diagram closely resembles that of the well-known Asakura-Oosawa model. The static and dynamic properties of the model are presented for systems at critical colloid density, varying the polymer density in the one-phase region. Applying Lees-Edwards boundary conditions, colloid-polymer mixtures exposed to shear deformation are considered, and the resulting anisotropy of correlations is studied. Whereas for the considered shear rate, [Formula: see text], radial distribution functions and static structure factors indicate only small structural changes under shear, an appropriate projection of these correlation functions onto spherical harmonics is presented that allows us to directly quantify the structural anisotropies. However, the considered shear rate is probably not high enough to see anisotropies in static structure factors at small wavenumbers that have been predicted by Onuki and Kawasaki (1979 Ann. Phys. 121 456) for the critical behavior of systems under shear. The anomalous dependence of the polymer's self-diffusion constant on polymer density is referred to the clustering of the colloid particles when approaching the critical point.
Journal of Physics Condensed Matter 03/2010; 22(10):104120. · 2.55 Impact Factor
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ABSTRACT: We investigate nanoflows through dilute disordered media by means of joint lattice Boltzmann (LB) and molecular dynamics (MD) simulations -when the size of the obstacles is comparable to the size of the flowing particles-for randomly located spheres and for a correlated particle-gel. In both cases at sufficiently low solid fraction, Phi < 0.01, LB and MD provide similar values of the permeability. However, for Phi > 0.01, MD shows that molecular-size effects lead to a decrease of the permeability, as compared to the Navier-Stokes predictions. For gels, the simulations highlights a surplus of permeability, which can be accommodated within a rescaling of the effective radius of the gel monomers.
EPL (Europhysics Letters) 07/2009; 89(4):44001. · 2.17 Impact Factor
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ABSTRACT: We propose a new coarse-grained model for the description of liquid-vapor phase separation of colloid-polymer mixtures. The hard-sphere repulsion between colloids, and between colloids and polymers, which is used in the well-known Asakura-Oosawa (AO) model, is replaced with Weeks-Chandler-Andersen potentials. Similarly, a soft potential of height comparable to thermal energy is used for the polymer-polymer interaction, rather than treating polymers as ideal gas particles. It is shown by grand-canonical Monte Carlo simulations that this model leads to a coexistence curve that almost coincides with that of the AO model and that the Ising critical behavior of static quantities is reproduced. Then the main advantage of the model is exploited-its suitability for Molecular Dynamics simulations-to study the dynamics of mean square displacements of the particles, transport coefficients such as the self-diffusion and interdiffusion coefficients, and dynamic structure factors. While the self-diffusion of polymers increases slightly when the critical point is approached, the self-diffusion of colloids decreases and at criticality the colloid self-diffusion coefficient is about a factor of 10 smaller than that of the polymers. Critical slowing down of interdiffusion is observed, which is qualitatively similar to symmetric binary Lennard-Jones mixtures, for which no dynamic asymmetry of self-diffusion coefficients occurs.
The Journal of chemical physics 03/2009; 130(6):064906. · 3.09 Impact Factor
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ABSTRACT: A description of phase separation kinetics for solid binary (A,B) mixtures in thin film geometry based on the Kawasaki spin-exchange kinetic Ising model is presented in a discrete lattice molecular field formulation. It is shown that the model describes the interplay of wetting layer formation and lateral phase separation, which leads to a characteristic domain size l(t) in the directions parallel to the confining walls that grows according to the Lifshitz-Slyozov t;{13} law with time t after the quench. Near the critical point of the model, the description is shown to be equivalent to the standard treatments based on Ginzburg-Landau models. Unlike the latter, the present treatment is reliable also at temperatures far below criticality, where the correlation length in the bulk is only of the order of a lattice spacing, and steep concentration variations may occur near the walls, invalidating the gradient square approximation. A further merit is that the relation to the interaction parameters in the bulk and at the walls is always transparent, and the correct free energy at low temperatures is consistent with the time evolution by construction.
Physical Review E 03/2009; 79(2 Pt 1):021602. · 2.26 Impact Factor
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ABSTRACT: The application of quasielastic neutron scattering and molecular dynamics simulation to the study of mass transport in silicate
melts is outlined. It is shown how the knowledge of atomic dynamics and structure reveals the mechanisms of mass transport.
Peculiar properties of atomic diffusion and viscous flow behaviour as a function of melt composition are discussed in terms
of the formation of alkali diffusion channels in the static structure. This non-homogeneous distribution of alkali ions in
a disrupted tetrahedral Si–O network is investigated in binary lithium, sodium and potassium silicate melts and in ternary
sodium aluminosilicates and sodium ironsilicates representing the main compositions of natural volcanic rocks.
12/2008: pages 189-209;
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ABSTRACT: Monte Carlo simulations of the Asakura-Oosawa model for colloid-polymer mixtures confined between two parallel repulsive structureless walls are presented and analyzed in the light of current theories on capillary condensation and interface localization transitions. Choosing a polymer-to-colloid size ratio of q=0.8 and studying ultrathin films in the range of D=3 to D=10 colloid diameters thickness, grand canonical Monte Carlo methods are used; phase transitions are analyzed via finite size scaling, as in previous work on bulk systems and under confinement between identical types of walls. Unlike the latter work, inequivalent walls are used here: While the left wall has a hard-core repulsion for both polymers and colloids, at the right-hand wall an additional square-well repulsion of variable strength acting only on the colloids is present. We study how the phase separation into colloid-rich and colloid-poor phases occurring already in the bulk is modified by such a confinement. When the asymmetry of the wall-colloid interaction increases, the character of the transition smoothly changes from capillary condensation type to interface localization type. For very thin films (i.e., for D=3 ) and a suitable choice of the wall-colloid interactions, evidence is found that the critical behavior falls in the universality class of the two-dimensional Ising model. Otherwise, we observe crossover scaling between different universality classes (namely, the crossover from the three-dimensional to the two-dimensional Ising model universality class). The colloid and polymer density profiles across the film in the various phases are discussed, as well as the correlation of interfacial fluctuations in the direction parallel to the confining walls. The broadening of the interface between the coexisting colloid-rich and polymer-rich phases (located parallel to the confining walls) is understood in terms of capillary wave fluctuations. The experimental observability of all these phenomena is briefly discussed.
Physical Review E 11/2008; 78(4 Pt 1):041604. · 2.26 Impact Factor
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ABSTRACT: When systems that can undergo phase separation between two coexisting phases in the bulk are confined in thin film geometry between parallel walls, the phase behavior can be profoundly modified. These phenomena shall be described and exemplified by computer simulations of the Asakura-Oosawa model for colloid-polymer mixtures, but applications to other soft matter systems (e.g. confined polymer blends) will also be mentioned. Typically a wall will prefer one of the phases, and hence the composition of the system in the direction perpendicular to the walls will not be homogeneous. If both walls are of the same kind, this effect leads to a distortion of the phase diagram of the system in thin film geometry, in comparison with the bulk, analogous to the phenomenon of "capillary condensation" of simple fluids in thin capillaries. In the case of "competing walls", where both walls prefer different phases of the two phases coexisting in the bulk, a state with an interface parallel to the walls gets stabilized. The transition from the disordered phase to this "soft mode phase" is rounded by the finite thickness of the film and not a sharp phase transition. However, a sharp transition can occur where this interface gets localized at (one of) the walls. The relation of this interface localization transition to wetting phenomena is discussed. Finally, an outlook to related phenomena is given, such as the effects of confinement in cylindrical pores on the phase behavior, and more complicated ordering phenomena (lamellar mesophases of block copolymers or nematic phases of liquid crystals under confinement).
Soft Matter. 05/2008; 4(2008):1555-1568.
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ABSTRACT: A fitting scheme is proposed to obtain effective potentials from Car-Parrinello molecular dynamics (CPMD) simulations. It is used to parameterize a new pair potential for silica. MD simulations with this new potential are done to determine structural and dynamic properties and to compare these properties to those obtained from CPMD and a MD simulation using the so-called BKS potential. The new potential reproduces accurately the liquid structure generated by the CPMD trajectories, the experimental activation energies for the self-diffusion constants and the experimental density of
amorphous silica. Also lattice parameters and elastic constants of alpha quartz are well-reproduced, showing the transferability of the new potential.
EPL (Europhysics Letters) 03/2008; 82(2008):17001. · 2.17 Impact Factor
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Computer Physics Communications. 01/2008; 179:1-7.
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ABSTRACT: We show that finite-range alternatives to the standard long-range pair potential for silica by van Beest et al. [Phys. Rev. Lett. 64, 1955 (1990)] might be used in molecular dynamics simulations. We study two such models that can be efficiently simulated since no Ewald summation is required. We first consider the Wolf method, where the Coulomb interactions are truncated at a cutoff distance rc such that the requirement of charge neutrality holds. Various static and dynamic quantities are computed and compared to results from simulations using Ewald summations. We find very good agreement for rc approximately 10 A. For lower values of rc, the long-range structure is affected which is accompanied by a slight acceleration of dynamic properties. In a second approach, the Coulomb interaction is replaced by an effective Yukawa interaction with two new parameters determined by a force fitting procedure. The same trend as for the Wolf method is seen. However, slightly larger cutoffs have to be used in order to obtain the same accuracy with respect to static and dynamic quantities as for the Wolf method.
The Journal of Chemical Physics 10/2007; 127(11):114512. · 3.33 Impact Factor
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ABSTRACT: An introduction to the Molecular Dynamics (MD) simulation of chemically realistic models for undercooled fluids and glasses
is given, emphasizing silicatic materials such as molten silicon dioxide and its mixtures with sodium oxide and aluminium
oxide, and comparing the simulation results to experimental data whenever possible.
A key ingredient to the computer simulation of materials is a sufficiently accurate description of the force fields with which
the atoms interact. The need to simulate large systems for sufficiently long times makes the use of effective potentials for
classical MD methods desirable. The validation of such effective potentials is best done studying the corresponding crystalline
states of the material. As an example, the use of the so-called BKS-potential studying the structural and thermal properties
of quartz crystals is described, and a comparison to other potentials is discussed.
When one studies undercooled fluids and glasses, a second problem enters, the disparity between experimental cooling rates
and the much larger rates of the simulation. The extent to which a meaningful comparison to experiments is nevertheless possible
is discussed. It is shown that the simulations can reproduce the structural and dynamic properties of molten silica (including
self diffusion coefficients, viscosity, sound velocity, etc.) and its mixtures with other oxides. Evidence for the formation
of sodium-rich channels responsible for anomalously large diffusion constants of sodium in mixtures containing sodium oxide
will be discussed. No enhanced diffusion of aluminium occurs, however, due to “tricluster” formation in such mixtures.
03/2007: pages 1-32;
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ABSTRACT: Using computer simulations, the electrophoretic motion of a positively charged colloid (macroion) in an electrolyte solution is studied in the framework of the primitive model. In this model, the electrolyte is considered as a system of negatively and positively charged microions (counterions and coions, respectively) that are immersed into a structureless medium. Hydrodynamic interactions are fully taken into account by applying a hybrid simulation scheme, where the charged ions (i.e., macroion and electrolyte), propagated via molecular dynamics, are coupled to a lattice Boltzmann (LB) fluid. In a recent electrophoretic experiment by Martin-Molina et al. [J. Phys. Chem. B 106, 6881 (2002)], it was shown that, for multivalent salt ions, the mobility mu initially increases with charge density sigma, reaches a maximum, and then decreases with further increase of sigma. The aim of the present work is to elucidate the behavior of mu at high values of sigma. Even for the case of monovalent microions, a decrease of mu with sigma is found. A dynamic Stern layer is defined that includes all the counterions that move with the macroion while subjected to an external electrical field. The number of counterions in the Stern layer, q(0), is a crucial parameter for the behavior of mu at high values of sigma. In this case, the mobility mu depends primarily on the ratio q(0)/Q (with Q the valency of the macroion). The previous contention that the increase in the distortion of the electric double layer (EDL) with increasing sigma leads to the lowering of mu does not hold for high sigma. In fact, it is shown that the deformation of the EDL decreases with the increase of sigma. The role of hydrodynamic interactions is inferred from direct comparisons to Langevin simulations where the coupling to the LB fluid is switched off. Moreover, systems with divalent counterions are considered. In this case, at high values of sigma the phenomenon of charge inversion is found.
The Journal of Chemical Physics 03/2007; 126(6):064907. · 3.33 Impact Factor
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ABSTRACT: Using computer simulations, the electrophoretic motion of a positively charged colloid (macroion) in an electrolyte solution is studied in the framework of the primitive model. Hydrodynamic interactions are fully taken into account by applying a hybrid simulation scheme, where the charged ions (i.e. macroion and electrolyte), propagated via molecular dynamics (MD), are coupled to a Lattice Boltzmann (LB) fluid. In a recent experiment it was shown that, for multivalent salt ions, the mobility $\mu$ initially increases with charge density $\sigma$, reaches a maximum and then decreases with further increase of $\sigma$. The aim of the present work is to elucidate the behaviour of $\mu$ at high values of $\sigma$. Even for the case of monovalent microions, we find a decrease of $\mu$ with $\sigma$. A dynamic Stern layer is defined that includes all the counterions that move with the macroion while subject to an external electrical field. The number of counterions in the Stern layer, $q_0$, is a crucial parameter for the behavior of $\mu$ at high values of $\sigma$. In this case, the mobility $\mu$ depends primarily on the ratio $q_0/Q$ (with $Q$ the valency of the macroion). The previous contention that the increase in the distortion of the electric double layer (EDL) with increasing $\sigma$ leads to the lowering of $\mu$ does not hold for high $\sigma$. In fact, we show that the deformation of the EDL decreases with increase of $\sigma$. The role of hydrodynamic interactions is inferred from direct comparisons to Langevin simulations where the coupling to the LB fluid is switched off. Moreover, systems with divalent counterions are considered. In this case, at high values of $\sigma$ the phenomenon of charge inversion is found.
01/2007;
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ABSTRACT: When simple or complex fluids are confined to ultrathin films or channels or other cavities of nanoscopic linear dimensions, the interplay of finite size and surface controls the phase behavior, and may lead to phase transitions rather different from the corresponding phenomena in the bulk. Monte Carlo simulation is a very suitable tool to clarify the complex behavior of such systems, since the boundary conditions providing the confinement can be controlled and arbitrarily varied, and detailed structural information on the inhomogeneous states of the considered systems is available. Examples used to illustrate these concepts include simple Ising models in pores and double-pyramid-shaped cavities with competing surface fields, where novel types of interface localization–delocalization phenomena occur accompanied by “macroscopic” fluctuations, and colloid-polymer mixtures confined in slit pores. Finite size scaling concepts are shown to be a useful tool also for such systems “in between” the dimensionalities.
Computer Physics Communications. 01/2007;
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ABSTRACT: We report comprehensive simulations of the critical dynamics of a symmetric binary Lennard-Jones mixture near its consolute point. The self-diffusion coefficient exhibits no detectable anomaly. The data for the shear viscosity and the mutual-diffusion coefficient are fully consistent with the asymptotic power laws and amplitudes predicted by renormalization-group and mode-coupling theories provided finite-size effects and the background contribution to the relevant Onsager coefficient are suitably accounted for. This resolves a controversy raised by recent molecular simulations.
Physical Review Letters 08/2006; 97(2):025702. · 7.37 Impact Factor
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ABSTRACT: A fluid flow in a simple dense liquid, passing an obstacle in a two-dimensional thin film geometry, is simulated by molecular dynamics (MD) computer simulation and compared to results of lattice Boltzmann (LB) simulations. By the appropriate mapping of length and time units from LB to MD, the velocity field as obtained from MD is quantitatively reproduced by LB. The implications of this finding for prospective LB-MD multiscale applications are discussed.
Physical Review Letters 07/2006; 96(22):224503. · 7.37 Impact Factor
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ABSTRACT: A symmetrical binary, A+B Lennard-Jones mixture is studied by a combination of semi-grand-canonical Monte Carlo (SGMC) and molecular dynamics (MD) methods near a liquid-liquid critical temperature T(c). Choosing equal chemical potentials for the two species, the SGMC switches identities (A-->B-->A) to generate well-equilibrated configurations of the system on the coexistence curve for T<T(c) and at the critical concentration, x(c)=12, for T>T(c). A finite-size scaling analysis of the concentration susceptibility above T(c) and of the order parameter below T(c) is performed, varying the number of particles from N=400 to 12 800. The data are fully compatible with the expected critical exponents of the three-dimensional Ising universality class. The equilibrium configurations from the SGMC runs are used as initial states for microcanonical MD runs, from which transport coefficients are extracted. Self-diffusion coefficients are obtained from the Einstein relation, while the interdiffusion coefficient and the shear viscosity are estimated from Green-Kubo expressions. As expected, the self-diffusion constant does not display a detectable critical anomaly. With appropriate finite-size scaling analysis, we show that the simulation data for the shear viscosity and the mutual diffusion constant are quite consistent both with the theoretically predicted behavior, including the critical exponents and amplitudes, and with the most accurate experimental evidence.
The Journal of Chemical Physics 07/2006; 125(2):24506. · 3.33 Impact Factor
