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We present the results of large-scale computer simulations in order to discuss the structural and dynamic properties of sodium silicate melts with the compositions (Na2O)2(SiO2) (NS2) and (Na2O)20(SiO2) (NS20). We show that, compared to silica (SiO2), these systems exhibit additional intermediate range order on intermediate length scales that stem from the tetrahedral network structure. By means of intermediate-scattering functions, we characterize the dynamics of sodium in the system under consideration. Whereas in NS2 the incoherent scattering functions for Na decay much faster to zero than the coherent ones for Na–Na, in NS20 this different behaviour of the incoherent and coherent functions is not very pronounced. On the other hand, the incoherent functions of the two systems share a very peculiar feature: their long-time decay can be described by a Kohlrausch law with a constant exponent β for q_{th}$>q > qth where qth is significantly below the location of the main peak in the static structure factor for the Na–Na correlations.

We use molecular dynamics computer simulations to study the relaxation dynamics of Na2O-2(SiO2) in its molten, highly viscous state. We find that at low temperatures the incoherent intermediate scattering function for Na relaxes about 100 times faster than the one of the Si and O atoms. In contrast to this all coherent functions relax on the same time scale if the wave-vector is around 1AA^-1. This anomalous relaxation dynamics is traced back to the channel-like structure for the Na atoms that have been found for this system. We find that the relaxation dynamics for Si and O as well as the time dependence of the coherent functions for Na can be rationalized well by means of mode-coupling theory. In particular we show that the diffusion constants as well as the alpha-relaxation times follow the power-law predicted by the theory and that in the beta-relaxation regime the correlators obey the factorization property with a master curve that is described well by a von Schweidler-law. The value of the von Schweidler exponent $b$ is compatible with the one found for the mentioned power-law of the relaxation times/diffusion constants. Finally we study the wave-vector dependence of f_s(q) and f(q), the coherent and incoherent non-ergodicity parameters. For the Si and O atoms these functions look qualitatively similar to the ones found in simple liquids or pure silica, in that the coherent function oscillates (in phase with the static structure factor) around the incoherent one and in that the latter is approximated well by a Gaussian function. In contrast to this, f(q) for Na-Na is always smaller than f_s(q) for Na and the latter can be approximated by a Gaussian only for relatively large q., Comment: 20 pages of Latex, 8 figures

Extensive molecular dynamics simulations are used to investigate the phase separation kinetics in a glass-forming binary Lennard-Jones mixture. The focus is on the two-phase region at low temperatures (i.e. below the glass transition line), where coexistence between a low-density gas with a metastable amorphous solid, i.e. a glass occurs. Two different quench paths are chosen to get into the two-phase region starting from a structurally homogeneous state, one along which temperature is lowered at a fixed density, and in the other case, the volume is expanded to reach lower densities at fixed temperatures. Both paths are explored by tuning the rates of cooling or expansion, respectively. We analyze thermodynamic and structural properties of the phase-separating systems, in particular with respect to differences in the morphologies that are obtained from the different quench protocols.

Crystal-liquid interfaces in nickel are investigated by molecular-dynamics computer simulations. Inhomogeneous systems of size Lx×Ly×Lz with Lz=5Ly are prepared where the crystal fcc phase at different orientations coexists with the liquid phase, separated by planar interfaces in the xy-plane. The lateral dimensions are varied, using two different geometries with Lx=Ly and with LyLx. In the framework of capillary wave theory (CWT), anisotropic interfacial stiffnesses and tensions are determined using different predictions of CWT with respect to the spectrum, finite-size broadening and different geometries. From a parameterization in terms of cubic harmonics up to 8th order, the anisotropic interfacial free energy is obtained.

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 q0 moving along with the spherical macroion. We observe in our simulations that q0 increases with the increase of bare charge Q, such that the effective charge Qeff = Q - q0 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 RM of the colloid but fixed bare charge Q, the effective charge Q - q0 decreases with the increase of σ. This behavior is qualitatively different when σ is increased by increasing Q keeping RM 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 Qeff 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.

The critical behavior of the two-dimensional Ising Model with non-conserved order parameter in steady-state shear is studied by large-scale Monte Carlo simulations. Studying the structure factor S(qx,qy) in the disordered phase, the ratio of correlation length exponents νx/νy in the two lattice directions (x,y) is estimated, and the critical temperature is determined as a function of the shear rate as Tc() − Tc(0) ∝ with ≈0.45. Critical exponents β≈0.37, γ≈1.1, ; ν⊥≈0.46, ν∥≈1.38 are roughly compatible with anisotropic hyperscaling.

Molecular-dynamics computer simulations in conjunction with Lees-Edwards boundary conditions are used to investigate a glass-forming binary Yukawa fluid under shear. The transition from the elastic response to plastic flow is elucidated by studying the stress relaxation after switching off the shear. We find a slow stress relaxation starting from states in the elastic regime and a fast one starting from states in the plastic-flow regime. We show that these relaxation patterns are related to a different distribution of local microscopic stresses in both cases.

Using Molecular Dynamics (MD) and Monte Carlo (MC) simulations interfacial properties of crystal-fluid interfaces are investigated for the hard sphere system and the one-component metallic system Ni (the latter modeled by a potential of the embedded atom type). Different local order parameters are considered to obtain order parameter profiles for systems where the crystal phase is in coexistence with the fluid phase, separated by interfaces with (100) orientation of the crystal. From these profiles, the mean-squared interfacial width w^2 is extracted as a function of system size. We rationalize the prediction of capillary wave theory that w^2 diverges logarithmically with the lateral size of the system. We show that one can estimate the interfacial stiffness from the interfacial broadening, obtaining 0.5 k_B T/sigma^2 for hard spheres and 0.18 J/m^2 for Ni., 15 pages, 7 figures

The high-pressure dynamics of a computer-modeled silica melt is studied in the framework of the mode-coupling theory of the glass transition (MCT) using static-structure input from molecular-dynamics (MD) computer simulation. The theory reproduces the experimentally known viscosity minimum (diffusivity maximum) as a function of density or pressure and explains it in terms of a corresponding minimum in its critical temperature. This minimum arises from a gradual change in the equilibrium static structure which shifts from being dominated by tetrahedral ordering to showing the cageing known from high-density liquids. The theory is in qualitative agreement with computer simulation results., Presented at ESF EW Glassy Liquids under Pressure, to be published in Journal of Physics

The melting and crystallization of Al50Ni50} are studied by means of molecular dynamics computer simulations, using a potential of the embedded atom type to model the interactions between the particles. Systems in a slab geometry are simulated where the B2 phase of AlNi in the middle of an elongated simulation box is separated by two planar interfaces from the liquid phase, thereby considering the (100) crystal orientation. By determining the temperature dependence of the interface velocity, an accurate estimate of the melting temperature is provided. The value k=0.0025 m/s/K for the kinetic growth coefficient is found. This value is about two orders of magnitude smaller than that found in recent simulation studies of one-component metals. The classical Wilson-Frenkel model is not able to describe the crystal growth kinetics on a quantitative level. We argue that this is due to the neglect of diffusion processes in the liquid-crystal interface., 6 pages, 6 figures