312 research outputs found

    Implicit Finite-Size Effects in Computer Simulations

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    The influence of periodic boundary conditions (implicit finite-size effects) on the anisotropy of pair correlations in computer simulations is studied for a dense classical fluid of pair-wise interacting krypton atoms near the triple point. Molecular dynamics simulation data for the pair distribution function of N-particle systems, as a function of radial distance, polar angle, and azimuthal angle are compared directly with corresponding theoretical predictions [L. R. Pratt and S. W. Haan, J. Chem. Phys. 74, 1864 (1981)]. For relatively small systems of N=60, 80, and 108 atoms, significant angular variation is observed, which is qualitatively, and in several cases quantitatively, well predicted by theory. Finite-size corrections to the spherically-averaged radial distribution function, however, are found to be comparable to random statistical errors for runs of 10^5 time steps.Comment: plain TeX, 14 pages + 16 postscript figures, to appear Z. Phys.

    Dynamics of a Rigid Rod in a Glassy Medium

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    We present simulations of the motion of a single rigid rod in a disordered static 2d-array of disk-like obstacles. The rotational, DRD_{\rm R}, and center-of-mass translational, DCMD_{\rm CM}, diffusion constants are calculated for a wide range of rod length LL and density of obstacles ρ\rho. It is found that DCMD_{\rm CM} follows the behavior predicted by kinetic theory for a hard disk with an effective radius R(L)R(L). A dynamic crossover is observed in DRD_{\rm R} for LL comparable to the typical distance between neighboring obstacles dnnd_{\rm nn}. Using arguments from kinetic theory and reptation, we rationalize the scaling laws, dynamic exponents, and prefactors observed for DRD_{\rm R}. In analogy with the enhanced translational diffusion observed in deeply supercooled liquids, the Stokes-Einstein-Debye relation is violated for L>0.6dnnL > 0.6d_{\rm nn}.Comment: 8 pages, 4 figures. Major changes. To be published in Europhysics Letter

    Fractional Stokes-Einstein and Debye-Stokes-Einstein relations in a network forming liquid

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    We study the breakdown of the Stokes-Einstein (SE) and Debye-Stokes-Einstein (DSE) relations for translational and rotational motion in a prototypical model of a network-forming liquid, the ST2 model of water. We find that the emergence of ``fractional'' SE and DSE relations at low temperature is ubiquitous in this system, with exponents that vary little over a range of distinct physical regimes. We also show that the same fractional SE relation is obeyed by both mobile and immobile dynamical heterogeneities of the liquid

    Long wavelength structural anomalies in jammed systems

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    The structural properties of static, jammed packings of monodisperse spheres in the vicinity of the jamming transition are investigated using large-scale computer simulations. At small wavenumber kk, we argue that the anomalous behavior in the static structure factor, S(k)kS(k) \sim k, is consequential of an excess of low-frequency, collective excitations seen in the vibrational spectrum. This anomalous feature becomes more pronounced closest to the jamming transition, such that S(0)0S(0) \to 0 at the transition point. We introduce an appropriate dispersion relation that accounts for these phenomena that leads us to relate these structural features to characteristic length scales associated with the low-frequency vibrational modes of these systems. When the particles are frictional, this anomalous behavior is suppressed providing yet more evidence that jamming transitions of frictional spheres lie at lower packing fractions that that for frictionless spheres. These results suggest that the mechanical properties of jammed and glassy media may therefore be inferred from measurements of both the static and dynamical structure factors.Comment: 8 pages, 6 figure captions. Completely revised version to appear in Phys. Rev.

    Lattice gas with ``interaction potential''

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    We present an extension of a simple automaton model to incorporate non-local interactions extending over a spatial range in lattice gases. {}From the viewpoint of Statistical Mechanics, the lattice gas with interaction range may serve as a prototype for non-ideal gas behavior. {}From the density fluctuations correlation function, we obtain a quantity which is identified as a potential of mean force. Equilibrium and transport properties are computed theoretically and by numerical simulations to establish the validity of the model at macroscopic scale.Comment: 12 pages LaTeX, figures available on demand ([email protected]

    Adding Salt to an Aqueous Solution of t-Butanol: Is Hydrophobic Association Enhanced or Reduced?

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    Recent neutron scattering experiments on aqueous salt solutions of amphiphilic t-butanol by Bowron and Finney [Phys. Rev. Lett. {\bf 89}, 215508 (2002); J. Chem. Phys. {\bf 118}, 8357 (2003)] suggest the formation of t-butanol pairs, bridged by a chloride ion via OH...Cl{O}-{H}...{Cl}^- hydrogen-bonds, and leading to a reduced number of intermolecular hydrophobic butanol-butanol contacts. Here we present a joint experimental/theoretical study on the same system, using a combination of molecular dynamics simulations and nuclear magnetic relaxation measurements. Both theory and experiment clearly support the more intuitive scenario of an enhanced number of hydrophobic contacts in the presence of the salt, as it would be expected for purely hydrophobic solutes [J. Phys. Chem. B {\bf 107}, 612 (2003)]. Although our conclusions arrive at a structurally completely distinct scenario, the molecular dynamics simulation results are within the experimental errorbars of the Bowron and Finney work.Comment: 15 pages twocolumn revtex, 11 figure

    Experimental observation of the intricate free-energy landscape for a soft glassy system

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    In the free energy landscape picture of glassy systems, the slow dynamics characteristic of these systems is believed to be due to the existence of a complicated free-energy landscape with many local minima. We show here that for a colloidal glassy material multiple paths can be taken through the free energy landscape, that can even lead to different 'final' non-ergodic states at the late stages of aging. We provide clear experimental evidence for the distinction of gel and glassy states in the system and show that for a range of colloid concentrations, the transition to non-ergodicity can occur in either direction (gel or glass), and may be accompanied by 'hesitations' between the two directions. This shows that colloidal gels and glasses are merely global free-energy minima in the same free energy landscape, and that the paths leading to these minima can indeed be complicated.Comment: 5 pages, 5 figure

    Microscopic dynamics and relaxation processes in liquid Hydrogen Fluoride

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    Inelastic x-ray scattering and Brillouin light scattering measurements of the dynamic structure factor of liquid hydrogen fluoride have been performed in the temperature rangeT=214÷283K T=214\div 283 K. The data, analysed using a viscoelastic model with a two timescale memory function, show a positive dispersion of the sound velocity c(Q)c(Q) between the low frequency value c0(Q)c_0(Q) and the high frequency value cα(Q)c_{\infty \alpha}(Q). This finding confirms the existence of a structural (α\alpha) relaxation directly related to the dynamical organization of the hydrogen bonds network of the system. The activation energy EaE_a of the process has been extracted by the analysis of the temperature behavior of the relaxation time τα(T)\tau_\alpha(T) that follows an Arrhenius law. The obtained value for EaE_a, when compared with that observed in another hydrogen bond liquid as water, suggests that the main parameter governing the α\alpha-relaxation process is the number of the hydrogen bonds per molecule.Comment: 9 pages and 12 figure

    Weak links between fast mobility and local structure in molecular and atomic liquids

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    We investigate by molecular-dynamics simulations, the fast mobility-the rattling amplitude of the particles temporarily trapped by the cage of the neighbors-in mildly supercooled states of dense molecular (linear trimers) and atomic (binary mixtures) liquids. The mixture particles interact by the Lennard-Jones potential. The non-bonded particles of the molecular system are coupled by the more general Mie potential with variable repulsive and attractive exponents in a range which is a characteristic of small n-alkanes and n-alcohols. Possible links between the fast mobility and the geometry of the cage (size and shape) are searched. The correlations on a per-particle basis are rather weak. Instead, if one groups either the particles in fast-mobility subsets or the cages in geometric subsets, the increase of the fast mobility with both the size and the asphericity of the cage is revealed. The observed correlations are weak and differ in states with equal relaxation time. Local forces between a tagged particle and the first-neighbour shell do not correlate with the fast mobility in the molecular liquid. It is concluded that the cage geometry alone is unable to provide a microscopic interpretation of the known, universal link between the fast mobility and the slow structural relaxation. We suggest that the particle fast dynamics is affected by regions beyond the first neighbours, thus supporting the presence of collective, extended fast modes

    Triplet correlations in two-dimensional colloidal model liquids

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    Three-body distribution functions in classical fluids have been theoretically investigated many times, but have never been measured directly. We present experimental three-point correlation functions that are computed from particle configurations measured by means of video-microscopy in two types of quasi-two-dimensional colloidal model fluids: a system of charged colloidal particles and a system of paramagnetic colloids. In the first system the particles interact via a Yukawa potential, in the second via a potential Γ/r3\Gamma/r^{3}. We find for both systems very similar results: on increasing the coupling between the particles one observes the gradual formation of a crystal-like local order due to triplet correlations, even though the system is still deep inside the fluid phase. These are mainly packing effects as is evident from the close resemblance between the results for the two systems having completely different pair-interaction potentials.Comment: many pages, 8 figures, contribution to the special issue in J.Phys. Cond. Mat. of the CECAM meeting in LYON ''Many-body....'
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