14 research outputs found
Overdamped van Hove function of atomic liquids
Using the generalized Langevin equation formalism and the process of
contraction of the description we derive a general memory function equation for
the thermal fluctuations of the local density of a simple atomic liquid. From
the analysis of the long-time limit of this equation, a striking equivalence is
suggested between the long-time dynamics of the atomic liquid and the dynamics
of the corresponding \emph{Brownian} liquid. This dynamic equivalence is
confirmed here by comparing molecular and Brownian dynamics simulations of the
self-intermediate scattering function and the long-time self-diffusion
coefficient for the hard-sphere liquid.Comment: 4 Figures, 23 page
Arrested dynamics of the dipolar hard-sphere model
We report the combined results of molecular dynamics simulations and
theoretical calculations concerning various dynamical arrest transitions in a
model system representing a dipolar fluid, namely, N (softcore) rigid spheres
interacting through a truncated dipole-dipole potential. By exploring different
regimes of concentration and temperature, we find three distinct scenarios for
the slowing down of the dynamics of the translational and orientational degrees
of freedom: At low ({} = 0.2) and intermediate ( = 0.4) volume
fractions, both dynamics are strongly coupled and become simultaneously
arrested upon cooling. At high concentrations ({} 0.6), the
translational dynamics shows the features of an ordinary glass transition,
either by compressing or cooling down the system, but with the orientations
remaining ergodic, thus indicating the existence of partially arrested states.
In this density regime, but at lower temperatures, the relaxation of the
orientational dynamics also freezes. The physical scenario provided by the
simulations is discussed and compared against results obtained with the
self-consistent generalized Langevin equation theory, and both provide a
consistent description of the dynamical arrest transitions in the system. Our
results are summarized in an arrested states diagram which qualitatively
organizes the simulation data and provides a generic picture of the glass
transitions of a dipolar fluid
Equilibration of Concentrated Hard Sphere Fluids
We report a systematic molecular dynamics study of the isochoric
equilibration of hard-sphere fluids in their metastable regime close to the
glass transition. The thermalization process starts with the system prepared in
a non-equilibrium state with the desired final volume fraction {\phi} but with
a prescribed non-equilibrium static structure factor S_0(k; {\phi}). The
evolution of the {\alpha}- relaxation time {\tau}{\alpha} (k) and long-time
self-diffusion coefficient DL as a function of the evolution time tw is then
monitored for an array of volume fractions. For a given waiting time the plot
of {\tau}{\alpha} (k; {\phi}, tw) as a function of {\phi} exhibits two regimes
corresponding to samples that have fully equilibrated within this waiting time
({\phi} \leq {\phi}(c) (tw)), and to samples for which equilibration is not yet
complete ({\phi} \geq {\phi}(c) (tw)). The crossover volume fraction {\phi}(c)
(tw) increases with tw but seems to saturate to a value {\phi}(a) \equiv
{\phi}(c) (tw \rightarrow \infty) \approx 0.582. We also find that the waiting
time t^(eq)_w({\phi}) required to equilibrate a system grows faster than the
corresponding equilibrium relaxation time, t^(eq)({\phi}) \approx 0.27 \times
[{\tau}{\alpha} (k; {\phi})]^1.43, and that both characteristic times increase
strongly as {\phi} approaches {\phi}^(a), thus suggesting that the measurement
of equilibrium properties at and above {\phi}(a) is experimentally impossible
Spherical harmonic projections of the static structure factor of the dipolar hard sphere model: Theory vs simulations
We investigate the static correlations of a dipolar fluid in terms of the irreducible coefficients of the spherical harmonic expansion of the static structure factor. To this end, we develop a theoretical framework based on a soft-core version of Wertheim's solution of the mean spherical approximation (MSA), which renders the analytical determination of such coefficients possible. The accuracy of this approximation is tested by a comparison against the results obtained with the assistance of extensive molecular dynamics simulations at different regimes of concentration and temperature. Crucial aspects for the comparison of the results provided by the two methods are carefully discussed, concerning the different reference frames used in theory and simulations to describe rotations and orientations, and leading to important differences in the behavior of correlation functions with the same combination of spherical harmonic indices. We find a remarkable agreement between the two approaches in the fluid regime, thus providing a first stringent comparison of the irreducible coefficients of the spherical harmonic expansion of the dipolar fluid's static structure factor, provided by the MSA theory and molecular dynamics simulations