15 research outputs found
Heterogeneous Diffusion in Highly Supercooled Liquids
The diffusivity of tagged particles is demonstrated to be very heterogeneous
on time scales comparable to or shorter than the relaxation time
( the stress relaxation time) in a highly supercooled
liquid via 3D molecular dynamics simulation. The particle motions in the
relatively active regions dominantly contribute to the mean square
displacement, giving rise to a diffusion constant systematically larger than
the Einstein-Stokes value. The van Hove self-correlation function is
shown to have a long distance tail which can be scaled in terms of
for t \ls 3\tau_{\alpha}. Its presence indicates heterogeneous diffusion in
the active regions. However, the diffusion process eventually becomes
homogeneous on time scales longer than the life time of the heterogeneity
structure ().Comment: 4 pages, 5 figure
Dynamical heterogeneities in a supercooled Lennard-Jones liquid
We present the results of a large scale molecular dynamics computer
simulation study in which we investigate whether a supercooled Lennard-Jones
liquid exhibits dynamical heterogeneities. We evaluate the non-Gaussian
parameter for the self part of the van Hove correlation function and use it to
identify ``mobile'' particles. We find that these particles form clusters whose
size grows with decreasing temperature. We also find that the relaxation time
of the mobile particles is significantly shorter than that of the bulk, and
that this difference increases with decreasing temperature.Comment: 8 pages of RevTex, 4 ps figure
Nanometer Scale Dielectric Fluctuations at the Glass Transition
Using non-contact scanning probe microscopy (SPM) techniques, dielectric
properties were studied on 50 nanometer length scales in poly-vinyl-acetate
(PVAc) films in the vicinity of the glass transition. Low frequency (1/f) noise
observed in the measurements, was shown to arise from thermal fluctuations of
the electric polarization. Anomalous variations observed in the noise spectrum
provide direct evidence for cooperative nano-regions with heterogeneous
kinetics. The cooperative length scale was determined. Heterogeneity was
long-lived only well below the glass transition for faster than average
processes.Comment: 4 pages, 4 embedded PS figures, RevTeX - To appear in Phys. Rev. Let
String-like Clusters and Cooperative Motion in a Model Glass-Forming Liquid
A large-scale molecular dynamics simulation is performed on a glass-forming
Lennard-Jones mixture to determine the nature of dynamical heterogeneities
which arise in this model fragile liquid. We observe that the most mobile
particles exhibit a cooperative motion in the form of string-like paths
(``strings'') whose mean length and radius of gyration increase as the liquid
is cooled. The length distribution of the strings is found to be similar to
that expected for the equilibrium polymerization of linear polymer chains.Comment: 6 pages of RevTex, 6 postscript figures, uses epsf.st