467 research outputs found

### Rheology of a Supercooled Polymer Melt

Molecular dynamics simulations are performed for a polymer melt composed of
short chains in quiescent and sheared conditions. The stress relaxation
function $G(t)$ exhibits a stretched exponential form in a relatively early
stage and ultimately follows the Rouse function in quiescent supercooled state.
Transient stress evolution after application of shear obeys the linear growth
$\int_0^t dt'G(t')$ for strain less than 0.1 and then saturates into a
non-Newtonian viscosity. In steady states, strong shear-thinning and elongation
of chains into ellipsoidal shapes are found at extremely small shear. A glassy
component of the stress is much enhanced in these examples.Comment: 4 pages, 5 figure

### Velocity relaxation of a particle in a confined compressible fluid

The velocity relaxation of an impulsively forced spherical particle in a
fluid confined by two parallel plane walls is studied using a direct numerical
simulation approach. During the relaxation process, the momentum of the
particle is transmitted in the ambient fluid by viscous diffusion and sound
wave propagation, and the fluid flow accompanied by each mechanism has a
different character and affects the particle motion differently. Because of the
bounding walls, viscous diffusion is hampered, and the accompanying shear flow
is gradually diminished. However, the sound wave is repeatedly reflected and
spreads diffusely. As a result, the particle motion is governed by the sound
wave and backtracks differently in a bulk fluid. The time when the backtracking
of the particle occurs changes non-monotonically with respect to the
compressibility factor and is minimized at the characteristic compressibility
factor. This factor depends on the wall spacing, and the dependence is
different at small and large wall spacing regions based on the different
mechanisms causing the backtracking.Comment: 8 pages, 9 figure

### Synchronized molecular-dynamics simulation for the thermal lubrication of a polymeric liquid between parallel plates

The Synchronized Molecular-Dynamics simulation which was recently proposed by
authors [Phys. Rev. X {\bf 4}, 041011 (2014)] is applied to the analysis of
polymer lubrication between parallel plates. The rheological properties,
conformational change of polymer chains, and temperature rise due to the
viscous heating are investigated with changing the values of thermal
conductivity of the polymeric liquid. It is found that at a small applied shear
stress on the plate, the temperature of polymeric liquid only slightly
increases in inverse proportion to the thermal conductivity and the apparent
viscosity of polymeric liquid is not much affected by changing the thermal
conductivity. However, at a large shear stress, the transitional behaviors of
the polymeric liquid occur due to the interplay of the shear deformation and
viscous heating by changing the thermal conductivity. This transition is
characterized by the Nahme-Griffith number $Na$ which is defined as the ratio
of the viscous heating to the thermal conduction at a characteristic
temperature. When the Nahme-Griffith number exceeds the unity, the temperature
of polymeric liquid increases rapidly and the apparent viscosity also
exponentially decreases as the thermal conductivity decreases. The conformation
of polymer chains is stretched and aligned by the shear flow for $Na<1$, but
the coherent structure becomes disturbed by the thermal motion of molecules for
$Na>1$.Comment: 19 pages, 3 figures. arXiv admin note: substantial text overlap with
arXiv:1401.124

### Replica-exchange molecular dynamics simulation for supercooled liquids

We investigate to what extend the replica-exchange Monte Carlo method is able
to equilibrate a simple liquid in its supercooled state. We find that this
method does indeed allow to generate accurately the canonical distribution
function even at low temperatures and that its efficiency is about 10-100 times
higher than the usual canonical molecular dynamics simulation.Comment: 6 pages, 5 figure

### Lifetime of dynamical heterogeneity in a highly supercooled liquid

We numerically examine dynamical heterogeneity in a highly supercooled
three-dimensional liquid via molecular-dynamics simulations. To define the
local dynamics, we consider two time intervals, $\tau_\alpha$ and
$\tau_{\text{ngp}}$. $\tau_\alpha$ is the $\alpha$ relaxation time, and
$\tau_{\text{ngp}}$ is the time at which non-Gaussian parameter of the van Hove
self-correlation function is maximized. We determine the lifetimes of the
heterogeneous dynamics in these two different time intervals,
$\tau_{\text{hetero}}(\tau_\alpha)$ and
$\tau_{\text{hetero}}(\tau_{\text{ngp}})$, by calculating the time correlation
function of the particle dynamics, i.e., the four-point correlation function.
We find that the difference between $\tau_{\text{hetero}}(\tau_\alpha)$ and
$\tau_{\text{hetero}}(\tau_{\text{ngp}})$ increases with decreasing
temperature. At low temperatures, $\tau_{\text{hetero}}(\tau_\alpha)$ is
considerably larger than $\tau_{\alpha}$, while
$\tau_{\text{hetero}}(\tau_{\text{ngp}})$ remains comparable to
$\tau_{\alpha}$. Thus, the lifetime of the heterogeneous dynamics depends
strongly on the time interval.Comment: 4pages, 6figure

### Tumbling motion of a single chain in shear flow: a crossover from Brownian to non-Brownian behavior

We present numerical results for the dynamics of a single chain in steady
shear flow. The chain is represented by a bead-spring model, and the smoothed
profile method is used to accurately account for the effects of thermal
fluctuations and hydrodynamic interactions acting on beads due to host fluids.
It is observed that the chain undergoes tumbling motions and that its
dimensionless frequency F depends only on the Peclet number Pe with a power
law. The exponent of Pe clearly changes from 2/3 to 1 around the critical
Peclet number, indicating that the crossover reflects the competition of
thermal fluctuation and shear flow. The presented numerical results agree well
with our theoretical analysis based on Jeffery's work

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