1,923 research outputs found
Kinetic Theory of Response Functions for the Hard Sphere Granular Fluid
The response functions for small spatial perturbations of a homogeneous
granular fluid have been described recently. In appropriate dimensionless
variables, they have the form of stationary state time correlation functions.
Here, these functions are expressed in terms of reduced single particle
functions that are expected to obey a linear kinetic equation. The functional
assumption required for such a kinetic equation, and a Markov approximation for
its implementation are discussed. If, in addition, static velocity correlations
are neglected, a granular fluid version of the linearized Enskog kinetic theory
is obtained. The derivation makes no a priori limitation on the density, space
and time scale, nor degree of inelasticity. As an illustration, recently
derived Helfand and Green-Kubo expressions for the Navier-Stokes order
transport coefficients are evaluated with this kinetic theory. The results are
in agreement with those obtained from the Chapman-Enskog solution to the
nonlinear Enskog kinetic equation.Comment: Submitted to J. Stat. Mec
Solidity of viscous liquids. IV. Density fluctuations
This paper is the fourth in a series exploring the physical consequences of
the solidity of highly viscous liquids. It is argued that the two basic
characteristics of a flow event (a jump between two energy minima in
configuration space) are the local density change and the sum of all particle
displacements. Based on this it is proposed that density fluctuations are
described by a time-dependent Ginzburg-Landau equation with rates in k-space of
the form with where is the average
intermolecular distance. The inequality expresses a long-wavelength dominance
of the dynamics which implies that the Hamiltonian (free energy) may be taken
to be ultra local. As an illustration of the theory the case with the simplest
non-trivial Hamiltonian is solved to second order in the Gaussian
approximation, where it predicts an asymmetric frequency dependence of the
isothermal bulk modulus with Debye behavior at low frequencies and an
decay of the loss at high frequencies. Finally, a general
formalism for the description of viscous liquid dynamics, which supplements the
density dynamics by including stress fields, a potential energy field, and
molecular orientational fields, is proposed
Solidity of viscous liquids. V. Long-wavelength dominance of the dynamics
This paper is the fifth in a series exploring the physical consequences of
the solidity of glass-forming liquids. Paper IV proposed a model where the
density field is described by a time-dependent Ginzburg-Landau equation of the
nonconserved type with rates in space of the form . The
model assumes that where is the average intermolecular
distance; this inequality expresses a long-wavelength dominance of the dynamics
which implies that the Hamiltonian (free energy) to a good approximation may be
taken to be ultralocal. In the present paper we argue that this is the simplest
model consistent with the following three experimental facts: 1) Viscous
liquids approaching the glass transition do not develop long-range order; 2)
The glass has lower compressibility than the liquid; 3) The alpha process
involves several decades of relaxation times shorter than the mean relaxation
time. The paper proceeds to list six further experimental facts characterizing
equilibrium viscous liquid dynamics and shows that these are readily understood
in terms of the model; some are direct consequences, others are quite natural
when viewed in light of the model
Therapeutic efficacy of favipiravir against Bourbon virus in mice
Bourbon virus (BRBV) is an emerging tick-borne RNA virus in the orthomyxoviridae family that was discovered in 2014. Although fatal human cases of BRBV have been described, little is known about its pathogenesis, and no antiviral therapies or vaccines exist. We obtained serum from a fatal case in 2017 and successfully recovered the second human infectious isolate of BRBV. Next-generation sequencing of the St. Louis isolate of BRBV (BRBV-STL) showed >99% nucleotide identity to the original reference isolate. Using BRBV-STL, we developed a small animal model to study BRBV-STL tropism in vivo and evaluated the prophylactic and therapeutic efficacy of the experimental antiviral drug favipiravir against BRBV-induced disease. Infection of Ifnar1-/- mice lacking the type I interferon receptor, but not congenic wild-type animals, resulted in uniformly fatal disease 6 to 10 days after infection. RNA in situ hybridization and viral yield assays demonstrated a broad tropism of BRBV-STL with highest levels detected in liver and spleen. In vitro replication and polymerase activity of BRBV-STL were inhibited by favipiravir. Moreover, administration of favipiravir as a prophylaxis or as post-exposure therapy three days after infection prevented BRBV-STL-induced mortality in immunocompromised Ifnar1-/- mice. These results suggest that favipiravir may be a candidate treatment for humans who become infected with BRBV
Phonon-like and single particle dynamics in liquid lithium
The dynamic structure factor, S(Q,E), of liquid lithium (T=475 K) has been
determined by inelastic x-ray scattering (IXS) in the momentum transfer region
(Q = 1.4-110 nm-1). These data allow to observe how, in a simple liquid, a
phonon-like collective mode evolves towards the single particle dynamics. As a
function of Q, one finds: i) at low Q's, a sound mode with a positive
dispersion of the sound velocity, ii) at intermediate Q's, excitations whose
energy oscillates similarly to phonons in the crystal Brillouin zones, and iii)
at high Q's, the S(Q,E) approaches a Gaussian shape, indicating that the single
particle dynamics has been reached.Comment: 3 pages and 5 figure
Nonlinear Hydrodynamics of a Hard Sphere Fluid Near the Glass Transition
We conduct a numerical study of the dynamic behavior of a dense hard sphere
fluid by deriving and integrating a set of Langevin equations. The statics of
the system is described by a free energy functional of the
Ramakrishnan-Yussouff form. We find that the system exhibits glassy behavior as
evidenced through stretched exponential decay and two-stage relaxation of the
density correlation function. The characteristic times grow with increasing
density according to the Vogel-Fulcher law. The wavenumber dependence of the
kinetics is extensively explored. The connection of our results with
experiment, mode coupling theory, and molecular dynamics results is discussed.Comment: 34 Pages, Plain TeX, 12 PostScript Figures (not included, available
on request
Short-time inertial response of viscoelastic fluids measured with Brownian motion and with active probes
We have directly observed short-time stress propagation in viscoelastic
fluids using two optically trapped particles and a fast interferometric
particle-tracking technique. We have done this both by recording correlations
in the thermal motion of the particles and by measuring the response of one
particle to the actively oscillated second particle. Both methods detect the
vortex-like flow patterns associated with stress propagation in fluids. This
inertial vortex flow propagates diffusively for simple liquids, while for
viscoelastic solutions the pattern spreads super-diffusively, dependent on the
shear modulus of the medium
Universal divergenceless scaling between structural relaxation and caged dynamics in glass-forming systems
On approaching the glass transition, the microscopic kinetic unit spends
increasing time rattling in the cage of the first neighbours whereas its
average escape time, the structural relaxation time , increases
from a few picoseconds up to thousands of seconds. A thorough study of the
correlation between and the rattling amplitude, expressed by the
Debye-Waller factor (DW), was carried out. Molecular-dynamics (MD) simulations
of both a model polymer system and a binary mixture were performed by varying
the temperature, the density , the potential and the polymer length to
consider the structural relaxation as well as both the rotational and the
translation diffusion. The simulations evidence the scaling between the
and the Debye-Waller factor. An analytic model of the master
curve is developed in terms of two characteristic length scales pertaining to
the distance to be covered by the kinetic unit to reach a transition state. The
model does not imply divergences. The comparison with the
experiments supports the numerical evidence over a range of relaxation times as
wide as about eighteen orders of magnitude. A comparison with other scaling and
correlation procedures is presented. The study suggests that the equilibrium
and the moderately supercooled states of the glassformers possess key
information on the huge slowing-down of their relaxation close to the glass
transition. The latter, according to the present simulations, exhibits features
consistent with the Lindemann melting criterion and the free-volume model.Comment: 8 pages, 11 figure
Numerical Methods for the Stochastic Landau-Lifshitz Navier-Stokes Equations
The Landau-Lifshitz Navier-Stokes (LLNS) equations incorporate thermal
fluctuations into macroscopic hydrodynamics by using stochastic fluxes. This
paper examines explicit Eulerian discretizations of the full LLNS equations.
Several CFD approaches are considered (including MacCormack's two-step
Lax-Wendroff scheme and the Piecewise Parabolic Method) and are found to give
good results (about 10% error) for the variances of momentum and energy
fluctuations. However, neither of these schemes accurately reproduces the
density fluctuations. We introduce a conservative centered scheme with a
third-order Runge-Kutta temporal integrator that does accurately produce
density fluctuations. A variety of numerical tests, including the random walk
of a standing shock wave, are considered and results from the stochastic LLNS
PDE solver are compared with theory, when available, and with molecular
simulations using a Direct Simulation Monte Carlo (DSMC) algorithm
Static and Dynamic Properties of a Viscous Silica Melt Molecular Dynamics Computer Simulations
We present the results of a large scale molecular dynamics computer
simulation in which we investigated the static and dynamic properties of a
silica melt in the temperature range in which the viscosity of the system
changes from O(10^-2) Poise to O(10^2) Poise. We show that even at temperatures
as high as 4000 K the structure of this system is very similar to the random
tetrahedral network found in silica at lower temperatures. The temperature
dependence of the concentration of the defects in this network shows an
Arrhenius law. From the partial structure factors we calculate the neutron
scattering function and find that it agrees very well with experimental neutron
scattering data. At low temperatures the temperature dependence of the
diffusion constants shows an Arrhenius law with activation energies which
are in very good agreement with the experimental values. With increasing
temperature we find that this dependence shows a cross-over to one which can be
described well by a power-law, D\propto (T-T_c)^gamma. The critical temperature
T_c is 3330 K and the exponent gamma is close to 2.1. Since we find a similar
cross-over in the viscosity we have evidence that the relaxation dynamics of
the system changes from a flow-like motion of the particles, as described by
the ideal version of mode-coupling theory, to a hopping like motion. We show
that such a change of the transport mechanism is also observed in the product
of the diffusion constant and the life time of a Si-O bond, or the space and
time dependence of the van Hove correlation functions.Comment: 30 pages of Latex, 14 figure
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