1,650 research outputs found
Kinetic Theory of Flocking: Derivation of Hydrodynamic Equations
It is shown how to explicitly coarse-grain the microscopic dynamics of the
Vicsek model for self-propelled agents. The macroscopic transport equations are
derived by means of an Enskog-type kinetic theory. Expressions for all
transport coefficients at large particle speed are given. The phase transition
from a disordered to a flocking state is studied numerically and analytically.Comment: 4 pages, 1 figur
Computational study of the thermal conductivity in defective carbon nanostructures
We use non-equilibrium molecular dynamics simulations to study the adverse
role of defects including isotopic impurities on the thermal conductivity of
carbon nanotubes, graphene and graphene nanoribbons. We find that even in
structurally perfect nanotubes and graphene, isotopic impurities reduce thermal
conductivity by up to one half by decreasing the phonon mean free path. An even
larger thermal conductivity reduction, with the same physical origin, occurs in
presence of structural defects including vacancies and edges in narrow graphene
nanoribbons. Our calculations reconcile results of former studies, which
differed by up to an order of magnitude, by identifying limitations of various
computational approaches
Configurational temperatures and interactions in charge-stabilized colloid
We demonstrate that the configurational temperature formalism can be derived
from the classical hypervirial theorem, and introduce a hierarchy of
hyperconfigurational temperature definitions, which are particularly well
suited for experimental studies. We then use these analytical tools to probe
the electrostatic interactions in monolayers of charge-stabilized colloidal
spheres confined by parallel glass surfaces. The configurational and
hyperconfigurational temperatures, together with a novel thermodynamic sum
rule, provide previously lacking self-consistency tests for interaction
measurements based on digital video microscopy, and thereby cast new light on
controversial reports of confinement-induced like-charge attractions. We
further introduce a new method for measuring the pair potential directly that
uses consistency of the configurational and hyperconfigurational temperatures
as a set of constraints for a model-free search.Comment: 15 pages, 12 figures, submitted to J. Chem. Phy
Calculations of canonical averages from the grand canonical ensemble
Grand canonical and canonical ensembles become equivalent in the
thermodynamic limit, but when the system size is finite the results obtained in
the two ensembles deviate from each other. In many important cases, the
canonical ensemble provides an appropriate physical description but it is often
much easier to perform the calculations in the corresponding grand canonical
ensemble. We present a method to compute averages in canonical ensemble based
on calculations of the expectation values in grand canonical ensemble. The
number of particles, which is fixed in the canonical ensemble, is not
necessarily the same as the average number of particles in the grand canonical
ensemble
A Simple Three-Parameter Model Potential For Diatomic Systems: From Weakly and Strongly Bound Molecules to Metastable Molecular Ions
Based on a simplest molecular orbital theory of H, a
three-parameter model potential function is proposed to describe ground-state
diatomic systems with closed-shell and/or S-type valence-shell constituents
over a significantly wide range of internuclear distances. More than 200 weakly
and strongly bound diatomics have been studied, including neutral and
singly-charged diatomics (e.g., H, Li, LiH, Cd, Na,
and RbH), long-range bound diatomics (e.g., NaAr, CdNe, He, CaHe,
SrHe, and BaHe), metastable molecular dications (e.g., BeH, AlH,
Mg, and LiBa), and molecular trications (e.g., YHe
and ScHe).Comment: 5 pages, 4 figures, accepted by Physical Review Letter
Ionic Capillary Evaporation in Weakly Charged Nanopores
Using a variational field theory, we show that an electrolyte confined to a
neutral cylindrical nanopore traversing a low dielectric membrane exhibits a
first-order ionic liquid-vapor pseudo-phase-transition from an
ionic-penetration "liquid" phase to an ionic-exclusion "vapor" phase,
controlled by nanopore-modified ionic correlations and dielectric repulsion.
For weakly charged nanopores, this pseudotransition survives and may shed light
on the mechanism behind the rapid switching of nanopore conductivity observed
in experiments.Comment: This version is accepted for publication in PR
Dynamics of a Rigid Rod in a Glassy Medium
We present simulations of the motion of a single rigid rod in a disordered
static 2d-array of disk-like obstacles. The rotational, , and
center-of-mass translational, , diffusion constants are calculated
for a wide range of rod length and density of obstacles . It is found
that follows the behavior predicted by kinetic theory for a hard
disk with an effective radius . A dynamic crossover is observed in
for comparable to the typical distance between neighboring
obstacles . Using arguments from kinetic theory and reptation, we
rationalize the scaling laws, dynamic exponents, and prefactors observed for
. In analogy with the enhanced translational diffusion observed in
deeply supercooled liquids, the Stokes-Einstein-Debye relation is violated for
.Comment: 8 pages, 4 figures. Major changes. To be published in Europhysics
Letter
Transport and Helfand moments in the Lennard-Jones fluid. I. Shear viscosity
We propose a new method, the Helfand-moment method, to compute the shear
viscosity by equilibrium molecular dynamics in periodic systems. In this
method, the shear viscosity is written as an Einstein-like relation in terms of
the variance of the so-called Helfand moment. This quantity, is modified in
order to satisfy systems with periodic boundary conditions usually considered
in molecular dynamics. We calculate the shear viscosity in the Lennard-Jones
fluid near the triple point thanks to this new technique. We show that the
results of the Helfand-moment method are in excellent agreement with the
results of the standard Green-Kubo method.Comment: Submitted to the Journal of Chemical Physic
Symmetry relations in chemical kinetics arising from microscopic reversibility
It is shown that the kinetics of time-reversible chemical reactions having
the same equilibrium constant but different initial conditions are closely
related to one another by a directly measurable symmetry relation analogous to
chemical detailed balance. In contrast to detailed balance, however, this
relation does not require knowledge of the elementary steps that underlie the
reaction, and remains valid in regimes where the concept of rate constants is
ill-defined, such as at very short times and in the presence of low activation
barriers. Numerical simulations of a model of isomerization in solution are
provided to illustrate the symmetry under such conditions, and potential
applications in protein folding-unfolding are pointed out.Comment: 4 pages, 1 figure, accepted to Phys Rev Let
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