535 research outputs found
Analytical Rescaling of Polymer Dynamics from Mesoscale Simulations
We present a theoretical approach to scale the artificially fast dynamics of
simulated coarse-grained polymer liquids down to its realistic value. As
coarse-graining affects entropy and dissipation, two factors enter the
rescaling: inclusion of intramolecular vibrational degrees of freedom, and
rescaling of the friction coefficient. Because our approach is analytical, it
is general and transferable. Translational and rotational diffusion of
unentangled and entangled polyethylene melts, predicted from mesoscale
simulations of coarse-grained polymer melts using our rescaling procedure, are
in quantitative agreement with united atom simulations and with experiments.Comment: 6 pages, 2 figures, 2 table
Random walk approach to the d-dimensional disordered Lorentz gas
A correlated random walk approach to diffusion is applied to the disordered
nonoverlapping Lorentz gas. By invoking the Lu-Torquato theory for chord-length
distributions in random media [J. Chem. Phys. 98, 6472 (1993)], an analytic
expression for the diffusion constant in arbitrary number of dimensions d is
obtained. The result corresponds to an Enskog-like correction to the Boltzmann
prediction, being exact in the dilute limit, and better or nearly exact in
comparison to renormalized kinetic theory predictions for all allowed densities
in d=2,3. Extensive numerical simulations were also performed to elucidate the
role of the approximations involved.Comment: 5 pages, 5 figure
Stability of adhesion clusters under constant force
We solve the stochastic equations for a cluster of parallel bonds with shared
constant loading, rebinding and the completely dissociated state as an
absorbing boundary. In the small force regime, cluster lifetime grows only
logarithmically with bond number for weak rebinding, but exponentially for
strong rebinding. Therefore rebinding is essential to ensure physiological
lifetimes. The number of bonds decays exponentially with time for most cases,
but in the intermediate force regime, a small increase in loading can lead to
much faster decay. This effect might be used by cell-matrix adhesions to induce
signaling events through cytoskeletal loading.Comment: Revtex, 4 pages, 4 Postscript files include
Phonon Band Structure and Thermal Transport Correlation in a Layered Diatomic Crystal
To elucidate the relationship between a crystal's structure, its thermal
conductivity, and its phonon dispersion characteristics, an analysis is
conducted on layered diatomic Lennard-Jones crystals with various mass ratios.
Lattice dynamics theory and molecular dynamics simulations are used to predict
the phonon dispersion curves and the thermal conductivity. The layered
structure generates directionally dependent thermal conductivities lower than
those predicted by density trends alone. The dispersion characteristics are
quantified using a set of novel band diagram metrics, which are used to assess
the contributions of acoustic phonons and optical phonons to the thermal
conductivity. The thermal conductivity increases as the extent of the acoustic
modes increases, and decreases as the extent of the stop bands increases. The
sensitivity of the thermal conductivity to the band diagram metrics is highest
at low temperatures, where there is less anharmonic scattering, indicating that
dispersion plays a more prominent role in thermal transport in that regime. We
propose that the dispersion metrics (i) provide an indirect measure of the
relative contributions of dispersion and anharmonic scattering to the thermal
transport, and (ii) uncouple the standard thermal conductivity
structure-property relation to that of structure-dispersion and
dispersion-property relations, providing opportunities for better understanding
of the underlying physical mechanisms and a potential tool for material design.Comment: 30 pages, 10 figure
Fermion Quasi-Spherical Harmonics
Spherical Harmonics, , are derived and presented (in a
Table) for half-odd-integer values of and . These functions are
eigenfunctions of and written as differential operators in the
spherical-polar angles, and . The Fermion Spherical Harmonics
are a new, scalar and angular-coordinate-dependent representation of fermion
spin angular momentum. They have symmetry in the angle , and hence
are not single-valued functions on the Euclidean unit sphere; they are
double-valued functions on the sphere, or alternatively are interpreted as
having a double-sphere as their domain.Comment: 16 pages, 2 Tables. Submitted to J.Phys.
A First Principle Approach to Rescale the Dynamics of Simulated Coarse-Grained Macromolecular Liquids
We present a detailed derivation and testing of our approach to rescale the
dynamics of mesoscale simulations of coarse-grained polymer melts (I. Y.
Lyubimov et al. J. Chem. Phys. \textbf{132}, 11876, 2010). Starting from the
first-principle Liouville equation and applying the Mori-Zwanzig projection
operator technique, we derive the Generalized Langevin Equations (GLE) for the
coarse-grained representations of the liquid. The chosen slow variables in the
projection operators define the length scale of coarse graining. Each polymer
is represented at two levels of coarse-graining: monomeric as a bead-and-spring
model and molecular as a soft-colloid. In the long-time regime where the
center-of-mass follows Brownian motion and the internal dynamics is completely
relaxed, the two descriptions must be equivalent. By enforcing this formal
relation we derive from the GLEs the analytical rescaling factors to be applied
to dynamical data in the coarse-grained representation to recover the monomeric
description. Change in entropy and change in friction are the two corrections
to be accounted for to compensate the effects of coarse-graining on the polymer
dynamics. The solution of the memory functions in the coarse-grained
representations provides the dynamical rescaling of the friction coefficient.
The calculation of the internal degrees of freedom provides the correction of
the change in entropy due to coarse-graining. The resulting rescaling formalism
is a function of the coarse-grained model and thermodynamic parameters of the
system simulated. The rescaled dynamics obtained from mesoscale simulations of
polyethylene, represented as soft colloidal particles, by applying our
rescaling approach shows a good agreement with data of translational diffusion
measured experimentally and from simulations. The proposed method is used to
predict self-diffusion coefficients of new polyethylene samples.Comment: 21 pages, 6 figures, 6 tables. Submitted to Phys. Rev.
Efficiency of quantum and classical transport on graphs
We propose a measure to quantify the efficiency of classical and quantum
mechanical transport processes on graphs. The measure only depends on the
density of states (DOS), which contains all the necessary information about the
graph. For some given (continuous) DOS, the measure shows a power law behavior,
where the exponent for the quantum transport is twice the exponent of its
classical counterpart. For small-world networks, however, the measure shows
rather a stretched exponential law but still the quantum transport outperforms
the classical one. Some finite tree-graphs have a few highly degenerate
eigenvalues, such that, on the other hand, on them the classical transport may
be more efficient than the quantum one.Comment: 5 pages, 3 figure
Inductive Construction of 2-Connected Graphs for Calculating the Virial Coefficients
In this paper we give a method for constructing systematically all simple
2-connected graphs with n vertices from the set of simple 2-connected graphs
with n-1 vertices, by means of two operations: subdivision of an edge and
addition of a vertex. The motivation of our study comes from the theory of
non-ideal gases and, more specifically, from the virial equation of state. It
is a known result of Statistical Mechanics that the coefficients in the virial
equation of state are sums over labelled 2-connected graphs. These graphs
correspond to clusters of particles. Thus, theoretically, the virial
coefficients of any order can be calculated by means of 2-connected graphs used
in the virial coefficient of the previous order. Our main result gives a method
for constructing inductively all simple 2-connected graphs, by induction on the
number of vertices. Moreover, the two operations we are using maintain the
correspondence between graphs and clusters of particles.Comment: 23 pages, 5 figures, 3 table
Dynamic of a non homogeneously coarse grained system
To study materials phenomena simultaneously at various length scales,
descriptions in which matter can be coarse grained to arbitrary levels, are
necessary. Attempts to do this in the static regime (i.e. zero temperature)
have already been developed. In this letter, we present an approach that leads
to a dynamics for such coarse-grained models. This allows us to obtain
temperature-dependent and transport properties. Renormalization group theory is
used to create new local potentials model between nodes, within the
approximation of local thermodynamical equilibrium. Assuming that these
potentials give an averaged description of node dynamics, we calculate thermal
and mechanical properties. If this method can be sufficiently generalized it
may form the basis of a Molecular Dynamics method with time and spatial
coarse-graining.Comment: 4 pages, 4 figure
Audio Description Washes Brighter? A Study in Brand Names and Advertising
Dealing with objects in audio description, and particularly with those objects that have a clear designer imprint or branding, is a complex matter when a brand name is part of the scene in a film. Deciding whether to describe or not, and how, becomes more than a technical matter that depends on text–image synchronization: it is a complex decision-making process comparable to other forms of audiovisual translation that needs to be approached as a paradigmatic example of intersemiotic translation. Dávila-Montes and Orero address the audio description of branded objects in movies, and their intersemiotic translation from images to spoken words, a complex issue that may harbour additional insights into topics of a wider scope
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