340 research outputs found
Dissipative particle dynamics for interacting systems
We introduce a dissipative particle dynamics scheme for the dynamics of
non-ideal fluids. Given a free-energy density that determines the
thermodynamics of the system, we derive consistent conservative forces. The use
of these effective, density dependent forces reduces the local structure as
compared to previously proposed models. This is an important feature in
mesoscopic modeling, since it ensures a realistic length and time scale
separation in coarse-grained models. We consider in detail the behavior of a
van der Waals fluid and a binary mixture with a miscibility gap. We discuss the
physical implications of having a single length scale characterizing the
interaction range, in particular for the interfacial properties.Comment: 25 pages, 12 figure
Dynamics of Polydisperse Polymer Mixtures
We develop a general analysis of the diffusive dynamics of polydisperse
polymers in the presence of chemical potential gradients, within the context of
the tube model (with all species entangled). We obtain a set of coupled
dynamical equations for the time evolution of the polymeric densities with
explicitly derived coefficients. For the case of chemical polydispersity (a set
of chains that are identical except for having a continuous spectrum of
enthalpic interaction strengths) the coupled equations can be fully solved in
certain cases. For these we study the linearised mode spectrum following a
quench through the spinodal, with and without a passive (polymeric) solvent. We
also study the more conventional case of length polydisperse chains in a poor
solvent.Comment: 21 pages, 3 figures,revised versio
Negative fluctuation-dissipation ratios in the backgammon model
We analyze fluctuation-dissipation relations in the Backgammon model: a
system that displays glassy behavior at zero temperature due to the existence
of entropy barriers. We study local and global fluctuation relations for the
different observables in the model. For the case of a global perturbation we
find a unique negative fluctuation-dissipation ratio that is independent of the
observable and which diverges linearly with the waiting time. This result
suggests that a negative effective temperature can be observed in glassy
systems even in the absence of thermally activated processes.Comment: 32 pages, 10 figures. Accepted in PR
A practical density functional for polydisperse polymers
The Flory Huggins equation of state for monodisperse polymers can be turned
into a density functional by adding a square gradient term, with a coefficient
fixed by appeal to RPA (random phase approximation). We present instead a model
nonlocal functional in which each polymer is replaced by a deterministic,
penetrable particle of known shape. This reproduces the RPA and square gradient
theories in the small deviation and/or weak gradient limits, and can readily be
extended to polydisperse chains. The utility of the new functional is shown for
the case of a polydisperse polymer solution at coexistence in a poor solvent.Comment: 9 pages, 3 figure
Bistability, oscillations and bidirectional motion of ensemble of hydrodynamically-coupled molecular motors
We analyze the collective behavior of hydrodynamically coupled molecular
motors. We show that the local fluxes induced by motors displacement can induce
the experimentally observed bidirectional motion of cargoes and vesicles. By
means of a mean--field approach we show that sustained oscillations as well as
bistable collective motor motion arise even for very large collection of
motors, when thermal noise is irrelevant. The analysis clarifies the physical
mechanisms responsible for such dynamics by identifying the relevant coupling
parameter and its dependence on the geometry of the hydrodynamic coupling as
well as on system size. We quantify the phase diagram for the different phases
that characterize the collective motion of hydrodynamically coupled motors and
show that sustained oscillations can be reached for biologically relevant
parameters, hence demonstrating the relevance of hydrodynamic interactions in
intracellular transport
Local size segregation in polydisperse hard sphere fluids
The structure of polydisperse hard sphere fluids, in the presence of a wall,
is studied by the Rosenfeld density functional theory. Within this approach,
the local excess free energy depends on only four combinations of the full set
of density fields. The case of continuous polydispersity thereby becomes
tractable. We predict, generically, an oscillatory size segregation close to
the wall, and connect this, by a perturbation theory for narrow distributions,
with the reversible work for changing the size of one particle in a
monodisperse reference fluid.Comment: RevTeX, 4 pages, 3 figures, submitted to Phys. Rev. Let
Universality of Fluctuation-Dissipation Ratios: The Ferromagnetic Model
We calculate analytically the fluctuation-dissipation ratio (FDR) for Ising
ferromagnets quenched to criticality, both for the long-range model and its
short-range analogue in the limit of large dimension. Our exact solution shows
that, for both models, if the system is unmagnetized while
if the initial magnetization is non-zero. This indicates that
two different classes of critical coarsening dynamics need to be distinguished
depending on the initial conditions, each with its own nontrivial FDR. We also
analyze the dependence of the FDR on whether local and global observables are
used. These results clarify how a proper local FDR (and the corresponding
effective temperature) should be defined in long-range models in order to avoid
spurious inconsistencies and maintain the expected correspondence between local
and global results; global observables turn out to be far more robust tools for
detecting non-equilibrium FDRs.Comment: 14 pages, revtex4, published version. Changes from v1: added
discussion of refs [16,36,37], other observables and local
correlation/response in short-range mode
Test of the fluctuation theorem for stochastic entropy production in a nonequilibrium steady state
We derive a simple closed analytical expression for the total entropy
production along a single stochastic trajectory of a Brownian particle
diffusing on a periodic potential under an external constant force. By
numerical simulations we compute the probability distribution functions of the
entropy and satisfactorily test many of the predictions based on Seifert's
integral fluctuation theorem. The results presented for this simple model
clearly illustrate the practical features and implications derived from such a
result of nonequilibrium statistical mechanics.Comment: Accepted in Phys. Rev.
Universality class of fiber bundles with strong heterogeneities
We study the effect of strong heterogeneities on the fracture of disordered
materials using a fiber bundle model. The bundle is composed of two subsets of
fibers, i.e. a fraction 0<\alpha<1 of fibers is unbreakable, while the
remaining 1-\alpha fraction is characterized by a distribution of breaking
thresholds. Assuming global load sharing, we show analytically that there
exists a critical fraction of the components \alpha_c which separates two
qualitatively different regimes of the system: below \alpha_c the burst size
distribution is a power law with the usual exponent \tau=5/2, while above
\alpha_c the exponent switches to a lower value \tau=9/4 and a cutoff function
occurs with a diverging characteristic size. Analyzing the macroscopic response
of the system we demonstrate that the transition is conditioned to disorder
distributions where the constitutive curve has a single maximum and an
inflexion point defining a novel universality class of breakdown phenomena
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