1,036 research outputs found
Kinetics of Fragmenting Freely Evolving Granular Gases
We investigate the effect of fragmentation on the homogeneous free cooling of
inelastic hard spheres, using Boltzmann kinetic theory and Direct Monte Carlo
simulations. We analyze in detail a model where dissipative collisions may
subsequently lead to a break-up of the grains. With a given probability, two
off-springs are then created from one of the two colliding partners, with
conservation of mass, momentum and kinetic energy. We observe a scaling regime
characterized by a single collisional average, that quantifies the deviations
from Gaussian behaviour for the joint size and velocity distribution function.
We also discuss the possibility of a catastrophe whereby the number of
particles diverges in a finite time. This phenomenon appears correlated to a
``shattering'' transition marked by a delta singularity at vanishingly small
grains for the rescaled size distribution.Comment: 22 pages, 8 figure
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
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
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
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