44 research outputs found
Superstatistics of Brownian motion: A comparative study
The dynamics of temperature fluctuations of a gas of Brownian particles in
local equilibrium with a nonequilibrium heat bath, are described using an
approach consistent with Boltzmann-Gibbs statistics (BG). We use mesoscopic
nonequilibrium thermodynamics (MNET) to derive a Fokker-Planck equation for the
probability distribution in phase space including the local intensive variables
fluctuations. We contract the description to obtain an effective probability
distribution (EPD) from which the mass density, van Hove's function and the
dynamic structure factor of the system are obtained. The main result is to show
that in the long time limit the EPD exhibits a similar behavior as the
superstatistics distribution of nonextensive statistical mechanics (NESM),
therfore implying that the coarse-graining procedure is responsible for the so
called nonextensive effects.Comment: 14 pages,5 figure
A Nonequilibrium Thermodynamic Approach to Generalized Statistics for Brownian Motion
We analyze the dynamics of a Brownian gas in contact with a heat bath in
which large temperature fluctuations occur. There are two distinct time scales
present, one describes the decay of the fluctuations in the temperature and the
other one is associated with the establishment of local equilibrium. Although
the gas has reached local equilibrium, there exist large fluctuations in an
intensive parameter (temperature) which break the thermodynamic equilibrium
with the heat bath. Thus the decay of the fluctuations in the intensive
parameter is larger than the characteristic time for the establishment of local
equilibrium. We show that the dynamics of such large and intensive fluctuations
may be described by adopting a nonequilibrium thermodynamics approach with an
adequate formulation of local equilibrium. A coarsening procedure is then used
to contract the space of mesoscopic variables needed to describe the dynamics
of the gas and the extensive character of the description is lost. This
procedure allows us to derive an effective Maxwell-Boltzmann factor (EMBF) for
the Brownian gas, as has been recently proposed in the literature. Furthermore,
we use this local equilibrium distribution and an entropy functional to derive
a nonequilibrium probability distribution and a hydrodynamic description for
the Brownian gas which contains fluctuating transport coefficients. The ensuing
description is nonextensive and our analysis shows that the coarse-graining
procedure is responsible for the nonextensivity property.Comment: 13 page
Finite-size effects in intracellular microrheology
We propose a model to explain finite-size effects in intracellular
microrheology observed in experiments. The constrained dynamics of the
particles in the intracellular medium, treated as a viscoelastic medium, is
described by means of a diffusion equation in which interactions of the
particles with the cytoskeleton are modelled by a harmonic force. The model
reproduces the observed power-law behavior of the mean-square displacement in
which the exponent depends on the ratio between
particle-to-cytoskeleton-network sizes.Comment: 6 pages 2 figures. To appear in the Journal of Chemical Physic
Mesoscopic constitutive relations for dilute polymer solutions
A novel approach to the dynamics of dilute solutions of polymer molecules
under flow conditions is proposed by applying the rules of mesoscopic
nonequilibrium thermodynamics (MNET). The probability density describing the
state of the system is taken to be a function of the position and velocity of
the molecules, and on a local vector parameter accounting for its deformation.
This function obeys a generalized Fokker-Planck equation, obtained by
calculating the entropy production of the system, and identifying the
corresponding probability currents in terms of generalized forces. In simple
form, this coarse-grained description allows one to derive hydrodynamic
equations where molecular deformation and diffusion effects are coupled. A
class of non-linear constitutive relations for the pressure tensor are
obtained. Particular models are considered and compared with experiments.Comment: To be published in Physica A (16 pages, 2 figures
The transition to irreversibility in sheared suspensions: An analysis based on a mesoscopic entropy production
We study the shear-induced diffusion effect and the transition to
irreversibility in suspensions under oscillatory shear flow by performing an
analysis of the entropy production associated to the motion of the particles.
We show that the Onsager coupling between different contributions to the
entropy production is responsible for the scaling of the mean square
displacement on particle diameter and applied strain. We also show that the
shear-induced effective diffusion coefficient depends on the volume fraction
and use Lattice-Boltzmann simulations to characterize the effect through the
power spectrum of particle positions for different Reynolds numbers and volume
fractions. Our study gives a thermodynamic explanation of the the transition to
irreversibility through a pertinent analysis of the second law of
thermodynamics.Comment: 17 pages, 3 figures, paper submitted tp phys rev
The rheology of hard sphere suspensions at arbitrary volume fractions: An improved differential viscosity model
We propose a simple and general model accounting for the dependence of the
viscosity of a hard sphere suspension at arbitrary volume fractions. The model
constitutes a continuum-medium description based on a recursive-differential
method that assumes a hierarchy of relaxation times. Geometrical information of
the system is introduced through an effective volume fraction that approaches
the usual filling fraction at low concentrations and becomes one at maximum
packing. The agreement of our expression for the viscosity with experiments at
low- and high-shear rates and in the high-frequency limit is remarkable for all
volume fractions.Comment: 16 pages, 6 figures, submitte