159 research outputs found
Kinetic Density Functional Theory: A microscopic approach to fluid mechanics
In the present paper we give a brief summary of some recent theoretical
advances in the treatment of inhomogeneous fluids and methods which have
applications in the study of dynamical properties of liquids in situations of
extreme confinement, such as nanopores, nanodevices, etc. The approach obtained
by combining kinetic and density functional methods is microscopic, fully
self-consistent and allows to determine both configurational and flow
properties of dense fluids.
The theory predicts the correct hydrodynamic behavior and provides a
practical and numerical tool to determine how the transport properties are
modified when the length scales of the confining channels are comparable with
the size of the molecules. The applications range from the dynamics of simple
fluids under confinement, to that of neutral binary mixtures and electrolytes
where the theory in the limit of slow gradients reproduces the known
phenomenological equations such as the Planck-Nernst-Poisson and the
Smoluchowski equations. The approach here illustrated allows for fast numerical
solution of the evolution equations for the one-particle phase-space
distributions by means of the weighted density lattice Boltzmann method and is
particularly useful when one considers flows in complex geometries.Comment: 14 page
Multicomponent Diffusion in Nanosystems
We present the detailed analysis of the diffusive transport of spatially
inhomogeneous fluid mixtures and the interplay between structural and dynamical
properties varying on the atomic scale. The present treatment is based on
different areas of liquid state theory, namely kinetic and density functional
theory and their implementation as an effective numerical method via the
Lattice Boltzmann approach. By combining the first two methods it is possible
to obtain a closed set of kinetic equations for the singlet phase space
distribution functions of each species. The interactions among particles are
considered within a self-consistent approximation and the resulting effective
molecular fields are analyzed. We focus on multispecies diffusion in systems
with short-range hard-core repulsion between particles of unequal sizes and
weak attractive long-range interactions. As a result, the attractive part of
the potential does not contribute explicitly to viscosity but to diffusivity
and the thermodynamic properties. Finally, we obtain a practical scheme to
solve the kinetic equations by employing a discretization procedure derived
from the Lattice Boltzmann approach. Within this framework, we present
numerical data concerning the mutual diffusion properties both in the case of a
quiescent bulk fluid and shear flow inducing Taylor dispersion.Comment: 19 pages + 5 figure
Clausius relation for active particles: what can we learn from fluctuations?
Many kinds of active particles, such as bacteria or active colloids, move in
a thermostatted fluid by means of self-propulsion. Energy injected by such a
non-equilibrium force is eventually dissipated as heat in the thermostat. Since
thermal fluctuations are much faster and weaker than self-propulsion forces,
they are often neglected, blurring the identification of dissipated heat in
theoretical models. For the same reason, some freedom - or arbitrariness -
appears when defining entropy production. Recently three different recipes to
define heat and entropy production have been proposed for the same model where
the role of self-propulsion is played by a Gaussian coloured noise. Here we
compare and discuss the relation between such proposals and their physical
meaning. One of these proposals takes into account the heat exchanged with a
non-equilibrium active bath: such an "active heat" satisfies the original
Clausius relation and can be experimentally verified.Comment: 10 pages, submitted to Entropy journal for the special issue
"Thermodynamics and Statistical Mechanics of Small Systems" (see
http://www.mdpi.com/journal/entropy/special_issues/small_systems
Controlling electroosmotic flows by polymer coatings: A joint experimental-theoretical investigation
We analyze the electroosmotic flow (EOF) of an electrolytic solution in a polymer coated capillary electrophoresis tube. The polymeric density, charge, thickness, and the capillary tube charge vary as a function of pH and produce a non-trivial modulation of the EOF, including a flow reversal at acid pH conditions. By means of a theoretical argument and numerical simulations, we recover the experimental curve for the EOF, providing a firm approach for predictive analysis of electroosmosis under different polymeric coating conditions. A proposed application of the approach is to determine the near-wall charge of the coating to be used for further quantitative analysis of the electroosmotic flow and mobility
Non-local kinetic theory of inhomogeneous liquid mixtures
In this work we investigate the dynamical properties of a mixture of mutually
interacting spherical molecules of different masses and sizes. From an analysis
of the microscopic laws governing the motion of the molecules we derive a set
of non-local self-consistent equations for the singlet phase-space distribution
functions. The theory is shown to reproduce the hydrodynamic equations for the
densities of each species, the total momentum and the local temperature. The
non ideal gas interaction term is separated into a contribution due to the
repulsive part, which is treated by means of the revised Enskog theory for hard
spheres, and an attractive contribution treated within the random phase
approximation. The present formulation accounts for the effects of the density
and velocity inhomogeneities both on the thermodynamic and transport properties
of the fluid. In a special limit, where one species is massive and diluted, the
theory leads to a description which is formally identical to the dynamic
density functional equation governing the time evolution of a colloidal system.
The derivation also determines the dependence of the friction coefficient,
appearing in the dynamic density functional theory, on the microscopic
parameters of the solvent. However, the predicted value takes into account only
the collisional contributions to the friction and not the Stokes friction of
hydrodynamic origin, suggesting that velocity correlations should be
incorporated in a more complete treatment.Comment: 12 pages To appear in Molecular Physic
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