114 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
Electro-osmotic flow in coated nanocapillaries: a theoretical investigation
Motivated by recent experiments, we present a theoretical investigation of
how the electro-osmotic flow occurring in a capillary is modified when its
charged surfaces are coated by charged polymers. The theoretical treatment is
based on a three dimensional model consisting of a ternary fluid-mixture,
representing the solvent and two species for the ions, confined between two
parallel charged plates decorated by a fixed array of scatterers representing
the polymer coating. The electro-osmotic flow, generated by a constant electric
field applied in a direction parallel to the plates, is studied numerically by
means of Lattice Boltzmann simulations. In order to gain further understanding
we performed a simple theoretical analysis by extending the Stokes-Smoluchowski
equation to take into account the porosity induced by the polymers in the
region adjacent the walls. We discuss the nature of the velocity profiles by
focusing on the competing effects of the polymer charges and the frictional
forces they exert. We show evidence of the flow reduction and of the flow
inversion phenomenon when the polymer charge is opposite to the surface charge.
By using the density of polymers and the surface charge as control variables,
we propose a phase diagram that discriminates the direct and the reversed flow
regimes and determine its dependence on the ionic concentration.Comment: 15 pages, 6 figures in Physical Chemistry Chemical Physics, 201
Steric modulation of ionic currents in DNA translocation through nanopores
Ionic currents accompanying DNA translocation strongly depend on molarity of
the electrolyte solution and the shape and surface charge of the nanopore. By
means of the Poisson-Nernst-Planck equations it is shown how conductance is
modulated by the presence of the DNA intruder and as a result of competing
electrostatic and confinement factors. The theoretical results reproduce
quantitatively the experimental ones and are summarized in a conductance
diagram that allows distinguishing the region of reduced conductivity from the
region of enhanced conductivity as a function of molarity and the pore
dimension.Comment: 22 pages, 7 figure
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