133 research outputs found
Wetting, roughness and hydrodynamic slip
The hydrodynamic slippage at a solid-liquid interface is currently at the
center of our understanding of fluid mechanics. For hundreds of years this
science has relied upon no-slip boundary conditions at the solid-liquid
interface that has been applied successfully to model many macroscopic
experiments, and the state of this interface has played a minor role in
determining the flow. However, the problem is not that simple and has been
revisited recently. Due to the change in the properties of the interface, such
as wettability and roughness, this classical boundary condition could be
violated, leading to a hydrodynamic slip. In this chapter, we review recent
advances in the understanding and expectations for the hydrodynamic boundary
conditions in different situations, by focussing mostly on key papers from past
decade. We highlight mostly the impact of hydrophobicity, roughness, and
especially their combination on the flow properties. In particular, we show
that hydrophobic slippage can be dramatically affected by the presence of
roughness, by inducing novel hydrodynamic phenomena, such as giant interfacial
slip, superfluidity, mixing, and low hydrodynamic drag. Promising directions
for further research are also discussed.Comment: 36 pages, 19 figures. This chapter would be a part of "Nanoscale
liquid interfaces" boo
Wetting, roughness and flow boundary conditions
We discuss how the wettability and roughness of a solid impacts its
hydrodynamic properties. We see in particular that hydrophobic slippage can be
dramatically affected by the presence of roughness. Owing to the development of
refined methods for setting very well-controlled micro- or nanotextures on a
solid, these effects are being exploited to induce novel hydrodynamic
properties, such as giant interfacial slip, superfluidity, mixing, and low
hydrodynamic drag, that could not be achieved without roughness.Comment: 28 pages, 14 figures, 4 tables; accepted for publication in Journal
of Physics: Condensed Matte
Hydrodynamic interaction with super-hydrophobic surfaces
Patterned surfaces with large effective slip lengths, such as
super-hydrophobic surfaces containing trapped gas bubbles, have the potential
to reduce hydrodynamic drag. Based on lubrication theory, we analyze an
approach of a hydrophilic disk to such a surface. The drag force is predicted
analytically and formulated in terms of a correction function to the Reynolds
equation, which is shown to be the harmonic mean of corrections expressed
through effective slip lengths in the two principal (fastest and slowest)
orthogonal directions. The reduction of drag is especially pronounced for a
thin (compared to texture period) gap. It is not really sensitive to the
pattern geometry, but depends strongly on the fraction of the gas phase and
local slip length at the gas area.Comment: 20 pages, 7 figure
Effective slip in pressure-driven flow past super-hydrophobic stripes
Super-hydrophobic array of grooves containing trapped gas (stripes), have the
potential to greatly reduce drag and enhance mixing phenomena in microfluidic
devices. Recent work has focused on idealized cases of stick-perfect slip
stripes, with limited guidance. Here, we analyze the experimentally relevant
situation of a pressure-driven flow past striped slip-stick surfaces with
arbitrary local slip at the gas sectors. We derive analytical formulas for
maximal (longitudinal) and minimal (transverse) directional effective slip
lengths that can be used for any surface slip fraction (validated by numerical
calculations). By representing eigenvalues of the slip length-tensor, they
allow us to obtain the effective slip for any orientation of stripes with
respect to the mean flow. Our results imply that flow past stripes is
controlled by the ratio of the local slip length to texture size. In case of a
large (compared to the texture period) slip at the gas areas, surface
anisotropy leads to a tensorial effective slip, by attaining the values
predicted earlier for a perfect local slip. Both effective slip lengths and
anisotropy of the flow decrease when local slip becomes of the order of texture
period. In the case of small slip, we predict simple surface-averaged,
isotropic flows (independent of orientation). These results provide a framework
for the rational design of super-hydrophobic surfaces and devices.Comment: 10 pages, 4 figures, revised versio
Disjoining Pressure of an Electrolyte Film Confined between Semipermeable Membranes
We consider an electrolyte solution confined by semipermeable membranes in
contact with a salt-free solvent. Membranes are uncharged, but since small
counter-ions leak-out into infinite salt-free reservoirs, we observe a
distance-dependent membrane potential, which generates a repulsive
electrostatic disjoining pressure. We obtain the distribution of the potential
and of ions, and derive explicit formulas for the disjoining pressure, which
are validated by computer simulations. We predict a strong short-range
power-law repulsion, and a weaker long-range exponential decay. Our results
also demonstrate that an interaction between membranes does strongly depend on
the screening lengths, valency of an electrolyte solution, and an
inter-membrane film thickness. Finally, our analysis can be directly extended
to the study of more complex situations and some biological problems.Comment: 9 pages, 8 figure
Surface Roughness and Hydrodynamic Boundary Conditions
We report results of investigations of a high-speed drainage of thin aqueous
films squeezed between randomly nanorough surfaces. A significant decrease in
hydrodynamic resistance force as compared with predicted by Taylor's equation
is observed. However, this reduction in force does not represents the slippage.
The measured force is exactly the same as that between equivalent smooth
surfaces obeying no-slip boundary conditions, but located at the intermediate
position between peaks and valleys of asperities. The shift in hydrodynamic
thickness is shown to be independent on the separation and/or shear rate. Our
results disagree with previous literature data reporting very large and
shear-dependent boundary slip for similar systems.Comment: Revised versio
Electrophoresis of Janus Particles: a Molecular Dynamics simulation study
In this work, we use Molecular Dynamics and Lattice-Boltzmann simulations to
study the properties of charged Janus particles in an electric field. We show
that for relatively small net charge and thick electrostatic diffuse layer
mobilities of Janus particles and uniformly charged colloids of the same net
charge are identical. However, for higher charges and thinner diffuse layers
Janus particles always show lower electrophoretic mobility. We also demonstrate
that Janus particles align with the electric field and the angular deviation
from the field's direction are related to their dipole moment. We show that the
latter is affected by the thickness of the electrostatic diffuse layer and
strongly correlates with the electrophoretic mobility.Comment: Accepted to JC
Electrostatic Interaction of Heterogeneously Charged Surfaces with Semipermeable Membranes
In this paper we study the electrostatic interaction of a heterogeneously
charged wall with a neutral semipermeable membrane. The wall consists of
periodic stripes, where the charge density varies in one direction. The
membrane is in a contact with a bulk reservoir of an electrolyte solution and
separated from the wall by a thin film of salt-free liquid. One type of ions
(small counterions) permeates into the gap and gives rise to a
distance-dependent membrane potential, which translates into a repulsive
electrostatic disjoining pressure due to an overlap of counterion clouds in the
gap. To quantify it we use two complementary approaches. First, we propose a
mean-field theory based on a linearized Poisson-Boltzmann equation and Fourier
analysis. These calculations allow us to estimate the effect of a heterogeneous
charge pattern at the wall on the induced heterogeneous membrane potential, and
the value of the disjoining pressure as a function of the gap. Second, we
perform Langevin dynamics simulations of the same system with explicit ions.
The results of the two approaches are in good agreement with each other at low
surface charge and small gap, but differ due to nonlinearity at the higher
charge. These results demonstrate that a heterogeneity of the wall charge can
lead to a huge reduction in the electrostatic repulsion, which could
dramatically facilitate a self-assembly in complex synthetic and biological
systems.Comment: 14 pages, 6 figure
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