37 research outputs found
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
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
Drag force on a sphere moving towards an anisotropic super-hydrophobic plane
We analyze theoretically a high-speed drainage of liquid films squeezed
between a hydrophilic sphere and a textured super-hydrophobic plane, that
contains trapped gas bubbles. A super-hydrophobic wall is characterized by
parameters (texture characteristic length), and (local slip
lengths at solid and gas areas), and and (fractions of solid
and gas areas). Hydrodynamic properties of the plane are fully expressed in
terms of the effective slip-length tensor with eigenvalues that depend on
texture parameters and (local separation). The effect of effective slip is
predicted to decrease the force as compared with expected for two hydrophilic
surfaces and described by the Taylor equation. The presence of additional
length scales, , and , implies that a film drainage can be much
richer than in case of a sphere moving towards a hydrophilic plane. For a large
(compared to ) gap the reduction of the force is small, and for all textures
the force is similar to expected when a sphere is moving towards a smooth
hydrophilic plane that is shifted down from the super-hydrophobic wall. The
value of this shift is equal to the average of the eigenvalues of the
slip-length tensor. By analyzing striped super-hydrophobic surfaces, we then
compute the correction to the Taylor equation for an arbitrary gap. We show
that at thinner gap the force reduction becomes more pronounced, and that it
depends strongly on the fraction of the gas area and local slip lengths. For
small separations we derive an exact equation, which relates a correction for
effective slip to texture parameters. Our analysis provides a framework for
interpreting recent force measurements in the presence of super-hydrophobic
surface.Comment: 9 pages, 5 figures, submitted to PRE; EPAPS file include
Threshold of microvascular occlusion: injury size defines the thrombosis scenario
Damage to the blood vessel triggers formation of a hemostatic plug, which is
meant to prevent bleeding, yet the same phenomenon may result in a total
blockade of a blood vessel by a thrombus, causing severe medical conditions.
Here, we show that the physical interplay between platelet adhesion and
hemodynamics in a microchannel manifests in a critical threshold behavior of a
growing thrombus. Depending on the size of injury, two distinct dynamic
pathways of thrombosis were found: the formation of a nonocclusive plug, if
injury length does not exceed the critical value, and the total occlusion of
the vessel by the thrombus otherwise. We develop a mathematical model that
demonstrates that switching between these regimes occurs as a result of a
saddle-node bifurcation. Our study reveals the mechanism of self-regulation of
thrombosis in blood microvessels and explains experimentally observed
distinctions between thrombi of different physical etiology. This also can be
useful for the design of platelet-aggregation-inspired engineering solutions.Comment: 7 pages, 5 figures + Supplementary informatio
Anisotropic flow in striped superhydrophobic channels
We report results of dissipative particle dynamics simulations and develop a
semi-analytical theory of an anisotropic flow in a parallel-plate channel with
two superhydrophobic striped walls. Our approach is valid for any local slip at
the gas sectors and an arbitrary distance between the plates, ranging from a
thick to a thin channel. It allows us to optimize area fractions, slip lengths,
channel thickness and texture orientation to maximize a transverse flow. Our
results may be useful for extracting effective slip tensors from global
measurements, such as the permeability of a channel, in experiments or
simulations, and may also find applications in passive microfluidic mixing.Comment: 11 pages, 10 figures, submitted to J. Chem. Phy
Electro-osmosis on anisotropic super-hydrophobic surfaces
We give a general theoretical description of electro-osmotic flow at striped
super-hydrophobic surfaces in a thin double layer limit, and derive a relation
between the electro-osmotic mobility and hydrodynamic slip-length tensors. Our
analysis demonstrates that electro-osmotic flow shows a very rich behavior
controlled by slip length and charge at the gas sectors. In case of uncharged
liquid-gas interface, the flow is the same or inhibited relative to flow in
homogeneous channel with zero interfacial slip. By contrast, it can be
amplified by several orders of magnitude provided slip regions are uniformly
charged. When gas and solid regions are oppositely charged, we predict a flow
reversal, which suggests a possibility of huge electro-osmotic slip even for
electro-neutral surfaces. On the basis of these observations we suggest
strategies for practical microfluidic mixing devices. These results provide a
framework for the rational design of super-hydrophobic surfaces.Comment: 4 pages, 4 figures; submitted to PRL Revised version: several
references added, typos corrected. Supplementary file was restructured, the
second part of the original EPAPS was removed and is supposed to be published
as a separate pape