164 research outputs found
Slippage of water past superhydrophobic carbon nanotube forests in microchannels
We present in this letter an experimental characterization of liquid flow
slippage over superhydrophobic surfaces made of carbon nanotube forests,
incorporated in microchannels. We make use of a micro-PIV (Particule Image
Velocimetry) technique to achieve the submicrometric resolution on the flow
profile necessary for accurate measurement of the surface hydrodynamic
properties. We demonstrate boundary slippage on the Cassie superhydrophobic
state, associated with slip lengths of a few microns, while a vanishing slip
length is found in the Wenzel state, when the liquid impregnates the surface.
Varying the lateral roughness scale L of our carbon nanotube forest-based
superhydrophobic surfaces, we demonstrate that the slip length varies linearly
with L in line with theoretical predictions for slippage on patterned surfaces.Comment: under revie
Effect of Patterned Slip on Micro and Nanofluidic Flows
We consider the flow of a Newtonian fluid in a nano or microchannel with
walls that have patterned variations in slip length. We formulate a set of
equations to describe the effects on an incompressible Newtonian flow of small
variations in slip, and solve these equations for slow flows. We test these
equations using molecular dynamics simulations of flow between two walls which
have patterned variations in wettability. Good qualitative agreement and a
reasonable degree of quantitative agreement is found between the theory and the
molecular dynamics simulations. The results of both analyses show that
patterned wettability can be used to induce complex variations in flow. Finally
we discuss the implications of our results for the design of microfluidic
mixers using slip.Comment: 13 pages, 12 figures, final version for publicatio
Colloidal motility and pattern formation under rectified diffusiophoresis
In this letter, we characterize experimentally the diffusiophoretic motion of
colloids and lambda- DNA toward higher concentration of solutes, using
microfluidic technology to build spatially- and temporally-controlled
concentration gradients. We then demonstrate that segregation and spatial
patterning of the particles can be achieved from temporal variations of the
solute concentration profile. This segregation takes the form of a strong
trapping potential, stemming from an osmotically induced rectification
mechanism of the solute time-dependent variations. Depending on the spatial and
temporal symmetry of the solute signal, localization patterns with various
shapes can be achieved. These results highlight the role of solute contrasts in
out-of-equilibrium processes occuring in soft matter
Scaling laws for slippage on superhydrophobic fractal surfaces
We study the slippage on hierarchical fractal superhydrophobic surfaces, and
find an unexpected rich behavior for hydrodynamic friction on these surfaces.
We develop a scaling law approach for the effective slip length, which is
validated by numerical resolution of the hydrodynamic equations. Our results
demonstrate that slippage does strongly depend on the fractal dimension, and is
found to be always smaller on fractal surfaces as compared to surfaces with
regular patterns. This shows that in contrast to naive expectations, the value
of effective contact angle is not sufficient to infer the amount of slippage on
a fractal surface: depending on the underlying geometry of the roughness,
strongly superhydrophobic surfaces may in some cases be fully inefficient in
terms of drag reduction. Finally, our scaling analysis can be directly extended
to the study of heat transfer at fractal surfaces, in order to estimate the
Kapitsa surface resistance on patterned surfaces, as well as to the question of
trapping of diffusing particles by patchy hierarchical surfaces, in the context
of chemoreception
Dynamics of simple liquids at heterogeneous surfaces : Molecular Dynamics simulations and hydrodynamic description
In this paper we consider the effect of surface heterogeneity on the slippage
of fluid, using two complementary approaches. First, MD simulations of a
corrugated hydrophobic surface have been performed. A dewetting transition,
leading to a super-hydrophobic state, is observed for pressure below a
``capillary'' pressure. Conversely a very large slippage of the fluid on this
composite interface is found in this superhydrophobic state. Second, we propose
a macroscopic estimate of the effective slip length on the basis of continuum
hydrodynamics, in order to rationalize the previous MD results. This
calculation allows to estimate the effect of a heterogeneous slip length
pattern on the composite interface. Comparison between the two approaches are
in good agreement at low pressure, but highlights the role of the exact shape
of the liquid-vapor interface at higher pressure. These results confirm that
small variations in the roughness of a surface can lead to huge differences in
the slip effect. On the basis of these results, we propose some guidelines to
design highly slippery surfaces, motivated by potential applications in
microfluidics.Comment: submitted to EPJ
Dynamic clustering in active colloidal suspensions with chemical signaling
In this paper, we explore experimentally the phase behavior of a dense active
suspension of self- propelled colloids. In addition to a solid-like and a
gas-like phase observed for high and low densities, a novel cluster phase is
reported at intermediate densities. This takes the form of a stationary
assembly of dense aggregates, with an average size which grows with activity as
a linear function of the self-propelling velocity. While different possible
scenarii can be considered to account for these observations - such as a
generic velocity weakening instability recently put forward -, we show that the
experimental results are reproduced by a chemotactic aggregation mechanism,
originally introduced to account for bacterial aggregation, and accounting here
for diffusiophoretic chemical interaction between colloidal swimmers.Comment: supplementary video :http://
www-lpmcn.univ-lyon1.fr/~lbocquet/Movie-Theurkauff-SI.av
Nanorheology : an Investigation of the Boundary Condition at Hydrophobic and Hydrophilic Interfaces
t has been shown that the flow of a simple liquid over a solid surface can
violate the so-called no-slip boundary condition. We investigate the flow of
polar liquids, water and glycerol, on a hydrophilic Pyrex surface and a
hydrophobic surface made of a Self-Assembled Monolayer of OTS
(octadecyltrichlorosilane) on Pyrex. We use a Dynamic Surface Force Apparatus
(DSFA) which allows one to study the flow of a liquid film confined between two
surfaces with a nanometer resolution. No-slip boundary conditions are found for
both fluids on hydrophilic surfaces only. Significant slip is found on the
hydrophobic surfaces, with a typical length of one hundred nanometers.Comment: 8 pages, 7 figures, 2 tables. Accepted for European Physical Journal
E - Sofr Mate
Slip flow over structured surfaces with entrapped microbubbles
On hydrophobic surfaces, roughness may lead to a transition to a
superhydrophobic state, where gas bubbles at the surface can have a strong
impact on a detected slip. We present two-phase lattice Boltzmann simulations
of a Couette flow over structured surfaces with attached gas bubbles. Even
though the bubbles add slippery surfaces to the channel, they can cause
negative slip to appear due to the increased roughness. The simulation method
used allows the bubbles to deform due to viscous stresses. We find a decrease
of the detected slip with increasing shear rate which is in contrast to some
recent experimental results implicating that bubble deformation cannot account
for these experiments. Possible applications of bubble surfaces in microfluidic
devices are discussed.Comment: 4 pages, 4 figures. v2: revised version, to appear in Phys. Rev. Let
Evidence of slippage breakdown for a superhydrophobic microchannel
© 2014 AIP Publishing LLC.A full characterization of the water flow past a silicon superhydrophobic surface with longitudinal micro-grooves enclosed in a microfluidic device is presented. Fluorescence microscopy images of the flow seeded with fluorescent passive tracers were digitally processed to measure both the velocity field and the position and shape of the liquid-air interfaces at the superhydrophobic surface. The simultaneous access to the meniscus and velocity profiles allows us to put under a strict test the no-shear boundary condition at the liquid-air interface. Surprisingly, our measurements show that air pockets in the surface cavities can sustain non-zero interfacial shear stresses, thereby hampering the friction reduction capabilities of the surface. The effects of the meniscus position and shape as well as of the liquid-air interfacial friction on the surface performances are separately assessed and quantified
Hydrodynamic slip boundary condition at chemically patterned surfaces: A continuum deduction from molecular dynamics
We investigate the slip boundary condition for single-phase flow past a
chemically patterned surface. Molecular dynamics (MD) simulations show that
modulation of fluid-solid interaction along a chemically patterned surface
induces a lateral structure in the fluid molecular organization near the
surface. Consequently, various forces and stresses in the fluid vary along the
patterned surface. Given the presence of these lateral variations, a general
scheme is developed to extract hydrodynamic information from MD data. With the
help of this scheme, the validity of the Navier slip boundary condition is
verified for the chemically patterned surface, where a local slip length can be
defined. Based on the MD results, a continuum hydrodynamic model is formulated
using the Navier-Stokes equation and the Navier boundary condition, with a slip
length varying along the patterned surface. Steady-state velocity fields from
continuum calculations are in quantitative agreement with those from MD
simulations. It is shown that, when the pattern period is sufficiently small,
the solid surface appears to be homogeneous, with an effective slip length that
can be controlled by surface patterning. Such a tunable slip length may have
important applications in nanofluidics.Comment: 41 pages, 17 figure
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