2,252 research outputs found
Probing the nanohydrodynamics at liquid-solid interfaces using thermal motion
We report on a new method to characterize nano-hydrodynamic properties at the
liquid/solid interface relying solely on the measurement of the thermal motion
of confined colloids. Using Fluorescence Correlation Spectroscopy (FCS) to
probe the diffusion of the colloidal tracers, this optical technique
--equivalent in spirit to the microrheology technique used for bulk
properties-- is able to achieve nanometric resolution on the slip length
measurement. It confirms the no-slip boundary condition on wetting surfaces and
shows a partial slip b=18 +/- 5 nm on non-wetting ones. Moreover, in the
absence of external forcing, we do not find any evidence for large nano-bubble
promoted slippage on moderately rough non-wetting surfaces.Comment: 4 pages, 3 figure
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
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
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
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
Are gravitational waves from giant magnetar flares observable?
Are giant flares in magnetars viable sources of gravitational radiation? Few
theoretical studies have been concerned with this problem, with the small
number using either highly idealized models or assuming a magnetic field orders
of magnitude beyond what is supported by observations. We perform nonlinear
general-relativistic magnetohydrodynamics simulations of large-scale
hydromagnetic instabilities in magnetar models. We utilise these models to find
gravitational wave emissions over a wide range of energies, from 10^40 to 10^47
erg. This allows us to derive a systematic relationship between the surface
field strength and the gravitational wave strain, which we find to be highly
nonlinear. In particular, for typical magnetar fields of a few times 10^15 G,
we conclude that a direct observation of f-modes excited by global magnetic
field reconfigurations is unlikely with present or near-future gravitational
wave observatories, though we also discuss the possibility that modes in a
low-frequency band up to 100 Hz could be sufficiently excited to be relevant
for observation.Comment: 4 pages, 3 figures. Further information can be found at
http://www.physik.uni-tuebingen.de/institute/astronomie-astrophysik/institut/theoretische-astrophysik/forschung.htm
Contact angle measurements on superhydrophobic Carbon Nanotube Forests : effect of fluid pressure
In this paper the effect of pressure on the contact angle of a water drop on
superhydrophobic Carbon Nanotube (CNT) forests is studied. Superhydrophobic CNT
forests are obtained from a new and simple functionalization strategy, based on
the gold-thiol affinity. Using a specifically devised experimental setup, we
then show that these surfaces are able to withstand high excess pressures
(larger than 10 kPa) without transiting toward a roughness-invaded state,
therefore preserving their low adhesion properties. Together with the
relatively low technical cost of the process, this robustness versus pressure
makes such surfaces very appealing for practical integration into microfluidic
systems.Comment: accepted for publication in Europhysics Letter
Diffusion in pores and its dependence on boundary conditions
We study the influence of the boundary conditions at the solid liquid
interface on diffusion in a confined fluid. Using an hydrodynamic approach, we
compute numerical estimates for the diffusion of a particle confined between
two planes. Partial slip is shown to significantly influence the diffusion
coefficient near a wall. Analytical expressions are derived in the low and high
confinement limits, and are in good agreement with numerical results. These
calculations indicate that diffusion of tagged particles could be used as a
sensitive probe of the solid-liquid boundary conditions.Comment: soumis \`a J.Phys. Cond. Matt. special issue on "Diffusion in
Liquids, Polymers, Biophysics and Chemical Dynamics
Surfactant-Mediated Epitaxial Growth of Single-Layer Graphene in an Unconventional Orientation on SiC
We report the use of a surfactant molecule during the epitaxy of graphene on
SiC(0001) that leads to the growth in an unconventional orientation, namely
rotation with respect to the SiC lattice. It yields a very
high-quality single-layer graphene with a uniform orientation with respect to
the substrate, on the wafer scale. We find an increased quality and homogeneity
compared to the approach based on the use of a pre-oriented template to induce
the unconventional orientation. Using spot profile analysis low energy electron
diffraction, angle-resolved photoelectron spectroscopy, and the normal
incidence x-ray standing wave technique, we assess the crystalline quality and
coverage of the graphene layer. Combined with the presence of a
covalently-bound graphene layer in the conventional orientation underneath, our
surfactant-mediated growth offers an ideal platform to prepare epitaxial
twisted bilayer graphene via intercalation.Comment: 7 pages, 3 figure
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