556 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
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International audienceCet article décrit comment le logiciel AGORA a été amélioré afin de tenir compte des retours d'usages obtenus lors d'une utilisation intensive de la version 1 du logiciel par les etudiants et les membres du CESR. La nouvelle version propose un module d'assistance aux utiilisateurs afin de rendre plus simple, conviviale et intuitive la phase de mise en place des scenarios d'indexation. L'utilisateur (non expert en traitement d'images) construit simplement des scénarios permettant d'étiqueter, de fusionner et de supprimer les éléments de contenus à indexer. Il localise ainsi les entités qui l'intéressent en ignorant les autres régions de l'image. Les scénarios élaborés peuvent ensuite être sauvegardés, modifiés et appliqués sur différentes images lors de traitements par lots
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
Impalement transitions in droplets impacting microstructured superhydrophobic surfaces
Liquid droplets impacting a superhydrophobic surface decorated with
micro-scale posts often bounce off the surface. However, by decreasing the
impact velocity droplets may land on the surface in a fakir state, and by
increasing it posts may impale droplets that are then stuck on the surface. We
use a two-phase lattice-Boltzmann model to simulate droplet impact on
superhydrophobic surfaces, and show that it may result in a fakir state also
for reasonable high impact velocities. This happens more easily if the surface
is made more hydrophobic or the post height is increased, thereby making the
impaled state energetically less favourable.Comment: 8 pages, 4 figures, to appear in Europhysics Letter
Piercing an interface with a brush: collaborative stiffening
The hairs of a painting brush withdrawn from a wetting liquid self-assemble
into clumps whose sizes rely on a balance between liquid surface tension and
hairs bending rigidity. Here we study the situation of an immersed carpet in an
evaporating liquid bath : the free extremities of the hairs are forced to
pierce the liquid interface. The compressive capillary force on the tip of
flexible hairs leads to buckling and collapse. However we find that the
spontaneous association of hairs into stronger bundles may allow them to resist
capillary buckling. We explore in detail the different structures obtained and
compare them with similar patterns observed in micro-structured surfaces such
as carbon nanotubes "forests".Comment: 9 pages, 6 figure
Making a splash with water repellency
A 'splash' is usually heard when a solid body enters water at large velocity.
This phenomena originates from the formation of an air cavity resulting from
the complex transient dynamics of the free interface during the impact. The
classical picture of impacts on free surfaces relies solely on fluid inertia,
arguing that surface properties and viscous effects are negligible at
sufficiently large velocities. In strong contrast to this large-scale
hydrodynamic viewpoint, we demonstrate in this study that the wettability of
the impacting body is a key factor in determining the degree of splashing. This
unexpected result is illustrated in Fig.1: a large cavity is evident for an
impacting hydrophobic sphere (1.b), contrasting with the hydrophilic sphere's
impact under the very same conditions (1.a). This unforeseen fact is
furthermore embodied in the dependence of the threshold velocity for air
entrainment on the contact angle of the impacting body, as well as on the ratio
between the surface tension and fluid viscosity, thereby defining a critical
capillary velocity. As a paradigm, we show that superhydrophobic impacters make
a big 'splash' for any impact velocity. This novel understanding provides a new
perspective for impacts on free surfaces, and reveals that modifications of the
detailed nature of the surface -- involving physico-chemical aspects at the
nanometric scales -- provide an efficient and versatile strategy for
controlling the water entry of solid bodies at high velocity.Comment: accepted for publication in Nature Physic
Low frequency Raman studies of multi-wall carbon nanotubes: experiments and theory
In this paper, we investigate the low frequency Raman spectra of multi-wall
carbon nanotubes (MWNT) prepared by the electric arc method. Low frequency
Raman modes are unambiguously identified on purified samples thanks to the
small internal diameter of the MWNT. We propose a model to describe these
modes. They originate from the radial breathing vibrations of the individual
walls coupled through the Van der Waals interaction between adjacent concentric
walls. The intensity of the modes is described in the framework of bond
polarization theory. Using this model and the structural characteristics of the
nanotubes obtained from transmission electron microscopy allows to simulate the
experimental low frequency Raman spectra with an excellent agreement. It
suggests that Raman spectroscopy can be as useful regarding the
characterization of MWNT as it is in the case of single-wall nanotubes.Comment: 4 pages, 2 eps fig., 2 jpeg fig., RevTex, submitted to Phys. Rev.
Interaction of carbon clusters with Ni(100) : Application to the nucleation of carbon nanotubes
In order to understand the first stages of the nucleation of carbon nanotubes
in catalytic processes, we present a tight-binding Monte Carlo study of the
stability and cohesive mechanisms of different carbon structures deposited on
nickel (100) surfaces. Depending on the geometry, we obtain contrasted results.
On the one hand, the analysis of the local energy distributions of flat carbon
sheets, demonstrate that dangling bonds remain unsaturated in spite of the
presence of the metallic catalyst. Their adhesion results from the energy gain
of the surface Ni atoms located below the carbon nanostructure. On the other
hand, carbon caps are stabilized by the presence of carbon atoms occupying the
hollow sites of the fcc nickel structure suggesting the saturation of the
dangling bonds
Grain legume–cereal intercropping systems
Chapitre 11Grain legume–cereal intercropping system
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