Effective hydrodynamic boundary conditions for microtextured surfaces

We report measurements of the hydrodynamic drag force acting on a smooth sphere falling down under gravity to a plane decorated with microscopic periodic grooves. Both surfaces are lyophilic, so that a liquid (silicone oil) invades the surface texture being in the Wenzel state. A significant decrease in the hydrodynamic resistance force as compared with that predicted for two smooth surfaces is observed. To quantify the effect of roughness we use the effective no-slip boundary condition, which is applied at the imaginary smooth homogeneous isotropic surface located at an intermediate position between top and bottom of grooves. Such an effective condition fully characterizes the force reduction measured with the real surface, and the location of this effective plane is related to geometric parameters of the texture by a simple analytical formula.Comment: 4 pages, submitted to Phys. Rev.

Rebond d'une sphère sur une paroi texturée

Nous présentons des résultats expérimentaux sur la collision d'une sphère sur une paroi texturée en milieu liquide. Le mouvement de la sphère est mesuré par interférométrie laser à nombre de Reynolds et nombre de Stokes modérés. Les résultats montrent que le rebond dépend bien sûr du nombre de Stokes mais aussi des paramètres de la paroi texturée, hauteur et densité des textures notamment. L'enfoncement maximal de la sphère ainsi que la durée correspondante de collision suivent une loi de Hertz modifiée prenant en compte la texture de la paroi

Edge effects on water droplet condensation

International audienceIn this study is investigated the effect of geometrical or thermal discontinuities on the growth of water droplets condensing on a cooled substrate. Edges, corners, cooled/non cooled boundaries can have a strong effect on the vapor concentration profile and mass diffusion around the drops. In comparison to growth in a pattern where droplets have to compete to catch vapor, which results in a linear water concentration profile directed perpendicularly to the substrate, droplets near discontinuities can get more vapor (outer edges, corners), resulting in faster growth or less vapor (inner edges), giving lower growth. When the cooling heat flux limits growth instead of mass diffusion (substrate with low thermal conductivity, strong heat exchange with air), edge effects can be canceled. In certain cases, growth enhancement can reach nearly 500% on edges or corners

Dynamique d'une sphère à l'approche d'une paroi texturée dans un fluide visqueux

Nous présentons des résultats expérimentaux sur le mouvement d'une sphère tombant dans un fluide visqueux vers une paroi micro-texturée. Le déplacement de la sphère dans la région proche paroi est mesuré par interférométrie laser à haute fréquence. Les résultats montrent que la force de lubrification sur la sphère est diminuée par la présence des textures par rapport au cas d'une paroi lisse. Pour des petits nombres de Reynolds et de Stokes, cette diminution est modélisée à l'aide d'une longueur de glissement effective, qui dépend des paramètres géométriques des textures. Cette modélisation est étendue au cas des nombres de Reynolds et de Stokes modérés

Percolation-induced frost formation

We report the observation of an unconventional mechanism for frost formation. On a smooth hydrophobic surface cooled much below the water freezing temperature (−9 °C), we find that, instead of the classical freezing of individual supercooled condensed droplets, frost can occur through a multi-step 2-dimensional percolation-driven mechanism. This in-plane propagation process provides a model to investigate more complex bulk phase transformations such as those occurring in atmospheric supercooled clouds. It can also lead to a new method to control and design in-plane solidification at a nanoscale level

Dew condensation on desert beetle skin

Some tenebrionind beetles inhabiting the Namib desert are known for using their body to collect water droplets from wind-blown fogs. We aim to determine whether dew water collection is also possible for desert insects. For this purpose, we investigated the infra-red emissivity, and the wetting and structural properties, of the surface of the elytra of a preserved specimen of Physasterna cribripes (Tenebrionidæ) beetle, where the macro-structure appears as a series of “bumps”, with “valleys” between them. Dew formation experiments were carried out in a condensation chamber. The surface properties (infra-red emissivity, wetting properties) were dominated by the wax at the elytra surface and, to a lower extent, its micro-structure. We performed scanning electron microscope on histological sections and determined the infra-red emissivity using a scanning pyrometer. The emissivity measured (0.95 ± 0.07 between 8–14 μm) was close to the black body value. Dew formation occurred on the insect’s elytra, which can be explained by these surface properties. From the surface coverage of the condensed drops it was found that dew forms primarily in the valleys between the bumps. The difference in droplet nucleation rate between bumps and valleys can be attributed to the hexagonal microstructure on the surface of the valleys, whereas the surface of the bumps is smooth. The drops can slide when they reach a critical size, and be collected at the insect’s mouth

Étude expérimentale de suspensions de particules anisotropes en écoulement élongationnel

Ce travail comporte deux parties indépendantes: la première concerne les suspensions de particules anisotropes en écoulement élongationnel, la seconde porte sur l'étude en lois d'échelle des couches limites en convection thermosolutale. L'écoulement de suspensions de particules anisotropes présente en déformation élongationnelle des propriétés spécifiques liées à la géométrie des particules. Après la mise en oeuvre d'une cellule d'écoulement à orifice relisant une sollicitation élongationnelle bien définie, des suspensions de fibres non-browniennes sont étudiées dans deux régimes de concentration. En régime dilué, les résultats expérimentaux concernant la dynamique d'orientation d'une fibre sont bien décrits par les équations d'orientations du vecteur directeur obtenues analytiquement. En régime semi-dilué, l'écoulement présente une structuration remarquable, qui est étudiée en fonction de la fraction volumique et du rapport de forme des particules. Un modèle d'écoulement obtenu en minimisant l'énergie dissipée permet d'interpréter les résultats expérimentaux en termes de viscosité élongationnelle, et en relation avec les propriétés de la suspension à l'échelle des particules. Les écoulements de couche limite au voisinage d'une paroi verticale dans un milieu fluide soumis a des gradients de température et de concentration horizontaux présentent une grande variété de situations suivant la valeur des paramètres adimensionnels du problème. Une analyse combinant raisonnements en loi d'échelle et méthode intégrale permet de prédire complètement les différents régimes d'écoulement et les variations des couches limites. Les résultats sont corroborés par des solutions de similitude obtenues numériquement

Sticking collision between a sphere and a textured wall in a viscous fluid

International audienceThe collision of a sphere with a wettable micro-textured wall in a viscous fluid is investigated experimentally, focusing on the region close to the contact with the wall. High frequency laser interferometry is used for measuring small displacements of the sphere in that region. The wall texture consists of an a array of square micro-pillars, whose geometrical parameters (height, width and spacing of the pillars) are varied. The wall texture decreases the hydrodynamic resistance, and hence the drag on the sphere, compared to the case of a smooth wall. At small Reynolds and Stokes numbers, this drag reduction is quantified in terms of an equivalent plane boundary, shifted down from the top of the pillars. The shift length depends on the geometrical parameters of the pillars array, and is compared to available predictions of effective slip length for a flow over arrays of micro-pillars in the Wenzel state. At finite Stokes number, below the bouncing transition, the wall texture influences the relative importance of sphere inertia and drag force in the near wall region. As a result, a great diversity of sphere dynamics are obtained by varying the texture geometrical parameters. These dynamics can be captured considering a shift-length-modified drag force in the equation of motion of the sphere

Squeeze flow between a sphere and a textured wall

International audienceThe motion of a millimetric sphere, translating in a viscous fluid towards a wettable textured wall, is investigated experimentally. The textures consist of square arrays of cylindrical or square micro-pillars, the height, width and spacing of which are varied, keeping the periodicity small compared to the sphere radius. An interferometric device is used to measure the sphere vertical displacement, for distances between the sphere and the base of the pillars smaller than 0.1 sphere radius, and with a resolution of 200 nm. At a given distance from the top of the pillars, the sphere velocity is found to be significantly larger than the corresponding velocity for a smooth solid wall. A squeeze flow model of two adjacent fluid layers is developed in the lubrication approximation, one fluid layer having an effective viscosity that reflects the viscous dissipation through the array of pillars. The pressure field in the gap between the sphere and the textured surface is then used to obtain the drag force on the sphere and hence its velocity. Adjustment of the model to the velocity measurements yields the effective viscosity for a given texture. Finally, a correlation between the effective viscosity and the geometry of the pillar array is proposed

Roughness-enhanced collection of condensed droplets

Gravity shedding of droplets is limited by droplet pinning, a major limitation for low condensation processes and in particular passive dew harvesting in its use as an alternative source of water. We present experiments showing that, paradoxically, a simple surface treatment increasing roughness (sand-blasting) favors droplet shedding compared to the original substrate, provided that sand-blasting does not increase too much the surface roughness. Sand-blasting ensures the high density of nucleation sites and enhances drops coalescence and growth at a sub-micron scale, thus lowering the lag-time to obtain drop sliding during condensation. Early nucleation indeed overcompensates the delay increase due to roughness. Edges of the substrate, where drops grow faster, also improve water collection, thanks to the early sliding of edge drops that behave as natural wipers. Combining the effects of sand-blasting and edges increases significantly the rate of collection of dew condensation on a substrate at a given time, gains of about 30% can be commonly obtained