Etienne Reyssat

Wicking within forests of micropillars

We describe how a wetting liquid brought into contact with a forest of micropillars impregnates this forest. Both the driving and the viscous forces depend on the parameters of the texture (radius b and height h of the pillars, pitch p of the network) and it is found that two different limits characterize the dynamics of wicking. For small posts ( h p ), the speed of impregnation becomes independent of the pillar height, and becomes mainly fixed by the radius of the posts. Copyright c EPLA, 2007

Imbibition in geometries with axial variations

When surface wetting drives liquids to invade porous media or microstructured materials with uniform channels, the penetration distance is known to increase as the square root of time. We demonstrate, experimentally and theoretically, that shape variations of the channel, in the flow direction, modify this ‘diffusive’ response. At short times, the shape variations are not significant and the imbibition is still diffusive. However, at long times, different power-law responses occur, and their exponents are uniquely connected to the details of the geometry. Experiments performed with conical tubes clearly show the two theoretical limits. Several extensions of these ideas are described.

Shape and instability of free-falling liquid globules

The velocity of a falling raindrop depends on its size, and thus so does its shape. Here we describe the different simple shapes which model drops falling in air. While millimetric drops remain spherical, owing to the action of surface tension, drops larger than the capillary length get flattened, as sessile drops on solids. Air penetrates still larger globules, which are observed to be unstable. They inflate till they burst, generating myriads of fragments.

Marangoni bursting: evaporation-induced emulsification of binary mixtures on a liquid layer

Adjusting the wetting properties of water through the addition of a miscible liquid is commonly used in a wide variety of industrial processes involving interfaces. We investigate experimentally the evolution of a drop of water and volatile alcohol deposited on a bath of oil: The drop spreads and spontaneously fragments into a myriad of minute droplets whose size strongly depends on the initial concentration of alcohol. Marangoni flows induced by the evaporation of alcohol play a key role in the overall phenomenon. The intricate coupling of hydrodynamics, wetting, and evaporation is well captured by analytical scaling laws. Our scenario is confirmed by experiments involving other combinations of liquids that also lead to this fascinating phenomenon.

Viscous jet drawing air into a bath

When filling a glass with water, air bubbles often appear.1 Similarly, the impact of a viscous jet on a bath of the same liquid may trigger air entrainment see Figs. 1 and 2. As the impact velocity is increased, viscous stresses induce deformations of the bath interface, up to the point where surface tension fails to maintain the surface integrity.2 Air is then entrained in the bath Fig. 2, as a thin film coating the jet before decaying into bubbles Fig. 3. The entrainment threshold velocity is observed to depend on the fluids,3 but also on the radius of the jet.4 As seen in Fig. 1, the impacting liquid is deformed by the bath before reaching it: The jet spreads out due to its viscosity, which prevents entrainment from occurring; the thinner the jet low Reynolds numbers, the stronger this effect. Above the entrainment threshold, a micrometric film of air coats the jet, before breaking into bubbles after a few centimeters. Measuring the rate of air entrainment shows that the film thickness is proportional to the jet radius. This is due to the Laplace pressure inside the film of air, which pushes air outward stronger if the jet is thinner.