A drop of water thrown on a super-hydrophobic solid will often bounce off. Here we discuss the conditions to be fulfilled on the surface design (which provides superhydrophobicity) to observe such a behavior. This allows us to precise how a material can be made water-repellent. We show in particular how the reduction of the scale of the microstructure provides a robust water repellency, and describe some peculiarities of violent shocks on such surfaces.

Water drops deposited on hydrophobic materials decorated with dilute micro-posts generally form pearls. Owing to the hydrophobicity of the material, the drop sits on the top of the posts. However, this "fakir state" is often metastable: if the drop impales inside the texture, its surface energy is lowered. Here we discuss the transition between these two states, considering the drop size as a parameter for inducing this transition: remarkably, it is found that a drop impales when it becomes small, which is interpreted by considering its curvature. This interpretation allows us to propose different recipes for avoiding this detrimental effect.

Water on solid decorated with hydrophobic defects (such as micropillars) often stays at the top of the defects in a so-called fakir state, which explains the superhydrophobicity observed in such case, provided that the density of defects is small enough. Here we show that this situation provides an ideal frame for studying the contact angle hysteresis; the phase below the liquid is "perfect" and slippery (it is air), contrasting with pillars' tops whose edges form strong pining sites for the contact line. This model system thus allows us to study the hysteresis as a function of the density of defects and to compare it to the classical theory by Joanny and de Gennes, which is based on very similar hypothesis.

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 film progresses all the faster since the posts are high, allowing a simple control of this dynamics. For tall pillars (h > p), the speed of impregnation becomes independent of the pillar height, and becomes mainly fixed by the radius of the posts.

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.

We report the influence of the nature of boundaries on the dynamics of wetting. We review some work recently published and highlight new experimental observations. Our paper begins with the spreading of drops on substrates and demonstrates how the exponents of the spreading laws are affected either by the surface chemistry or by the droplet shape. We then discuss the imbibition of completely and partially wetting fluids into channels and over microtextured surfaces. Starting with the one-dimensional imbibition of completely wetting liquids in tubes and surface textures, we show that (i) shape variations of channels change the power-law response of the imbibition and (ii) the geometrical parameters of a surface roughness change the spreading behavior. For partially wetting fluids, we observe directionally dependent spreading: polygonal wetted domains can be obtained. We conclude with a tabular summary of our findings, allowing us to draw connections between the different systems investigated, and shed light on open questions that remain to be addressed.

Superhydrophobicity is mainly remarkable for the special dynamical behaviours it generates: low adhesion, giant hydrodynamic slip, frictionless motion, rebounds after impacts. Here we discuss most of these properties. We first recall how contact angle hysteresis can be minimized in this state. Then, we show that a water drop first follows the Galilean law of free fall on an incline, before reaching a stationary state, for which we discuss the associated friction. Finally, the property of water repellency (that is, rebounds after impact) is presented. We describe in particular how the texture responsible for superhydrophobicity can also influence the figure of impact at a very large scale.

Hydrophobic microtextures on solids provide water repellency: drops hardly stick on these materials and bounce off after impacts. Here we achieved solids decorated with a texture of variable density. Impacting water drops were observed to bounce off obliquely, demonstrating a transfer of vertical momentum in the horizontal direction, after rebound. This allows us to understand why vibrated drops move on such surface: an asymmetric dewetting takes place for each cycle of the vibration, which leads to an incremental drift of the liquid towards regions of high texture density.