The ``fakir drop'': a (un)stable situation

Hydrophobic surfaces can be made super-hydrophobic by creating a texture on them. This effect, sometimes referred to as the ``fakir effect,'' is due to air trapping in the structure, which provides a composite surface made of solid and air on which the deposited drop sits. Here we give evidence for this effect using forests of micro-pillars, made by photolithography and deep etching. This allows us to control the density of solid and air under the drop, and thus the degree of super-hydrophobicity. However, the ``fakir state'' is not always the most stable situation for a drop on a hydrophobic surface. The drop may instead fill the microstructure and this other state, called the ``Wenzel state,'' has very different properties. We show how to observe transitions between these two states. We also achieve materials with a density gradient of micropillars, and discuss the possibility of inducing spontaneous drop motion on such surfaces.