Pearl drops

– If deposited on a hydrophobic rough substrate, a small drop of water can look like a pearl, with a contact angle close to 180. We examine the conditions for observing such a phenomenon and show practical achievements where the contact angle can be predicted and thus quantitatively tuned by the design of the surface microstructure. Presentation. – The contact angle of a drop on a solid is ideally (for a flat homogeneous solid) given by the classical Young’s equation: cos θ = γSV − γSL γ , (1) where γSV, γSL and γ are the different surface tensions (solid/vapor, solid/liquid and liquid/vapor) involved in the system. Here we describe how the contact angle can be modified by the surface texture and focus on the limit of practical interest where very large contact angles can be obtained [1, 2]. Such surfaces were recently achieved by the Kao group [3,4]: a water drop put on fractal hydrophobic surfaces was found to have contact angles of order 170◦. On some leaves (for example lotus or lily-water), water drops are also found to be nearly spherical (contact angles of order 160◦) [5]. Here, we exhibit the relevant parameters for obtaining such angles and present new data obtained with model surfaces. Simple models. – Wenzel proposed the first approach to caracterize the influence of the surface roughness on the wettability of a solid [6]. The contact angle θ∗ on a rough surface can be evaluated by considering a little displacement dx of the contact line parallel to the surface (fig. 1).