| Abstract: | The wetting/non-wetting properties of a liquid drop in contact with achemically hydrophobic rough surface (thermodynamic contact angle  e> /2) are studied for the case of an extremely idealized rough profile:the liquid drop is considered to lie on a simple sinusoidal profile. Dependingon surface geometry and pressure values, it is found that the Cassie andWenzel states can coexist. But if the amplitude h of the substrate issufficiently large the only possible stable state is the Cassie one, whereasif h is below a certain critical value hcr a transition to the Wenzelstate occurs. Since in many potential applications of such super-hydrophobicsurfaces, liquid drops often collide with the substrate (e.g. vehiclewindscreens), in the paper the critical drop pressure pW is calculated atwhich the Cassie state is no longer stable and the liquid jumps into fullcontact with the substrate (Wenzel state). By analyzing the asymptoticbehavior of the systems in the limiting case of a large substrate corrugation,a simple criterion is also proposed to calculate the minimum height asperity h necessary to prevent the Wenzel state from being formed, to preserve thesuper-hydrophobic properties of the substrate, and, hence, to design a robustsuper-hydrophobic surface. |
