Contact angle measurement on rough surfaces: the missing link

Wettability of rough surfaces is critical for many processes and materials. The most prevalent technique for characterizing wettability is the measurement of contact angles. Unfortunately, the routine assessment of wettability by contact angle measurement is not as simple as it appears to be. The main reason is that most real surfaces are characterized by a range of seemingly stable contact angles, and not by a single value. Moreover, the technique of contact angle measurement has not been updated to utilize current theoretical understanding with regard to the most stable state of the drop and its symmetry. Here we experimentally demonstrate the validity of the theoretical observation that a sufficiently large drop must be axisymmetric at the most stable thermodynamic state of the system. This is the link missing in current methodology between the measurement of the apparent contact angle and its interpretation in terms of the ideal (Young) contact angle.