Wetting and roughness – What to do if Wenzel does not work?
How contact angle measurements and confocal microscopy can provide an empirical solution
The fact that the roughness of a surface can influence its wetting behavior is well known and described by Wenzel in his theory with a simple equation. But how well can this theory be transferred to industrial practice? We did the test and microstructured silicon wafers very precisely by reactive ion etching, but without changing the chemical structure of the surface and thus its surface free energy.
The microstructure of the surfaces was measured by confocal microscopy to determine the parameters relevant to Wenzel's theory. Contact angle measurements on the differently structured samples surprisingly showed a correlation contrary to Wenzel's prediction. This example shows that a correction of contact angle data, which uses Wenzel's assumptions as sole basis, is not recommended. However, by measuring both roughness and contact angle, an empirical solution can often be found in practice to separately investigate and ultimately predict the effects of roughness and surface free energy on wetting behavior.