Recent advances regarding the measurement and theory of equilibrium contact angles on real surfaces are presented. The intrinsic contact angle is discussed in terms of the Young equation and the line tension concept. The fundamental question that is presented and discussed is the relationship between the intrinsic, actual and apparent contact angles. Apparent contact angle measurement using the Capillary Bridge System (CBS) is explained. The main advantages of this approach are the use of force measurements rather than direct optical measurements, and the ability to calculate an average apparent contact angle. The Wenzel equation for rough surfaces and the Cassie equation for heterogeneous surfaces are shown to be true only for drops that are very large compared to the scale of roughness/heterogeneity. Contact angle hysteresis is explained. Of special interest is the predicted difference between the drop and captive bubble techniques, which stems from contact angle hysteresis.
Abraham Marmur, "Wetting on Real Surfaces" in Proc. IS&T Int'l Conf. on Digital Printing Technologies (NIP15), 1999, pp 22 - 25, https://doi.org/10.2352/ISSN.2169-4451.1999.15.1.art00007_1