The characterization of wettability of substrates by liquid nanodrops

Wettability of substrates is characterized by a contact angle. Applicability of the simple formula developed by Derjaguin, which relates to the contact angle and disjoining pressure to nano-scale liquid droplets is reconsidered within the framework of the theory of the first-order wetting transition of volatile liquids. It is concluded that his formula is generally correct for large droplets on an incompletely-wettable substrate. But it cannot be applied to nanodroplets, in particular, on a completely-wettable substrate. An effective contact angle can be defined even for the nanodroplet. A formula similar to the Derjaguin formula is proposed by which we can calculate the contact angle of nanodoroplets on both an incompletely- and a completely-wettable substrate for which the whole volume of the droplet is under the influence of the disjoining pressure.