When a drop of liquid contacts a surface, one of two phenomena
generally4 occurs (1) the liquid wets the surface; a circular three-
phase contact line is formed that continues to increase in radius
and liquid spreads over the entire surface, (2) the droplet contacts
the surface and the three-phase contact line advances to a certain
radius and stops, creating a sessile droplet in the shape of a
section of a sphere with a contact angle that depends on the
chemical compositions of the liquid and the surface and the
topography of the surface. Which of these phenomena occurs
and the value of the contact angle that forms in the second case
are described in quantitative terms by Young’s equation (eq 1),
which relates the contact angle to the three interfacial free energies