The wetting transition is generally first-order (discontinuous),
implying a discontinuity in the first derivative of the surface free
energy. At the wetting transition, a discontinuous jump in film
thickness occurs from a molecularly thin to a thick film. We show
that the first-order nature of the transition can lead to the
observation of metastable surface states and an accompanying
hysteresis.
The second part of my talk deals with the exceptions to the
first-order nature of the wetting transition. We have reported two
different types of continuous or critical wetting transitions, for
which a discontinuity in a higher derivative of the surface free
energy occurs. This consequently leads to a continuous divergence of
the film thickness. The first type is long-range critical wetting,
due to the long-range van der Waals forces. We show that this
transition is preceded by the usual first-order wetting transition,
which however is not achieved completely. This leads to the existence
of a new intermediate wetting state, in which droplets coexist with a
mesoscopic film: frustrated complete wetting. The film thickness
diverges continuously from this mesoscopic film to a thick film.
The second type of continuous transition is short-range critical
wetting, for which the layer thickness diverges continuously all the
way from a microscopic to a macroscopically thick film. This
transition is interesting, as renormalization-group studies predict
non-universal behavior for the critical exponents characterizing the
wetting transition. The experimental results however, show mean field
behavior, the reason for which remains unclear.