When a liquid droplet is put onto a surface, two situations distinguishable
by the contact angle may result. If the contact angle is zero, the droplet
spreads across the surface, a situation referred to as complete wetting. O
n the other hand, if the contact angle is between 0 degrees and 180 degrees
, the droplet does not spread, a situation called partial wetting. A wettin
g transition is a surface phase transition from partial wetting to complete
wetting. We review the key experimental findings on this transition, toget
her with simple theoretical models that account for the experiments.
The wetting transition is generally first order (discontinuous), implying a
discontinuity in the first derivative of the surface free energy. In this
case, if one measures the thickness of the adsorbed film beside the droplet
, at the wetting transition a discontinuous jump in film thickness occurs f
rom a microscopically thin to a thick film. We show that this can lead to t
he observation of metastable surface states and an accompanying hysteresis.
The observed hysteresis poses, in turn, a number of questions concerning t
he nucleation of wetting films that we also consider here. In addition, we
consider the equilibrium wetting film thickness that results from a competi
tion between the long-range van der Waals forces and gravity.
Finally, the first-order character of the wetting transition can lead to a
similar transition even when the phase that does the wetting is not (yet) s
table in the bulk. For such prewetting transitions, a discontinuous thin-to
-thick film transition occurs off bulk coexistence. We show that, for the l
arge variety of systems for which prewetting transitions have been observed
, the behaviour is surprisingly uniform, and can be mapped onto a simple ge
neric phase diagram.
The second part of the review deals with the exceptions to the first-order
nature of the wetting transition. Two different types of continuous or crit
ical wetting transition have been reported, for which a discontinuity in a
higher derivative of the surface free energy occurs. This consequently lead
s to a continuous divergence of the film thickness. The first type is the s
o-called long-range critical wetting transition, which is due to the long-r
ange van der Waals forces. We show under what circumstances such a transiti
on can occur, and that it is usually preceded by a first-order wetting tran
sition, which however is not achieved completely. This leads to the existen
ce of an intermediate wetting state, in which droplets coexist with a relat
ively-but not macroscopically-thick film. The second type of transition is
the short-range critical wetting transition, for which the layer thickness
diverges continuously from a microscopic to a macroscopically thick film. T
his transition is interesting, as a number of renormalization-group studies
predict non-universal behaviour for the critical exponents. The experiment
al results indicate, however, mean-field behaviour, the reason for which is
discussed in detail.