94 Liquid-gas and liquid-liquid interfaces
form two separate liquid-air interfaces (Figure 4.15a), and is given
by the Dupre equation
W, = TA + yB ~ TAB (4.29)
(b)
Figure 4.15 Work of adhesion (a) and of cohesion (b)
The work of cohesion for a single liquid corresponds to the work
required to pull apart a column of liquid of unit cross-sectional area
(Figure 4.15b) - i.e.
Wc = 2yA
Spreading of one liquid on another
(4.30)
When a drop of an insoluble oil is placed on a clean water surface, it
may behave in one of three ways:
- Remain as a lens, as in Figure 4.16 (non-spreading).
- Spread as a thin film, which may show interference colours, until it
is uniformly distributed over the surface as a 'duplex' film. (A
duplex film is a film which is thick enough for the two interfaces -
i.e. liquid-film and film-air - to be independent and possess
characteristic surface tensions.)
- Spread as a monolayer, leaving excess oil as lenses in equilibrium,
as in Figure 4.17.
