than its thicknessh, the interaction free energyVthen should be given per
unit surface area of the film, i.e., in J?m^2. In the case of particles, it is given
in joules per particle pair.
In some cases, the force involved is needed rather than the energy. This
is simply given bydV/dh(cf. Figure 3.1). For particle pairs the result is in
FIGURE12.1 Hypothetical example (solid curve) of the colloidal interaction free
energyVdivided bykBT, as a function of interparticle distanceh. The insert defines
the geometry involved for the case of two spheres of radiusR. The broken lines give
two other examples of interaction curves.