10.6 CONTACT ANGLES AND WETTING
If three different phases are in pairwise contact, thus giving three different
phase boundaries, there is acontact linewhere the three phases meet. Two
rather different cases can be distinguished:
A solid, a liquid, and a fluid phase (gas or liquid); for instance, a water
drop on a metal surface in air or in oil.
Three fluid phases, of which one (and not more than one) is mostly
gaseous; for instance, an oil drop on an air–water surface.10.6.1 Contact Angle
We will first consider Figure 10.24a. At equilibrium there must be a balance
of surface forces at any point on the contact line. Since all interfacial
tensions act over the same length, this means that the tensions must balance.
Considering the forces in the horizontal direction, the equilibrium condition
FIGURE10.23 (a) Representation of an amount of liquid confined in containers
with different wetting properties. (b) Cylindrical thread of liquid of original radiusr
that has undergone an axisymmetric varicose deformation; two radii of curvature
indicated.