92168.pdf

6 The solid-liquid

interface

Contact angles and wetting

75 77

Wetting is the displacement from a surface of one fluid by another. It

involves, therefore, three phases, at least two of which must be fluids.

The following account will be restricted to wetting in which a gas

(usually air) is displaced by a liquid at the surface of a solid. A

wetting agent is a (surface-active) substance which promotes this

effect.

Three types of wetting can be distinguished:

1. Spreading wetting.


2. Adhesional wetting.


3. Immersional wetting.

Spreading wetting

In spreading wetting, a liquid already in contact with the solid spreads
so as to increase the solid-liquid and liquid-gas interfacial areas and
decrease the solid-gas interfacial area. The spreading coefficient, S
(cf. equation 4.31), is defined by the expression

(7sL + TLG) <^6 '^1 )

where AGS is the free energy increase due to spreading. The liquid
spreads spontaneously over the solid surface when S is positive or
zero.