Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 39

Figure 1. Liquid drop in contact with a solid surface. The contact angle θ depends on three tension
forces acting at the three-phase line.

0 ◦ ). Figure 2. Complete wetting; liquid drop spreads over the surface ( θ ∼

Figure 3. Partial wetting (0 ◦ <θ< 90 ◦ ).

of the type and the intensity of the intermolecular forces acting inside the solid. A
general, empirical rule is that liquids with surface tension lower than the surface
energy of the solid effectively wet the substrate. Most metals and surfaces similar
to silica have high surface energies and are therefore wetted by most liquids, at
least when clean [2]. On the other hand, polymers and waxes have low surface
energies [3] and to what extent a liquid will wet a low-energy surface primarily
depends on the surface tension of the liquid. Hydrophobicity of the substrate is
usually characterised in terms of the contact angle that pure water drop exhibits
on the particular surface. When a liquid drop is brought in contact with the solid
surface, depending on its free energy, three wetting scenarios may occur.
Complete wetting (Fig. 2) happens in the case when the liquid drop spreads over
the surface, with the contact angle progressively decreasing to very low values,
theoretically zero. Otherwise, a drop forms a finite contact angle with the surface.
If the contact angle is between 0 ◦ and 90 ◦ , partial wetting takes place (Fig. 3).
Values of the contact angle bigger than 90 ◦ represent what is called non-wetting
(Fig. 4) and corresponding surfaces are referred to as hydrophobic, when the liquid
is aqueous.
The Young-Dupré equation is based on the equilibrium thermodynamics of an
ideal system and is therefore restricted by the following assumptions: line tension
contributions can be neglected; the solid surface is inert and flat; and all three in-
terfacial tensions have constant values. Therefore, the Derjaguin-Frumkin equation
[4] was proposed as an improved form. In real systems, there are certain irregulari-
ties of the solid surface and measured contact angle values vary from the calculated