wetting

Drip water on a clean glass plate and it spreads into a flat sheet; drip it on a waxy leaf and it stays as round beads. The first surface is well wetted, the second is poorly wetted. Wetting is the process by which a liquid spreads over a solid and displaces the air that was clinging to its surface.

Whether a liquid wets a solid is a tug-of-war between three interfacial tensions: solid-air, solid-liquid, and liquid-air. When the liquid sticks to the solid more strongly than it sticks to itself, it spreads and wetting is good; when the reverse holds, it beads up and wetting is poor. The outcome is summarised by the contact angle the liquid edge makes with the solid: small angles mean good wetting, large angles mean poor wetting.

Wetting is a constant concern in pharmaceutics because many drug powders are hydrophobic and float or clump rather than disperse when added to water. Poor wetting slows dissolution and disintegration and makes suspensions hard to reconstitute. Formulators improve it by adding wetting agents (surfactants) that lower the relevant tensions, by milling to expose fresh surface, or by levigation with a wetting liquid. The caveat is that excessive surfactant can cause foaming or affect stability, so the amount is balanced carefully.