Wetting refers to the dynamics of how a liquid, when deposited on a solid (or another liquid) substrate, is able to spread. This wetting of solid substrates by liquids is a basic element in many natural and commercial processes. Understanding the wetting phenomenon enables us to explain why water spreads nicely on clean glass, but does not spread on a sheet of polypropylene.
A wetting agent is a compound that causes water to spread over the surface of another material. A common observation is that the addition of a surface active agent (or surfactant) to water will enable a solution to wet.1