Relative ability of a fluid to spread readily and uniformly over the surface of another fluid or solid, forming a thin, continuous film. Fluids with high surface tension (such as mercury) have little or no wetting ability; those with low surface tension (such as petroleum solvents) have high wetting ability. Fluids with medium surface tension (such as water) acquire greater wetting ability with the addition of a wetting agent. Wetting ability is of critical importance in cleaning, dyeing, painting, and printing.