When a drop of liquid meets a surface, it spreads out partly. The extent to which the liquid spreads out can be summarized by the angle of contact
that the liquid surface makes with the the surface.
If the angle of contact is greater than 90 degrees, the liquid will tend to form droplets on the surface, above left – as mercury does on glass - and if the angle of contact is less than 90 degrees, above right, the liquid will tend to wet the surface – as pure water does on glass. If pure water is poured onto clean glass, the angle of contact is very close to zero. If the surface is waxed, the water will tend to form droplets and the angle of contact will be over 90 degrees. If soap is added to water, the angle of contact degrees – for this reason soap is used in cleaning.