When liquid contacts solid, there is mutual attraction between the molecules on the surface of the liquid and the molecules on the surface of the solid in contact. This attraction is called adhesion. When the adhesion between the solid and liquid surfaces is greater than the cohesive force inside the liquid, the solid surface will be wetted by the liquid. This phenomenon is called wetting. At this time, a liquid boundary layer attached to the solid surface will be formed (the liquid molecules in the boundary layer are often polar and arranged in a certain way). When there is relative motion between the liquid and the solid surface, the boundary layer has the same movement speed as the solid surface.
If the adhesion between the solid and liquid surfaces is less than the cohesive force of the liquid, it is called non-wetting. At this time, the solid surface is not wetted by the liquid. When the solid and liquid are separated, no liquid will remain attached to the solid surface, such as mercury and glass.
