Surface Tension

Surface tension is measured as the energy required to increase the surface area of a liquid by a unit of area. The surface tension of a liquid results from an imbalance of intermolecular attractive forces, the cohesive forces between molecules:

A microscopic view of water illustrates the difference between molecules at the surface of a liquid and water molecules within a liquid.

The molecules at the surface of this sample of liquid water are not surrounded by other water molecules. The molecules inside the sample are surrounded by other molecules. The unbalanced attraction of molecules at the surface of a liquid tends to pull the molecules back into the bulk liquid leaving the minimum number of molecules on the surface. It required energy to increase the surface area of a liquid because a larger surface area contains more molecules in the unbalanced situation.

Adhesive Forces

Forces of attraction between a liquid and a solid surface are called adhesive forces. The difference in strength between cohesive forces and adhesive forces determine the behavior of a liquid in contact with a solid surface.

Formation of a Meniscus

The mensicus of water The mensicus of mercury
When liquid water is confined in a tube, its surface (meniscus) has a concave shape because water wets the surface and creeps up the side. Mercury does not wet glass - the cohesive forces within the drops are stronger than the adhesive forces between the drops and glass. When liquid mercury is confined in a tube, its surface (meniscus) has a convex shape because the cohesive forces in liquid mercury tend to draw it into a drop.

Capillary Action

Capillary action is the rise of a liquid that wets a tube up the inside of a small diameter tube (i.e., a capillary) immersed in the liquid.