The ability of a liquid to wet a solid surface depends on the surface energies of the solid-vapor interface, the liquid-vapor interface, and the solidliquid interface. The surface energy across an interface is a measure of the energy required to form a unit area of new surface at the interface. The intermolecular bonds or cohesive forces between the molecules of a liquid cause surface tension. There is usually an attraction between the liquid and another substance when they are brought into contact, but the adhesive forces between the liquid and the second substance will compete against the cohesive forces of the liquid. Liquids with weak cohesive bonds and a strong attraction to the other material will tend to spread over the material. Liquids with strong cohesive bonds and weaker adhesive forces will tend to bead up and form a droplet when in contact with another material.