The contact angle is a measure of the ability of a liquid to wet the surface of a solid. The shape that a drop takes on a surface depends on the surface tension of the fluid and the nature of the surface. At the boundary between droplets and the gaseous environment, the surface tension causes a curved contour. At the edge of the drop, where the contour merges into the bearing surface, the contact angle between the interface liquid / solid and the tangent to the interface liquid / gaseous forms.