. . . Surface Tension Air F F Liquid As a molecule moves through the surface, we must “pull” to oppose the attractive forces between the emerging molecule and surrounding solvent molecules. As the surface of a liquid is extended, more molecules must come to the surface. Note that the magnitude of the force will depend on the depth or length(perpendicular to the figure. This relates to the number of molecules involved. In general, this is  = F/ l (dyn/cm). Whether or not we are actually extended the surface of a liquid, the forces parallel to the surface between molecules is present. This layer acts like a membrane in tension, similar to a balloon containing air.

Wilhelmy Plate is one method to measure surface tension. Balance The balance will read the weight of the plate plus the surface tension contribution. Contact angle  The contact angle indicates the affinity of the liquid for the solid. For θ less than 90o, the interaction is favorable and the force pulls down on the plate. For a contact angle greater than 90o, the force will be repulsive and will lower the apparent weight.  Liquid Wbalance = Wplate +  p cos  p is the perimeter of the plate  = Wbalance - Wplate p cos 