Surface Tension, condition existing at the free surface of a liquid, resembling the properties of an elastic skin under tension. The tension is the result of intermolecular forces exerting an unbalanced inward pull on the individual surface molecules; this is reflected in the considerable curvature at those edges where the liquid is in contact with the wall of a vessel. More specifically, the tension is the force per unit length of any straight line on the liquid surface that the surface layers on the opposite sides of the line exert upon each other.

The tendency of any liquid surface is to become as small as possible as a result of this tension, as in the case of mercury, which forms an almost round ball when a small quantity is placed on a horizontal surface. The near-perfect spherical shape of a soap bubble, which is the result of the distribution of tension on the thin film of soap, is another example of this force; surface tension alone can support a needle placed horizontally on a water surface.

Surface tension is important at zero gravity, as in space flight: Liquids cannot be stored in open containers because they run up the vessel walls.

get a small saucer of water, a sewing needle. very gently place the needle across the water. if you did it right, the needle will sit on top of the water. its not floating, its sitting on the surface tension of the water. if you put the needle point first, it will poke through the surface tension and sink