13.12 : Surface Tension and Surface Energy

When a paint brush is immersed in water, the bristles wave freely inside the water. When it is taken out, the bristles stick together. The reason behind this effect is surface tension.

Consider a beaker filled with liquid. The bulk molecules in the liquid experience equal attractive forces on all sides with the surrounding molecules. However, the surface molecules experience a net attractive force downward due to the bulk molecules. The surface of the liquid behaves like a stretched membrane, and it tends to minimize the surface area. This property of liquids is called surface tension. This is why liquid drops take a spherical shape, as a sphere has the minimum surface area for a given volume.

Like a needle, certain objects denser than water can float on water due to surface tension.

Consider a frame with a sliding arm dipped in soap solution. The soap bubble pulls the sliding arm inward due to surface tension. If the frame is kept in a vertical position, so that the sliding arm can move up and down, then a small weight can be hung on the sliding arm to keep the frame in equilibrium. This weight is equal to the force required to pull the arm back outward. Surface tension is expressed as force per unit length. Its unit is N/m or J/m².

Surface molecules have higher potential energy than molecules inside the bulk of a liquid. This energy is called surface energy, which is the product of force and displacement.