13.12:
Surface Tension and Surface Energy
Liquid surfaces behave like a stretched membrane, meaning they tend toward surface area minimization. This property is called surface tension.
Molecules inside the liquid experience attractive forces equally on all sides from the surrounding molecules. But, surface molecules experience asymmetric forces.
The net inward force on surface molecules pulls them down, causing surface tension.
Work is done to move bulk molecules toward the surface, increasing the surface molecules' potential energy. This additional energy is surface energy.
Consider a frame with a sliding wire dipped in soap solution to form a bubble with top and bottom surfaces. The surface tension acts inward perpendicular to the sliding wire, and external force is required to pull the wire outward. This force per unit length gives the surface tension.
If the force pulls the wire through a distance, increasing the surface area, then the surface energy is given by the work done. This shows that surface tension can also be defined as the surface energy per unit surface area.
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.
Suggested Reading
- Verma, H.C. (2008). Concepts of Physics Part I, Fourth Edition. pp. 284-286