SURFACE TENSION

Any liquid has a tendency to reduce its surface area to a minimum.Hence its free surface behaves like a streched elastic membrane and is in a state of tension.This pull or tension on the liquid surface is known as surface tension.Surface tension is measured as the energy required to increase the surface area of a liquid by a unit of area. The surface tension of a liquid results from an imbalance of intermolecular attractive forces, the cohesive forces between molecules:

 

• A molecule in the bulk liquid experiences cohesive forces with other molecules in all directions.

• A molecule at the surface of a liquid experiences only net inward cohesive forces.

For example gum has more adhesive force than water.

surface tension is defined as the tangential force per unit length acting at right angles to an imaginary line on the free surface of a liquid in equilibrium.At the boundary of a liquid surface the force of  surface tension acts inwards only.At other places on the surface it acts towards both sides of the imaginary line.The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above, and exhibit stronger attractive forces upon their nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called surface tension.

surface tension = force / length.

unit of surface tension is newtons per metre(N/m).

                                Surface Tension Examples

•  Walking on water

• Floating a needle

• Washing with cold water:- The major reason for using hot water for washing is that its surface tension is lower and it is a better wetting agent. But if the detergent lowers the surface tension, the heating may be unneccessary.

• Soaps and detergents:-help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas.

Surface Tension and the Water Strider

This interesting insect can freely run along the surface of a quiet pond. It’s tiny mass and the geometry of its legs allow it to be supported by the high surface tension of water.

                                          SURFACE ENERGY 

The potential energy of the molecules on the surface of the liquid is greater than that of the molecule inside the liquid.The excess of potential energy per unit area of the liquid surface is called surface energy.The surface area of a liquid  work has to be done aganist surface tension .This work has to be done againist surface tension.This work is stored up in the surface as part of surface energy of the new surface created.Surface energy is expressed in joules/metre2 .

 Link : http://www.way2science.com/surface-tension/