Surface Tension, $\sigma$

Surface Tension is said to act between two immiscible liquids or between a liquid and gas, say water and air. The interface between the two fluids is assumed to act like a stretched membrane under tensile stress. This stress is required to hold the membrane in position. Many instances from everyday life could be cited to justify this assumption. A steel needle will float on water due to the apparent tension developed at the water surface. Water drops form on smooth surfaces. Tiny spheres of mercury will form when it is poured on a smooth surface. Unbalanced cohesive forces acting on liquid molecules at the interface are balanced by the tensile force due to surface tension.

Surface Tension, $\sigma$ is expressed as force per unit distance (N/m) and has dimensions of $FL^{-1}$. Its magnitude depends upon the two fluids in contact and on temperature.

(c) Aerospace, Mechanical & Mechatronic Engg. 2005
University of Sydney