Surface Tension:
On account of the cohesive forces between the molecules of a liquid the free surface of a liquid always behaves like stretched membrane or sheet and tends to contract to the smallest possible area. Let us imagine a line AB drawn on the surface of the liquid. On account of the contractile tendency of the surface, the liquid molecules on the two opposite sides of the line pulled apart from each other at right angles to AB.
So surface tension of a liquid is defined as the force per unit length of a straight line imagined to be drawn on the surface of the liquid acting tangentially to the liquid surface and perpendicular to the line, which tends to contract the surface area of the liquid to a minimum.
The dimension of surface tension
\( \frac{force}{length}=\frac{MLT^{-2}}{L}=MT^{-2} \)And the unit of surface tension is dyne/cm(c.g.s.), poundal/ft(f.p.s.) and N/m(S.I.).