# Fluid Properties – Surface Tension

## Surface Tension Definition :

Surface tension is defined as the tensile force acting on the surface of a liquid in contact with a gas or on the surface between two immiscible liquids such that the contact surface behaves like a membrane under tension. The magnitude of this force per unit length of the free surface will have the same value as the surface energy per unit area. The phenomenon of surface tension is explained by the following figure. Consider three molecules A, B and C of a liquid in a mass of liquid. The molecule A is attracted by the surrounding molecules of the liquid. Thus the resultant force acting on the molecule A is zero. But the molecule B, which is situated near the free surface, is acted upon by upward and downward forces which are unbalanced. Thus a net resultant force on molecule B is acting in the downward direction. The molecule C, situated on the free surface of liquid, does experience a resultant downward force. All the molecules on the free surface experience a downward force. Thus the free surface of the liquid acts like a very thin film under tension of the surface of the liquid act as though it is an elastic membrane under tension.

σ (sigma)

## Surface Tension Equation :

$\sigma = \frac{F}{L}$

F : Surface tension force

L : Contact length

## Surface Tension Dimensions :

$M{T^{ - 2}}$

## Surface Tension Units :

SI units :

$N/m$

MKS units :

$k{g_f}/m$

CGS units :

$dyne/cm$

English units :

$l{b_f}/ft$