Surface Tension

The surface of a liquid is the interface between the liquid volume and the fluid above the liquid. Generally the liquid is water and the fluid above the liquid is air. The molecules within the liquid attract each other and at the interface there are more attractive forces towards the bulk of the liquid than there are towards the adjacent gas molecules. The molecular forces tend to pull into the fluid bulk. The surface of a liquid is apt to shrink, and its free surface is in such a state that each section pulls another as if an elastic film is being stretched. The surface behaves like a flexible membrane. This property is evident when overfilling a cup with water. The level of water in the cup will be higher than the cup edge before it overflows.

If a double line is drawn on the surface of a liquid there is a force normal to the lines holding the lines together.

The tensile strength per unit length of assumed section on the free surface is called the surface tension (symbon γ).

For large volumes of liquid the forces due to gravity and inertia are large compared to the surface tension forces. Therefore the surface tension is not considered in most hydrostatic and hydrodynamic calculations.

For small volumes and areas of fluid the surface tension becomes important and results in spherical water droplets and the capillary effect.

Liquid	Surface Fluid	Surface Tension N/m
Water	Air	0,0728
Mercury	Air	0,476
Mercury	Water	0,373
Paraffin	Air	0,027
Water	Paraffin	0,027
Methyl alcohol	Air	0,048>