Surface Tension On A Bubble

Surface tension comes from liquid's inter molecules forces located at the upper most of the liquid. The molecules of the liquid attract each other with relatively equal magnitude in all direction. The resultant of the forces can be assumed as zero.
But the molecules located at the upper most of the liquid are not attracted upward, since there is no liquid molecules above them. Thus they have a resultant of force downward. This force will give a reaction effect upon an external action force though it is small in magnitude.Greater force will break the "binding" and make the force acting on the surface penetrates the liquid.

The bubble is developed because the existence of the surface tension. The outward force from pressure difference between the inside gas and the environment is in equilibrium with the force developed by surface tension.



The force from surface tension iswhere gamma is the surface tension, r is the radius of the bubble. Notice that the hemisphere has two surface, the inner and the outer ones.
The force caused by pressure difference between the gas inside the bubble and the atmospheric pressure isThe effective area (A) here is the area perpendicular to the force. This hemisphere has the effective area of a circle across it.
Equating the equation (1) and (2) we have