Excess Pressure Inside a Drop and a Bubble

A liquid drop can be considered spherical, ignoring the effect of gravity. The two hemispheres of the drop attract each other due to surface tension. In addition, the forces due to the pressure of the liquid inside and air outside act on the hemispheres.

At equilibrium, the magnitude of forces inside and outside balance each other. Rewriting this expression indicates an excess pressure inside the liquid drop.

For drops of different liquids with the same radius, the excess pressure is greater for the liquid with higher surface tension. However, if the radius decreases, the excess pressure increases.

The scenario remains unchanged in the case of air bubbles inside a liquid, where the pressure inside the bubbles is greater than the pressure in the liquid.

In the case of a soap bubble, the pressure enclosed in the thickness of the bubble is greater than the air pressure outside and less than the air pressure inside the surface.

Adding these expressions gives the excess pressure inside a soap bubble.