Excess Pressure Inside a Drop and a Bubble

At equilibrium, forces from surface tension pulling the hemispheres together, internal liquid pressure, and external air pressure balance each other. The internal pressure must exceed external pressure to counteract the inward pull of surface tension and maintain the drop's spherical shape.

Excess pressure is directly proportional to surface tension. Liquids with higher surface tension create stronger inward forces on the hemispheres, requiring greater internal pressure to maintain equilibrium. This relationship is fundamental to understanding capillary phenomena and surface tension and surface energy effects.

Excess pressure increases as drop radius decreases. Smaller drops have greater curvature, intensifying the inward pull of surface tension. Therefore, smaller drops require proportionally higher internal pressure to maintain equilibrium and resist the stronger surface tension forces acting on the interface.

Air bubbles inside liquid experience excess pressure similar to drops: internal pressure exceeds surrounding liquid pressure. However, soap bubbles have two surfaces—one facing internal air and one facing external air—creating a different pressure relationship where internal pressure falls between the two air pressures.

The Young-Laplace relation describes excess pressure in a soap bubble by accounting for both its inner and outer surfaces. Since the bubble has two air-liquid interfaces, the total excess pressure equals the sum of pressure differences across each surface, yielding twice the pressure difference of a single drop.

Pressure difference across an air-liquid interface is proportional to the interface's curvature. Higher curvature creates larger pressure differences and determines bubble size. This relationship explains why smaller bubbles have higher internal pressures and why bubbles tend to merge into larger, lower-pressure structures.

Ignoring gravity's effects, a small liquid drop maintains a spherical shape due to surface tension. This sphere can be conceptually divided into two equal hemispheres that attract each other through surface tension forces. This model simplifies force analysis by allowing examination of pressure and tension forces acting on each hemisphere.