Excess Pressure

Due to the property of surface tension a drop or bubble tries to contract and so compresses the matter enclosed. This in turn increases the internal pressure which prevents further contraction and equilibrium is achieved. So in equilibrium the pressure inside a bubble or drop is greater than outside and the difference of pressure between two sides of the liquid surface is called excess pressure. In case of a drop excess pressure is provided by hydrostatic pressure of the liquid within the drop while in case of bubble the gauge pressure of the gas confined in the bubble provides it.

Excess pressure in different cases is given in the following table :

Plane surface

Concave surface

P = 0


                                    DP = 0






Convex surface






Bubble in air

Bubble in liquid






Bubble at depth h below the free surface of liquid of density d

Cylindrical liquid surface







Liquid surface of unequal radii

Liquid film of unequal radii






Related Keywords
11    PMT    Physics    Mechanical Properties of Fluids    Excess Pressure