The capillary number represents the relative effect of viscous forces and surface tension acting across an interface between a liquid and a gas, or between two immiscible liquids. It is defined as

<math>Ca \equiv \frac{\mu v}{\sigma} <math>

where <math>\mu<math> is the viscosity of the liquid, <math>v<math> is a characteristic velocity, and <math>\sigma<math> is the surface or interfacial tension between the two fluid phases.