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

\[\text{Ca} \ \stackrel{\mathrm{def}}{=}\ \frac{\mu V}{\gamma} \]

where \(\mu\) is the viscosity of the liquid, \(V\) is a characteristic velocity and \(\gamma\) is the surface or interfacial tension between the two fluid phases.

For low capillary numbers (a rule of thumb says less than \(10^{-5}\)), flow in porous media is dominated by capillary forces.

See also

bs:Kapilarni broj

de:Kapillarzahl es:Número de capilaridad fa:عدد موئینگی fr:Nombre capillaire it:Numero di capillarità pl:Liczba kapilarna ru:Число капиллярности tr:Kapilarite uk:Число капілярності