Capillary number

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

• Bond number
• Capillary pressure

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