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


 * $$\text{Ca} \ \stackrel{\mathrm{def}}{=}\ \frac{\mu v}{\sigma} $$

where $$\mu$$ is the viscosity of the liquid, $$v$$ is a characteristic velocity and $$\sigma$$ 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.