# Capillary number

$\text{Ca} \ \stackrel{\mathrm{def}}{=}\ \frac{\mu V}{\gamma}$
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.