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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
:
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.

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See also

See also

★ Bond number

★ Capillary pressure