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 is the viscosity of the liquid, is a characteristic velocity and is the surface or interfacial tension between the two fluid phases.
For low capillary numbers (a rule of thumb says less than ), flow in porous media is dominated by capillary forces.
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