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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“The two great points of difference between a democracy and a republic are: first, the delegation of the government, in the latter, to a small number of citizens elected by the rest; secondly, the greater number of citizens and greater sphere of country over which the latter may be extended.”
—James Madison (1751–1836)