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Capillary pressure

In fluid statics, capillary pressure is the difference in pressure across the interface between two immiscible fluids. The pressure difference is proportional to the surface tension, γ, and inversely proportional to the effective radius, r, of the interface, it also depends on the wetting angle, θ, of the liquid on the surface of the capillary.

$p_c=\frac{2\gamma \cos \theta}{r}$

The capillary pressure pcis defined as

pc = pgpw

for the case of one phase being gas with pressure pg and the other phase being water with pressure pw.

The equation for capillary pressure is only valid under capillary equilibrium, which means that there can not be any flowing phases.