In fluid statics, capillary pressure is the difference in pressure across the interface between two immiscible fluids, and thus defined as
In oil-water systems, water is typically the wetting phase, while for gas-oil systems, oil is typically the wetting phase.
The Young–Laplace equation states that this pressure difference is proportional to the surface tension, and inversely proportional to the effective radius, of the interface, it also depends on the wetting angle, of the liquid on the surface of the capillary.
The equation for capillary pressure is only valid under capillary equilibrium, which means that there can not be any flowing phases.
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