Capillary pressure

In fluid statics, capillary pressure is the difference in pressure across the interface between two immiscible fluids, and thus defined as

This pressure difference arises because the one of the fluids (the wetting phase) typically diffuses across the capillary walls before the other, which then remains in contact with a thin layer of the wetting phase. This pressure difference also results in a curved interface between the two fluids, where the curvature is a function of how easily the fluids wet the capillary walls, the fluid saturation properties, and the capillary pore properties. 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 interfacial tension, ${\displaystyle \gamma }$, and inversely proportional to the effective radius, ${\displaystyle r}$, of the interface, it also depends on the wetting angle, ${\displaystyle \theta }$, of the liquid on the surface of the capillary.

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