Capillary pressure is defined as:
where:
:is the capillary pressure
: is the pressure of the non-wetting phase
: is the pressure of the wetting phase
The wetting phase is identified by its ability to preferentially diffuse across the capillary walls before the non-wetting phase. The "wettability" of a fluid depends on its surface tension, the forces that drive a fluids tendency to take up the minimal amount of space possible, and it is determined by the contact angle of the fluid. A fluids "wettability" can be controlled by varying capillary surface properties (e.g. roughness, hydrophilicity). However, in oil-water systems, water is typically the wetting phase, while for gas-oil systems, oil is typically the wetting phase. Regardless of the system, a pressure difference arises at the resulting curved interface between the two fluids.