capillary pressure

Capillary pressure is the pressure difference that occurs at the boundary between two fluids—like water and air—inside a small tube or porous material, such as soil or a sponge. It results from surface tension, which is the cohesive force of the fluid molecules, causing the fluid to stick to the surfaces and form curved interfaces. This pressure difference determines how fluids move through narrow spaces, with the smaller the pore or tube, the higher the capillary pressure. Essentially, it influences the ability of fluids to rise, be retained, or drain through tiny channels in porous materials.