The pores of the microporous membrane act as small capillaries. When the hydrophilic membrane comes into contact with water, capillary action related to surface tension causes water to spontaneously enter and fill the pores. In this way, the membrane is easily wetted and allows a large amount of water to flow through the pores. Once wetted, the hydrophilic membrane will not allow air or other gases to flow in large amounts unless they are applied at a pressure greater than the bubble point of the membrane.
Hydrophilic membranes are usually used with water and aqueous solutions. They can also be used with compatible non-aqueous fluids. Hydrophilic membranes are generally not used for air, gas, or exhaust filtration because if they get wet accidentally, for example through condensation, the filter will block the flow.