The pores of the microporous membrane act as small capillaries. When the hydrophobic membrane comes into contact with water, the surface tension will act to drive the water out of the pores. Unless the applied water pressure is greater than the water inlet pressure of the membrane, water will not enter the pores and the membrane will act as a barrier to water flow. Liquids with low surface tension (such as alcohol) will spontaneously enter and fill the pores of the hydrophobic membrane. Once all the air in the hole is replaced, there is no longer any surface tension, and water can easily enter the hole, displacing the low surface tension fluid and pass through the membrane. Then, as long as the holes remain full of water, the membrane will allow large amounts of water to flow. If the membrane is allowed to dry (that is, air enters the pores), it should be pre-wetted again with a low surface tension fluid before using water.
Hydrophobic membranes are usually used with compatible non-aqueous fluids. They are also commonly used as air, gas, or ventilation filters. Hydrophobic membrane filters are sometimes used with water or aqueous solutions; and, in these applications, they are pre-wetted with a low surface tension water-miscible liquid before use.