Hydrophobic Syringe Filters

When the hydrophobic membrane comes into contact with water, surface tension works to drive the water out of the pores. Unless the applied water pressure is greater than the water entry 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, spontaneously enter and fill the pores of the hydrophobic membrane. Once all the air in the pores has been displaced, there is no longer any surface tension and water can easily enter the pores, displace the low surface tension fluid and pass through the membrane. Then, as long as the pores remain full of water, the membrane will allow a lot of water to flow. If the membrane is allowed to dry (i.e. air enters the pores), it must be pre-wetted again with a low surface tension fluid before water is used.

Hydrophobic filters are typically 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 must be pre-wetted with a low surface tension water-miscible liquid before use.