The principle of water selectivity takes advantage of the different susceptibilities to surface wetting (wettability) of different materials. "Wetting" is a term from surface science. It involves consideration of the forces of the inter-molecular behavior of phase boundaries and interfaces.
When a drop of liquid is placed on a solid, the liquid either spreads to form a thin, more or less uniform film or remains as a discrete drop, as shown to the right. The former behavior is generally described as complete wetting, and the latter as incomplete or partial wetting. The external measure of the degree of wetting is the contact angle, q, i.e. the angle formed between planes tangent to the surfaces of the solid and the liquid at the wetted perimeter.
A zero contact angle (q = 0) is the condition for complete wetting of a solid by a liquid (e.g. water on clean glass). Conversely, complete non-wetting dictates an angle of 180°, and this is only possible for a liquid in a liquid.
FSI has developed and patented an innovative fuel filter (FRS) that takes advantage of surface wetting tendencies to achieve water selection, i.e. separate emulsified water from hydrocarbons. The concept is illustrated in the illustration to the left. Here, an influx of fuel oil together with undissolved water is fed through a water selective membrane. Since the membrane is oil-wettable, oil will pass into the narrow pores, leaving water (being non-wettable to the membrane) trapped on the membrane surface, and thus water separation is achieved.