much as a sponge soaks up water from a wet surface. Nonwovens are often used with absorbent structures, to contain the absorbent material and to act as a transport layer for fluid to flow to the absorbent material. However, absorbent structures such as diaper fluff, hygienic pads, and wipes are not considered as filter media, because there is no separation involved. On the other hand, some materials are selective. Polyolefins, although hydrophobic, have an affinity for oil and are often used to absorb oil from water. Polyolefin materials are often used to control oil spills at sea.
Very often absorbency is an undesired quality in a filter medium. The tendency of cellulose media to absorb water tends to soften and weaken the filter structure and shorten filter life. In addition, the absorption of moisture causes fiber swelling and interference with filtration performance.