Wicking fabrics are modern technical fabrics which draw moisture away from the body. They are made of high-tech polyester, which, unlike cotton, absorbs very little water. Cotton will absorb 7% of its weight in water, polyester only 0.4%. Cotton will therefore hang onto your sweat, making your garment heavy and unpleasantly clammy. Wicking polyester has a special cross-section and a large surface area, which picks up moisture and carries it away from your body, spreading it out, to evaporate easily on the outside of the fabric. So you stay cool and dry.
Some people will refer to wicking fabrics as being breathable – that is, they let air in and sweat out. Breathable showerproof and waterproof fabrics have tiny pores in the fabric, larger than water vapour molecules (so these can get out) but much smaller than drops of rain (so these can’t get in).
You’ll see a lot written on garment labels about wicking properties, but read what they are telling you and it often amounts to no more than ‘this fabric moves moisture away from your body’.
The two properties normally used to predict wicking performance in a fabric are capillary pressure and permeability.
Capillary pressure is the main force responsible for the movement of moisture along or through a fabric, where the force of the surface tension between the liquid and the walls of a narrow gap or pore overcome the forces between the molecules of the liquid, moving it into empty gaps until the forces even out. Permeability is the measure of a fabric’s ability to transport moisture through itself, and is determined by a combination of sizes of spaces within it and the connections between the spaces.
Other properties that certainly do effect the wicking properties of a fabric include yarn twist (how threads in the fabric turn around each other), contact angle (between the fibre and the liquid), knit or weave (the larger scale construction of the fabric), yarn roughness and a whole lot more.