What is moisture wicking? The short answer is that moisture wicking moves moisture away from the skin. This means that moisture wicking fabrics play a crucial role in your layering system.
In order to understand how moisture wicking works, we first need to discuss a few properties that the individual fibers can have. These fiber properties work in tandem to achieve moisture wicking in the finished fabric.
Hydrophilic vs Hydrophobic
Our hardcore gear nerds will already know a lot about these terms from our articles about how waterproof fabrics work. But just in case you’re new to gear science, a basic description is that hydrophilic means “water-loving” and hydrophobic means “water-hating.” Hydrophilic materials cling to water, either through ionic bonds or by having lower surface energy. Hydrophobic materials, by contrast, do not mix with water, and won’t allow water to cover much of the surface. Instead, water beads up and rolls off of these materials.
Absorption vs Adsorption
Most people have an intuitive understanding of absorption. This is when a substance permeates another, inside and out. Think about how a sponge soaks up water. The water goes all the way through the sponge, so we can say that the sponge has absorbed the water. Absorbent materials tend to have lots of storage space that can let in a substance and hold it.
Adsorption is less commonly talked about, but it’s more important than absorption when it comes to explaining what moisture wicking is. When a substance, usually a liquid or gas, is spread evenly over the surface of another, usually a solid – we refer to it as adsorption. Adsorption holds water to a surface without letting it penetrate any deeper.
Moisture wicking is all about keeping water moving. If a fiber is able to absorb water, then it can’t transport that moisture effectively since it’s trapped in the fiber. There’s no quick-acting force strong enough to pull the water out of the fibers and get it to the surface. Fabrics that absorb a lot of water tend to remain wet. Evaporative cooling happens while moisture is still in contact with the skin, which steals your body heat and lets it go into the atmosphere.
Adsorption, on the other hand, doesn’t let water fully saturate the fiber. When the fibers are spun together and woven into a fabric, it creates a huge number of super narrow spaces between the individual fibers where water can flow. Using finer individual fibers creates even narrower spaces. Moisture wicking happens in these spaces by capillary action.
Capillary action is the propensity of a liquid to flow through narrow spaces without regards to outside forces like gravity. It depends on a complex series of interactive forces like osmotic pressure, the surface tension of the liquid, the surface energy of the solid material, and the strength of the molecular bonds between them.
Capillary action between the individual adsorbent fibers quickly pulls water toward the surface of the garment. Once the water has been transported to the surface it’s free to evaporate. Since the water is no longer in contact with your skin, the evaporative cooling does not take away as much body heat, which allows your fabric to keep on insulating.
Fiber Types: What is moisture wicking… and what is not
Since we know that moisture wicking is a result of properties of individual fibers, we can compare different fabrics based on what materials they comprise.
Cotton is highly porous. The structure of a cotton fiber resembles a drinking straw with all the air crushed out. Once this hydrophilic material is exposed to water, the straw shape fills up with water, allowing the cotton to absorb a huge amount of water compared to its weight.
Since cotton absorbs water and has a hard time letting it go, it’s not very good at wicking moisture.
Cotton is nice and breathable, but any amount of perspiration has trouble escaping this fiber. If you’re exerting a lot of effort or if you’re in a hot place, you’ll probably feel hot and humid while wearing cotton. Once temperatures cool or the wind picks up, evaporative cooling will leave you feeling clammy and cold. You could even be at risk for hypothermia while wearing wet cotton in somewhat chilly conditions.
Under a microscope, wool fibers look like they’re wearing a suit of armor – called the cuticle. This armor is actually made of a protein called keratin, which is found in abundance in human hair and fingernails. Wool fibers can still absorb some moisture (about 16-18% of its weight), but the cuticle provides a surface on which water can easily adsorb. This allows moisture wicking to take place in the spaces between fibers.
Of particular interest is Merino wool. Merino is much, much finer than other types of wool. Since the fine fibers can be packed together more tightly, this creates a larger number of narrower spaces. Capillary action works even more efficiently in these conditions. Merino wool is the most remarkable moisture wicking natural fiber that we know of.
Synthetic fibers such as polyester are virtually non-porous and hydrophobic. The amount of moisture that polyester absorbs is so low that it’s practically negligible. While it can’t absorb water very easily, it does adsorb quite well. Once the fibers are spun and woven, capillary action drives moisture to the surface. Synthetic fabrics that are designed for performance wear can use incredibly small fibers to more directly control the moisture wicking properties of the finished fabric.