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Fibers such as cotton absorb a substantial amount of water. Clothes made from cotton can therefore transport water from your skin to the outside of a garment where it can evaporate. This cools your skin because the heat of vaporization of water is quite large.

Cotton is mostly cellulose, a polysaccharide. It has hydroxyl (alcohol) groups in the repeat units, meaning that a single polymer chain has thousands of hydroxyl groups. Hydroxyl groups form hydrogen bonds with water, and are mostly responsible for the water absorbency of cotton.

Synthetic fibers such as polyesters are often co-woven with cotton to improve the mechanical properties of the cloth. Polyesters absorb much less water than cotton. Cloth made exclusively from polyester fiber is uncomfortable for this reason. There are synthetic polyester fibers such as poly(lactic acid) that absorb water and are more comfortable. They are more expensive, and therefore have not been widely adopted.

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Absorption and transpiration are two different processes, though. Cotton IS very good at absorbing water, which is why it sucks so much to wear it. When something Absorbs a fluid, it takes it into itself. Cotton wicks water into its structure - - like, into the actual individual strands of thread, where air and wind can't get to it, to evaporate it.

If you take something like wool, on the other hand, the hairs themselves do not actually absorb water in the same way that cotton does. This is wonderful, because they simply pull the water ALONG the hairs via surface tension and capillary forces, rather than pulling the water INTO itself. This exposes the water to air on the other side of the fabric, allowing it to evaporate.

The same is true of polyester, which is why poly is the go-to cheap sports fabric, and wool is the king of sports and sweating fabric when you don't need the structural properties of poly.

Try it for yourself. Wear a pair of merino wool socks for a few weeks, then go back to cotton. It feels like you're standing in a puddle by comparison. The cotton simply traps moisture against your skin.

Surface tension is the main driver of the movement of water across fabrics.

Some materials, like cotton, are very porous, and have low "contact angle", a property which governs the wettability (yes, a real word) of the material. This encourages the ABsorption of water, both along the length of the fibers, and into every nook and cranny of a given fiber as well. This is what makes cotton so absorbent, but it's also what makes it such a terrible fabric to have against your skin. It TRAPS moisture. Once the water gets into those little nooks and crannies, surface tension keeps it there, and very little of the water is exposed to the air, so it dries very slowly.

Other materials, like nylon, are inherently non-porous, so they can't ABsorb water into the individual fibers. They can, however, still ADsorb (also a real word) water along the length of the fibers. This means that the water is just sitting in top of the fiber, fully exposed to the air, allowing it to evaporate quickly. This tends to be true of most of the synthetic fibers, which is why they dry so quickly once they DO get wet. Getting them wet in the first place, however, is hard, because they tend to have higher contact angles, which makes it harder for water to get a hold, and some of them, like nylon, can be woven so tightly that they almost become hydrophobic. This is why synthetic clothes dry really fast, but also feel nasty against your skin when you sweat - the sweat doesn't get picked up off your skin, so you just feel the sensation of water on your skin.

Now take wool. A truly impressive fiber. It's not really porous in and of itself, so it doesn't absorb very much, but it has a lower contact angle and greater ability to ADsorb water, which makes it much better and pulling water off your skin. This is why all of the finest expedition, outdoor, and sports clothes are made of merino wool. It's naturally anti-bacterial, and it's proficiency at wicking away water, and then letting it evaporate, makes it very thermo-regulating. It keeps you warm in the winter, and cool in the summer.

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Yes I can't explain it but wool is the best. I am fortunate that I don't get "itchy". Wool can keep you warm soaking wet and frozen unlike most synthetics (and wool doesn't get stinky). That said I have plenty of synthetics, it just depends. Hat to wool lined boots most of the time for my local and adventures

Modern-day wools are way better than our grandparent's wools, too! They tend to be far less itchy if they're made well, because they go through a mechanical process that removes the scratchy bit. Merino wool products in particular have 0 itch to them!

Weave can factor into this some. A satin weave is worse for breathability because the threads are very close together and tightly packed to create the smooth surface satin is known for. There are various kinds of twills that are less tightly packed and more open, that are better for preventing the fabric from trapping heat.

I love my merino wool undergarments. I've had them for over 10 years and they're only now just starting to show some wear. A bit pricy, but non-itchy, lasts a decade, keeps me warm and doesn't smell much even after several days of camping. It's incredible stuff.

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