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Always when I'm wet after bathing and try to dress, or when I try to put a wet sock in my (dry or wet) feet, the friction skin-cloth is very high compared to both dry. Why does water increase the friction dynamic and static coefficients?

I've seen this question on reddit, 1, 2,and over here, without good answers.

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Part of the explanation lies in the composition of the epidermis (outer skin), which is made up of mainly keratin. Hair, nails, animal horn and hooves are also made of this material.

Keratin has the peculiar property of softening when wet. This is due to some of the chemical bonds (that keep the keratin protein strands together) breaking in the presence of water. This causes loss of hardness, as experienced also when wetting hair of soaking finger or toe nails.

Softer materials usually show higher coefficients of friction (due to better 'grip'), compare e.g. soft silicone rubber to hard plastics or soft, malleable metals like lead to hard, rigid ones like stainless steel.

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It isn't friction. the extra effort is due to water's surface tension. The fabric is soaked with water, and your skin is wet with water. Thus you have to break the surface tension of the water between a layer of wet skin and wet fabric.

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    $\begingroup$ I find it hard to believe surface tension makes a significant contribution here. $\endgroup$
    – Gert
    Nov 23, 2016 at 19:29
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The coefficient of friction on fabric over wet skin is higher because wet skin is softer. The softer skin conforms to the fabric more easily hence higher resistance to movement - aka the sensation of friction. A similar analogy can be observed in tires. Racing tires are designed to be extremely soft compared to street tires. The soft racing tire compound offers much greater surface grip or higher coefficient of friction (albeit at the expense of durability) compared to street tire compounds.

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Water wets the sock, so it fills the little holes in the fabric and won't easily fall off. The presence of water makes the fabric much more massive, so it does not change shape as easily when being put on a foot. But I think this is not the main reason.

Think of a wet plastic bag on flat surface, such as a glass desk. Two interesting things happen:

  • the water will cause the plastic bag to get very close to the table, it will even fill the little pockets of air if the air can escape and the water can get there, and straighten the plastic; this is due to net result of capillary forces acting on the water and the air in contact with it;

  • when the plastic is pulled along the table, the water that is in contact with the table will be attracted to it so much that it cannot move along it, even if the plastic does so. But water right near the plastic cannot move with respect to that plastic.

This means there will be gradient of velocity in quite a large but thin liquid layer. Such gradient is always accompanied with friction, acting on both the liquid layers and the bodies in contact with them. This means there will be force slowing down the movement of the bag.

Something similar happens when you pull wet clothes along your skin.

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