Why does water sometimes make things slipperier (e.g. hydroplaning) and other times make things “stickier” (e.g. putting socks on wet feet)?

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Hopefully the question is clear: basically, why is it that water on roads decreases friction, and water on a floor can make it easier to slip, but water droplets on feet make putting on socks harder?

In: Chemistry

It has to do with the molecular properties of water. When there is a lot of water such as on the road it reduces the friction of the 2 surfaces and makes them have less friction because there is a film of water on both objects. Water on water. But on your skin like taking off a wet t shirt. Water has a lot of friction on objects and water doesn’t sit on your skin so it’s friction against the wet t shirt.

It depends largely on the normal amount of friction. For road cars, generally there is a lot of friction between the tires and the road. If you stick a layer of water between them, there’s far less friction. With a dry sock and human skin, generally there’s not much friction; it’s enough to feel, but easy to overcome. When there’s a touch of water, the water tension kinda sucks the sock to your skin, giving it much more friction than normal. So it depends on how much friction is normal, and how much water you’re adding.

It’s a battle between the properties of fluids.

If you have lots of water, then the more influential property is the friction reduction *(Such as rain on roads)*, however, if you only have a few droplets as you mentioned with the sock, then the more influential properties are the cohesion between the water molecules. This means that the droplets on your sock and skin are trying to stick together.

Fun fact: Water is the most cohesive non-metallic liquid!

Aquaplaning specifically is caused by there being too much water for your tyre to displace. At this point the tyre isn’t in contact with the tarmac at all, but only with the plane on water between it and the tarmac hence aquaplaning.

Water on a road surface could also act as a lubricant without causing aquaplaning but that is a different scenario.

Clothes usually don’t fit very tight hence the friction is low. But wet clothes are forced onto your skin by the cohesion of water (that also sticks very well to most fabric) and the atmosphere as the water makes the fabric impermeable to air creating a suction effect. Since friction increases proportional to the normal force wet clothes are much stickier.

When slipping on a wet floor or hydroplaning the cohesion of the water doesn’t change the normal force and hence the friction much because the gravity of the object itself is much greater. Instead on smooth surfaces water fills in all unevenness of the surfaces with a layer in between that can’t transfer friction very well as the shear strength of low viscosity fluids is basically zero. On rough surfaces this effect usually isn’t pronounced enough to actually make you slip but a car can move faster through water on the road than the water can move out of the way increasing the layer of water between the road and tire – if the layer is thick enough you get hydroplaning.