eli5: Why do liquids sometimes increase grip?


So we all know how car tires are less grippy when it’s wet.

But why is it better to use water or saliva on my fingers when trying to open a plastic bag, or quickly turn paper pages over?

And why does applying washing soap/shower gel to my slippery yoga mat make it grippy again?

Does this means car tires would be grippier on a soapy surface? Is so do they apply this principle in (for example) motorsports?

Thanks in advance folks!

In: 3

You’re not making the page more grippy, you’re making the page wet and making a stronger bond with your finger while you turn it.

And soap doesn’t directly make your yoga mat more grippy, you’re just cleaning all the non-grippy dirt and grime off of it.

Multiple reasons. First, your fingers are oily due to natural oils produced and excreted by your skin (called sebum). Sometimes they’re too oily and you’ll get better contact with a layer of water or saliva between your fingers and whatever you’re trying to grip. Second is your fingers are not flat – there’s lots of grooves on them. Adding a bit of liquid can increase the surface area (actually the surface tension) between your fingers and whatever you’re grabbing, especially if what you’re grabbing is porous (like paper). But the biggest reason is that a bit of water is pretty sticky due to surface tension holding the water molecules together like glue. Get a lot of water, and the surface tension breaks and the water becomes more of a lubricant than a glue, which is why tires will tend to spin and slide when roads are wet.

As for your yoga mat, sweat and oils from your body likely accumulate on it, and then dirt and dust adheres to that coating. When you wash it, you expose the grippier material refreshing the grip.

Some motor sports use bleach and water to help with the burn out. The bleach helps heat up the tires and makes them sticky for quarter mile or eighth mile drag racing.

Radio control cars use soda on the track to help with traction. It’s usually only applied to the corners and the starting line. Some use a sugar water mix, but others use generic Sprite soda. Some racers will treat their tires with simple green type cleaners to make them sticker both on and off road.

You have two different mechanisms at play, *friction* (“grippiness”) and *adhesion* (basically, stickyness).

Water is actually relatively sticky to the right substances, because water molecules are relatively strongly attracted to each other due to how the electrons are distributed in them. As a result, a bit of water helps your fingers stick to the page or the bag.

When it comes to grip though, you’re not looking for adhesion – it would be useful for keeping your car from being pulled *up* off the road, but that’s rarely something you need to worry about. Instead, you want your car tires to not slide – which they do by having unevenness on them which interlocks with the unevenness on the ground. Water smooths out that unevenness, which reduces grip.

The sweat glands on your hands are somewhat different than in most other parts of the body, in that they excrete a much more watery sweat, and it’s been surmised that the purpose of that is to increase grip. Some evolutionary biologists have suggested that that is one of the mutations that help human ancestors with handling items and developing hand skills.