Why moistening your fingers helps increase their traction with respect to opening thin plastic bags or flipping through paper, but moisture on a road decreases the traction of your car tires?
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Your fingers and the paper can absorb some of the moisture, which prevents a layer of water from forming between the two surfaces. Tire rubber and road pavement are non-absorptive to water, so the water will form a lubricating layer between the rubber and the road. This is why tire treads have voids to let water flow out from between the two surfaces.
Moisture on your fingers only gives you better grip if they’re too dry in the first place. By adding moisture you make them softer which makes them stick better to paper and plastic bags.
But adding moisture in between your fingers actually makes them more slippery as well, especially if you’re rubbing your hands together your hands stick together better if they’re dry than if you dip them in water before doing so. This is because the water forms a layer on top of your skin which prevents direct contact with the skin on the other hand, as there will usually be a thin film made up of water in between the hands.
And this thin film of water is also the problem when it comes to tarmac, because what happens when tarmac is wet is that because the vehicle is moving so fast the water actually prevents direct contact between the tarmac and the tires to some degree. Especially if you start sliding as this pushes water in front of the tires which eventually forms a tiny wave that goes in between the tires and the tarmac.
**Anyhow, that’s a short explanation of some of what’s going on but it’s very far from being a comprehensive or perfect explanation, hopefully someone else will make a better one.**
Water is sticky to a lot of things. It’s slick against itself if there is enough of it.
When you lose traction on a wet road, it’s because there is enough water sticking to both the road and your tire, that it creates a film on both, so that the tire and road have enough water between the two of them to cause the water to slip against itself.
Think of it like sand. Sand on a surface can act as either a lubricant or as grip, depending on the amount of sand and the surface itself. A lot of sand and you basically swimming it it.
Water has something called surface tension. Basically water molecules are slightly attracted to other water molecules, like a super weak magnet.
When you lick your fingers, you’re using that weak attractive force to make light weight objects like paper stick to you.
That force isn’t nearly strong enough to stop a moving car and it doesn’t scale up with more water. So on a larger scale, water is just a fluid substance that gets between the car and the road, reducing friction.
I don’t experience this effect, is it even real, or a mind trick?
Your fingers are normally covered with body fat which works as grease. Adding water changes structure of flipping papers making it more adhesive to fingers. As to plastic covers I never saw it. Use a clean towel.
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