How come the same thing makes the situation either stickier or more slippery?

Question

When I ride my motorcycle in the rain I feel the roads of London (UK) more slippery and I noticed I take longer to break and stop

When the temperature goes below 1 degree celsius on the edge of the roads there is sometimes something we call it black ice, that is frozen water or ice mixed with the dust of the streets, that can be even more slippery than the normal rain.

so in the cases above the rain (or water) made things more slippery, however, when reading a book, or opening a plastic back, sometimes the fingers fit too slippery and wetting them down make them stickier to the paper or plastic, making it possible to handle them.

why is it so that the same thing - water - can make either things either more slippery or stickier depending on the situation?

Answer 1

In the case of fingers turning the pages of a book, the water or spit you put on your fingers is quickly absorbed into the paper, which pulls the paper into close contact with your skin and makes them stick together by surface tension forces. This increases the friction coupling between your fingertip and the paper, and helps lift the page you are turning from the other pages lying below it in the book.

In the case of rainwater on asphalt pavement, the water mixes with spilled oil, tire dust, and dirt and prevents the rubber surface of the tire from getting a good mechanical grip on the gravelly surface- in other words, it acts like a lubricant instead of a glue.

And if there's a lot of water on the road and you are going really fast, the water under the tire forms into a continuous film and the tire "skates" across the wet road like a speedboat skimming the surface of a lake.