Why does the rainwater sheet together where my windshield wipers have recently passed, but bead together where they haven’t?

Question

[Kind of crappy visual aid](https://i.imgur.com/zrzlYkW.jpeg)

I’m sitting in my car, procrastinating going inside because of the rain, and I noticed that the rainwater is behaving differently depending on the area where the windshield wipers passed and where they haven’t.

Why is that?

In: 91

Answer 1

Here’s my best guess – physical wetting of the glass surface at a microscopic level because the blade has wiped the water into the structure and also polishing due to road grit being captured and moved across the surface.

The wetting is similar to wiping out a dry glass with a wet cloth before pouring a beer. If the glass is dry, the head foams more. Wet? No foamy head.

The reverse effect is probably like Mentos and diet coke… something about the surface of the Mentos is just right to cause the carbonation to effervesce quickly.

I use Rain-X, and I see less of a difference between the wiped and unwiped area – probably because the coating stays put.