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

A truly clean surface of glass will always have a clean sheet of water with no beads and minimal “breaks” or obvious separations when wetted. Dirt or grease spots will cause water to bead, clump, and pull together away from the grease. This is surface tension, just like you see at the top of your glass before it overflows.

The clean, sheeting area is referred to as a “water-break free” surface and is usually what you want to see on dishware as an indicator it is clean and ready to dry.