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

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[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

14 Answers

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

Water is cohesive, meaning that it likes sticking together, which is how you can fill a glass slightly above the top and the water will form a curved bubble rising above the glass.

The water on your windshield is just droplets, separate little drops that stick together. That’s why they don’t just immediately run down the windshield as a bunch of little rivers. They’re sticking together as droplets. When you wipe them, the droplets all get pushed along together and combine, and even though a lot of that water gets flicked away, you’ve now added water all along the path of the blade.

Not *a lot* of water, very little, but enough so that any water that falls onto that area doesn’t need to stick together as a droplet. Because that new water gets to stick to all the water molecules already spread across the windshield by your wiper blade, so it just spreads itself out nice and comfy.

To put it in simpler (although not at all ELI5) terms, compare masturbating alone to being at an orgy. When you’re masturbating alone, you’re staying in one spot, but when dropped into an orgy you’ll make your way all around the room and have a great time.

TL;DR the water where your wipers pass are having an orgy before the wiper comes back.

Anonymous 0 Comments

Water is cohesive, meaning that it likes sticking together, which is how you can fill a glass slightly above the top and the water will form a curved bubble rising above the glass.

The water on your windshield is just droplets, separate little drops that stick together. That’s why they don’t just immediately run down the windshield as a bunch of little rivers. They’re sticking together as droplets. When you wipe them, the droplets all get pushed along together and combine, and even though a lot of that water gets flicked away, you’ve now added water all along the path of the blade.

Not *a lot* of water, very little, but enough so that any water that falls onto that area doesn’t need to stick together as a droplet. Because that new water gets to stick to all the water molecules already spread across the windshield by your wiper blade, so it just spreads itself out nice and comfy.

To put it in simpler (although not at all ELI5) terms, compare masturbating alone to being at an orgy. When you’re masturbating alone, you’re staying in one spot, but when dropped into an orgy you’ll make your way all around the room and have a great time.

TL;DR the water where your wipers pass are having an orgy before the wiper comes back.

Anonymous 0 Comments

Because the glass that doesn’t get wiped with the rubber blade is more perfectly smooth, & doesn’t break the surface tension of the droplets. The wiped area has been scratched by the blades & the droplets “wet” onto the surface more easily.

Anonymous 0 Comments

Because the glass that doesn’t get wiped with the rubber blade is more perfectly smooth, & doesn’t break the surface tension of the droplets. The wiped area has been scratched by the blades & the droplets “wet” onto the surface more easily.

Anonymous 0 Comments

Wear patterns on the hard-to-see microscopic dirt that’s on the glass. Near the edge it is fallen dirt, and in the wiper pattern, it’s smeared dirt

If you clean the windshield properly, really degrease it, get everything off, maybe even get it a wee polish with glass cleaner, then remove all the products back to only having pure water on it, then dry it and wax it (if you get this done professionally, they’ll call it ‘sealing’ but it’s just wax) then wax off, then you won’t be able to see water collecting anywhere

It’s because water has a ‘sticky’ property to it, where it doesn’t shed as well if it has something to grab onto, it will form beads or a thin surface covering

Anonymous 0 Comments

Wear patterns on the hard-to-see microscopic dirt that’s on the glass. Near the edge it is fallen dirt, and in the wiper pattern, it’s smeared dirt

If you clean the windshield properly, really degrease it, get everything off, maybe even get it a wee polish with glass cleaner, then remove all the products back to only having pure water on it, then dry it and wax it (if you get this done professionally, they’ll call it ‘sealing’ but it’s just wax) then wax off, then you won’t be able to see water collecting anywhere

It’s because water has a ‘sticky’ property to it, where it doesn’t shed as well if it has something to grab onto, it will form beads or a thin surface covering

Anonymous 0 Comments

Wiper push water off windshield. Wiper stop on left of windshield and then go back down. When Wiper stops, many waters from all its pushing becomes one water with one purpose

Me cave man. 5yo smarter

Anonymous 0 Comments

Wiper push water off windshield. Wiper stop on left of windshield and then go back down. When Wiper stops, many waters from all its pushing becomes one water with one purpose

Me cave man. 5yo smarter