How does Rain-X work to help you see better through your windscreen in the rain?

04 October 2011

Question

Here in the states, we have a liquid product called Rain-X. When you spread it on your windshield, then raindrops beat up. Rain-X seems to add an invisible waxy surface to the windshield. If the only light source is directly in front of you as with normal rural night driving, all of the water droplets vanish almost magically. If you drive under a street lamp when the light comes from the side, then you can see the water droplets, but just for a second until the light passes. I assume that Rain-X works by forcing the raindrops to be tiny hemispherical lenses and I'm not sure about the details. Does it use refraction or total internal reflection, or both?

Answer

The first thing it's doing is covering your windscreen with something called a fluorosilane. It's basically silicon-oxygen-silicon-oxygen, with fluorines on the side. It's incredibly hydrophobic so the water droplets, instead of forming kind of flattish lenses, form very, very circular things. I think what's going on is they're much, much stronger lenses. So they bring any light to a focus, very, very close to them and then the light spreads out, is evened out all over everywhere. So instead of using a really bright light, you see a kind of a much, much dimmer light and you hardly see them at all. And then probably, when light comes very close to you, something more complicated happens and you get the bright lens.

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