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Science Q&A

Nothing is more exciting than getting questions from listeners. This week, Everyday Einstein shares 3 of the more interesting queries he's received from science fans lately.

By
Lee Falin, PhD,
December 28, 2013
Episode #082

Page 1 of 2

Nothing is more exciting than getting questions from listeners. While I generally respond directly to the questioner, this week I wanted to share a few of the more interesting questions I've received from my fellow science fans:

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Question #1: Water on the Windshield

Our first question comes from Jess: 

"It's been raining a lot where I live lately. The rain has been off and on, alternating between light sprinkles and torrential downpours. So the other day my girlfriend and I were sitting in my car waiting for the rain to let up a bit before making a break for it and running inside. While waiting, I was looking at the rain fall on my windshield and observed something. Even with my windshield wipers off, the areas of the windshield outside the range of the windshield wipers would coalesce into droplets on the window, while on the area where the windshield wipers had wiped when they were on, the water didn't coalesce into droplets - wit just ran off in a sheet.

To my knowledge, the windshield hasn't been treated with anything - especially in any way differentiating the areas of the windshield. So, my question is: Why doesn't the rain coalesce into droplets uniformly across the whole windshield after using the wipers?"

Everyday Einstein: Generally, when water starts forming droplets like that, it's because it's sitting on some kind of hydrophobic surface which is preventing it from making hydrogen bonds. Hydrogen bonding is what gives water many of its special qualities, such as being able to form a liquid at room temperature.

Most wiper blades are made of rubber, and a few are coated with teflon. Both rubber and teflon are hydrophobic, so most likely the wipers are leaving a residue of rubber and/or teflon across the area of the windshield where they pass. This results in that portion of the window being partially hydrophobic, which causes the water to bead up, since it can't spread out very well.

More details on hydrophobic (and its opposite: hydrophilic) can be found here.

* The astute reader will notice a flaw in this response. For more details along with the corrected response, see this episode. *

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