Why is ice on my car harder to scrape off?

12 January 2016



As the weather is colder again, I wondered if you were interested in following up with the question, which was:

Each day I come out to my car and windows are frozen. Sometimes the ice seems to be welded to the glass, and sometimes if can be scraped off quite easily. I wondered why this was and (more importantly) how I can predict in advance the likely state of the car so I can get 5 minutes extra in bed?


Kat Arney put this troublesome question to phycist Stuart Higgins and chemist Ben Pilgrim...

Ben - So I've had a couple of initial thoughts about this. One thing might be how quickly the water freezes to ice. We know that things that freeze quickly tend to form crystals that are very, very small - think about snow, something like that. Whereas something that freezes over a long time, forms much larger pieces of ice and, I imagine, the larger bits of ice would be more difficult to get rid of than smaller bits. That's one of my first thoughts. And the other one might be whether the car was wet before the night, so if the droplets that are already on the car then they might be quite well adhered to the surface...

Kat - Or like if it's been misty or something like that?

Ben - Yes. Or just rained or something like that. Sort of above freezing during the day but then these droplets already on the car, they just freeze at night. Or whether it's just moisture that comes from the atmosphere during the night. My feeling is that if the car was wet already, that would lead to sort of ice that was a bit more stuck on.

Ginny - But would the easy answer not just be to boil the kettle and pour it over? Cause that would work on either wouldn't it?

Chris - Or deicer?

Ben - Yes, that would work, but you've got to be a bit careful with very hot water because you might lead to an expansion of your windscreen and crack your windscreen. But yes, certainly deicers and things like that would be good.

Kat - Stuart?

Stuart - Yes, one of thing you could do would be the surface chemistry of the windscreen itself. If you've got a layer of material on it that the ice doesn't want to stick to well to the glass, that might well help. So, for example, a thin layer of washing up liquid, something like surfactant that forms basically a layer in between the glass and the water would also be another way of doing it.

Chris - That's what's known as a tarp isn't it?

Stuart - Yes a tarp. Yes you could put a tarp over it.Kat - Does that answer your question Martin?

Martin - Yes, that's great. Thanks a lot.

Kat - Well, happy scrapping.


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