Ice crystals on cars are beautiful

As I was walking to the train sta­tion yes­terday I noticed that all the parked cars were cov­ered in the same beau­tiful pat­tern of swirling fan­like crys­tals. The hoods and roofs and wind­shields looked like an artist had drawn all over them with a mag­ical ice pen (does that sound cheesy? It’s exactly what it looked like). But I’m a sci­ence writer…I should know by now there’s no such thing as magic (sorry).

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All photos by yours truly.

So I got in touch with one of our top crystal growth con­nois­seurs to find out what was really behind these beau­tiful pat­terns. Mon­eesh Upmanyu is an asso­ciate pro­fessor of mechan­ical and indus­trial engi­neering who uses com­pu­ta­tional tech­niques to under­stand the struc­tures and prop­er­ties of emerging mate­rials. But he’s also pretty inter­ested in the ancient, nat­u­rally occur­ring ones, too (in par­tic­ular, rhodo­den­drons!).

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Here’s what he had to say about the crys­talline car art I observed yes­terday. “Frost on sur­faces, com­monly referred to as hoar frost, does not con­dense from the air,” he said. “Rather, it crys­tal­lizes from a thin liquid film of water on the sur­face.” This is usu­ally only pos­sible when a frosty day fol­lows a rainy one. The phe­nom­enon also requires three ingre­di­ents: A thin film of liquid water poised peace­fully on a sur­face is the first. The second is a single par­ticle of dust or other imper­fec­tion, like a pearl sit­ting unob­tru­sively in an oyster shell. Third, freezing cold temperatures.

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The par­ticle acts as a nucleus around which crys­tal­liza­tion begins. As the water mol­e­cules in its imme­diate vicinity slowly drop in tem­per­a­ture, they begin to solidify. This layer of hoar frost is already pat­terned at a much smaller scale since the solid­i­fi­ca­tion doesn’t happen uni­formly, Upmanyu said. It can grow in this branching manner due to the molecular-​​scale struc­ture of the crystal. Because the water layer is so very thin, the growing frost front sucks up the liquid in its vicinity. It often begins to meander and curve as it tries to accu­mu­late the liquid, which itself is still flowing.

Now, since the frosty regions are cooler, the water-​​saturated air above con­denses and they thicken. This trig­gers fur­ther evap­o­ra­tion of the already thin liquid layer until it dries out, leaving dry, exposed patches of the metal roof, the glass wind­shield, between these frosty pro­tru­sions, Upmanyu said. Without any water, crys­tal­liza­tion stops locally and we see the emer­gence of these fan-​​like frost patterns.

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The evap­o­ra­tion of the exposed sur­face makes it much colder (just as we feel cold when water evap­o­rates off our skin) that it trig­gers frost for­ma­tion from the remaining liquid at a dif­ferent loca­tion. This depends on how well the sur­face can con­duct heat, and the process repeats until the entire sur­face is dec­o­rated. Overnight, the dry sur­faces will accu­mu­late some frost as well, but they will always be thin and the pat­tern will persist.

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Upmanyu said he’s seen wind­shields where the whole thing was one giant set of con­cen­tric crys­talline cir­cles, instead of repeating fans like what I saw. “This is a sign of a very clean wind­shield,” he chuckled. Just a single par­ticle of debris is enough to set it off into a sym­metric growth pattern.

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What hap­pened on the cars is essen­tially the same way a snowflake forms up in the clouds, Upmanyu told me, although there are lots of things sci­en­tists are still scratching their heads about in both depart­ments. As water mol­e­cules hang out in the slowly cooling air (it could take a whole day for the tem­per­a­ture to drop just a couple of degrees), the crystal growth occurs very slowly. We know that the super-​​slow growth pat­tern of water favors a six-​​sided crystal ori­en­ta­tion, which just gets ampli­fied again and again until you can actu­ally see it on a macro­scopic scale as a snowflake. Upmanyu said that if you cool water down in the lab more rapidly, you will see a much dif­ferent pat­tern emerge.

This is of course a sim­pli­fied expla­na­tion,” Upmanyu said. “Pat­tern for­ma­tion is almost always com­plex, but it’s beau­tiful to watch.” I couldn’t agree more