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Q & A: sparks in melting ice

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Melting & Freezing
Stuff that Sparks
Q:
At 1 am last night I took 3 water bottles out of my freezer and put them in my lunch box for tommorow. I wanted to give them a few hours to thaw to mostly water. The freezer was below zero. It was dark in my kitchen and as put them in the lunch pail they were crackling as usual coming into room temperature. But then I saw three small sparks in the ice about one inch from the bottom. My question is what happened?
- Paul Byron (age 53)
kittery, ME USA
A:
I wish I'd been there to see that.  What follows is speculation.

It's easy to get sparks from certain types of materials, called piezoelectrics, when they're under strain. In fact, little static-canceling guns use such piezoelectrics to generate big electric fields. A piezoelectric crystal has an asymmetrical distribution of charge, which is why strain can create electric fields and hence sparks.

The cracking sounds you hear tell you that the ice is under strain, and that the strain is changing suddenly. That sounds like a good set-up to make piezoelectric sparks. There's just one problem: a good crystal of ice isn't piezoelectric. However, I read on the following link that ice that freezes under strain can be less symmetric than an ideal crystal, and have piezoelectric effects. So that's my best guess as to what's happening.

http://books.google.com/books?id=BgTQeW7C-sIC&pg=PA10&lpg=PA10&dq=ice+piezoelectric&source=bl&ots=551rypSScM&sig=lwCc9tqnc4k32RykIo2kpaeGlSg&hl=en&ei=PG7WTvTuJ6bb0QGppe3hAQ&sa=X&oi=book_result&ct=result&resnum=2&ved=0CEcQ6AEwAQ#v=onepage&q=ice%20piezoelectric&f=false.


Mike W.
(unchecked, Lee's in Paris.)

(published on 11/30/2011)

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