Microscopic Tribology

I read "Tribology All Around" with great interest (C&EN, Dec. 7, 2009, page 47). It was mentioned that the sparking seen when biting a Wint-O-Green Life Saver in the dark is an example of triboluminescence—light produced by two surfaces in relative motion.

An undergraduate researcher and I observed this phenomenon when heating wintergreen oil with sodium hydroxide and water in the microwave oven to produce salicylic acid. (We were working on developing a laboratory experiment for undergraduates that would start with wintergreen leaves to synthesize aspirin.) It sparked and produced "lightning" just as a metal-trimmed plate heated in a microwave oven does. After a little research, we theorized that this was an example of triboluminescence on a microscopic scale: The oscillating electromagnetic field produced by the microwave oven first aligned the molecules of salicylic acid and then allowed them to relax, resulting in relative motions and hence the sparking.

Pamela Brown
Brooklyn, N.Y.