Retorts

Fresh Light on Triboluminescence

My chance observation [Retorts, Dec] that pulling the flap of a self-sealing envelope can produce a flash of light has generated a wide response, including confirmation -- after initial skepticism -- by the manufacturers, Croxley. Recently the Listener entered the act by reprinting part of the story in its Upfront column of February 5th under the heading "Weird Science".

Connecting these leads has proved as interesting as the original observation. First a colleague from the chemistry department where I work pointed out that sellotape does the same thing -- we experimented and found that parcel tape is even better. Others remembered articles in Scientific American. The Amateur Scientist feature of the December 1987 issue explains how frictional forces generate electrically charged regions of the tape. Charged particles are accelerated by the resulting electric field, they collide with nitrogen molecules and this makes the nitrogen glow blue. The article goes on to tell how to capture on film the faint glow emitted when sticky tape is peeled off a surface.

Earlier, in the July 1982 column, an activity even more bizarre than opening one's bank statements in the dark is described, for the author had noticed triboluminescence while crunching a Lifesaver when standing in front of a mirror in a dark room. It turns out that Robert Boyle (of Boyle's Law fame) had recorded in the 17th century that "hard sugar being nimbly scraped with a knife would afford a sparkling light".

With help from readers of my report I soon found that a great deal is known about the production of light in ways like these, including the fact (proved by its spectrum) that the light originates not from the sugar or from the material of the adhesive but from nitrogen of the surrounding air. To quote from an authoritative source (An Introduction to the Luminescence of Solids by H.W. Leverenz): "An inrushing gas in the fracture may add to the excitational process by frictional ionisation and subsequent attraction to the charged faces (compare with the faint blue [nitrogen] glow observed when a roll of friction tape is unrolled in the dark)." To provide proof, when tape is unwound in neon gas, the glow produced by this experiment becomes the familiar red colour of a neon sign.

The ingredients necessary to conjure up this effect -- friction on a suitable chemical substance in contact with air molecules to emit the flash -- are common enough. The missing condition for it to be noticed easily is genuine darkness which, like total silence, has become a scarce commodity in the late 20th century. One can imagine the apothecary of former times in his dimly lit premises witnessing flashes while grinding sugar with pestle and mortar, though he would not have called it crystalloluminescence, and might have feared it to be the work of the devil.

Nowadays the photographic darkroom offers the ideal setting. Among those to respond to my report, J.C. Welch of Woodbourne wrote of having seen a marked flare when removing black and white film from its backing paper.

My note had also bee printed in the teachers' journal Chem NZ where it was read by Professor Ian Rae, Dean of Science of Monash University, Melbourne, who kindly drew my attention to the current research of Professor Linda Sweeting on this subject in the US. So if anyone really wants a lot more detail, I can now supply chapter and verse, or rather journal and page from Chemical Abstracts.

Michael Taylor, Associate Professor of Chemistry, University of Auckland