X-rays by triboluminescence?

Drs. Wilfred T. Tysoe & Nicholas D. Spencer | TLT Cutting Edge October 2009

A recent discovery shows that more than just light can be produced by rubbing.
 

The phenomenon of triboluminescence, the emission of light during rubbing, is well known and was first recorded by Francis Bacon in 1620 where he noted that, “It is well known that all sugar, whether candied or plain, if it be hard, will sparkle when broken or scraped in the dark.” Also, the emission of other particles such as electrons to produce a “triboplasma” at sliding interfaces is well documented.

One might expect that this phenomenon would relate to the strength of the chemical bonds that are broken. However, it was found as early as 1939 that an adhesive tape affixed to a surface, which is thought to be held together by rather weak van der Waals forces, provided, upon peeling apart, an example of a triboluminescent system and generated light emission that could also be detected by the naked eye.

Even more surprising was the recent discovery by professor Seth Putterman and his group in the department of physics and astronomy at UCLA that X-rays could be generated as well. In order to investigate this phenomenon, they constructed an apparatus that enabled them to peel a roll of commercial Scotch Tape® at a controlled velocity. Since they found that the effect was quenched by air, the experiment was performed under a moderate vacuum of about 10-3 Torr (about a millionth of atmospheric pressure) while detecting the resulting emitted X-rays using an efficient, high-speed X-ray detector.

They observed the emission of X-ray pulses lasting only about a billionth of a second that correlated with slips in the force that was required to peel the Scotch tape. While the X-ray photons had energies of about 20 keV, the pulses could contain more than 100,000 photons, resulting in a total energy per pulse of GeVs. Based on the length of the X-ray pulses, the authors suggested that the emission occurred from regions at the point of peeling that were much smaller than a millimeter.

The authors observe that, during peeling, the adhesive becomes positively charged, while the underlying polyethylene roll becomes negatively charged so that large electric fields are built up and become sufficiently strong to trigger discharges. Under the reduced pressures of the experiment, the electrons in the discharge are accelerated by the electric field between the roll and the adhesive to sufficiently high energies that they emit X-rays as they strike the positive side of the tape. While a sensitive X-ray detector was used to monitor the emitted X-rays and to measure their energy distribution and duration, the emitted X-ray flux is quite high.

A very striking demonstration of this significant intensity was made by taking X-ray images of one of the author’s fingers, where they were able to collect quite clear images of the bone in the finger with only a 20-second exposure while a tape was being peeled at 10 cm/s. The process by which the high-energy photons (X-rays) are emitted is still not completely understood, but the authors suggest that these effects might lead to insights into fundamental aspects of tribology.

In the meantime, fundamental research into triboluminescent X-ray emission might help to limit rising health-care costs!

FOR FURTHER READING:
Camara, C.G., Escobar, J.V., Hird, J.R. and Putterman, S.J. (2008), “Correlation Between Nanosecond X-ray Flashes and Stick-Slip Friction in Peeling Tape,” Nature, 445, pp. 1089-1092.

Dickinson, J.T., Scudiero, L., Yasuda, K., Kim, M.-W. and Langford, S. C. (1997), “Dynamic Tribological Probes: Particle Emission and Transient Electrical Measurements,” Tribology Letters, 3(1) pp. 53–67.


Eddy Tysoe is a Distinguished Professor of Physical Chemistry at the University of Wisconsin-Milwaukee. You can reach him at wtt@uwm.edu.


Nic Spencer is professor of surface science and technology at the ETH Zurich, Switzerland. Both serve as editors-in-chief of STLE-affiliated Tribology Letters journal. You can reach him at nspencer@ethz.ch.