One of the joys of physics, and science in general, is that even seemingly mundane objects occasionally yield physical surprises. A great example of this made the news about a month ago: the observation that, under the right circumstances, x-rays can be generated by the peeling of Scotch tape! The phenomenon is an extreme example of the phenomenon of triboluminescence, and I thought I would take a closer look at the research results, which appeared in Nature.
First, a quick but important notice: THERE’S NO REASON TO WORRY ABOUT USING STICKY TAPE AT HOME! As we will note below, the x-ray effect is only significant when tape is peeled in a high vacuum. Such a condition obviously does not occur without special preparation. So the wrapping of Christmas packages can continue without fear.
It’s worth taking a moment to explain why this seems like such a surprising result in the first place. Interaction energies in normal chemical interactions tend to be no greater than 10’s of electron volts; for instance, it takes 13.6 eV to ionize a hydrogen atom. If the reaction releases a photon, this puts the wavelength of the photon at best in the ultraviolet or visible range, with an energy several orders of magnitude lower than the keV or MeV of x-rays. X-ray emission from atoms under normal circumstances comes only from nuclear processes, e.g. the decay of an atomic nucleus. Chemical reactions seemingly don’t have enough ‘oomph!’ to generate x-rays.
Electromagnetic radiation can be generated by any acceleration of electric charges, however, and with the right system this can readily produce x-rays. X-rays were in fact discovered by Wilhelm Röntgen in 1895 while he was experimenting with cathode ray tubes (which is, in essence, a device which accelerates electrons along a vacuum tube). Electrons, traveling from a cathode to an anode, are decelerated sharply on interaction with the anode. An illustration of such a tube is shown in a Wikipedia picture below, with the cathode on the left:
This process of ‘braking’ releases radiation, known as Bremsstrahlung radiation. This radiation will include x-rays if the ‘braking’ is strong enough, i.e. if the electrons were moving fast enough to begin with.
Another example of x-ray emission from acceleration is the phenomena of synchrotron radiation, in which particles in high-energy accelerators shed x-rays as they circulate. Initially, such radiation was seen as a hindrance to performing high-energy physics experiments, but now it is used at places such as Argonne National Laboratory as a source of x-rays for other experiments.
These examples involve the acceleration of charged particles to high velocities. There seems at first glance to be no such process going on when one peels a roll of tape, which is what makes the result of the University of California research group so interesting.
The phenomena is an example of a process known as triboluminescence, which in short refers to the release of light from crystalline structures when they are rubbed, scratched or broken. The phenomenon has been well-known for quite some time, dating back to the early 1600s; Jennifer at Cocktail Party Physics has written a nice description of the history of triboluminescence and some of the current developments (and scooped me again!). A classic example (which is celebrating its 90th anniversary), is the ‘spark’ produced when biting down on a Wint-O-Green Life Savers (for your sanity, stop the video after the first 11 seconds):
Interestingly, the most sensational parts of the Nature paper are not new; in fact, they are quite old! As the authors point out, triboluminescence from Scotch tape was observed back in 1939*, and evidence that x-rays can appear in triboluminescence was suggested in 1930** and confirmed in 1953***.
The first hint of x-rays in triboluminescence appeared in a paper by J.W. Obreimoff on, “The splitting strength of mica.” In addition to studying the ability of polished glass plates to readhere to one another after splitting, Obreimoff describes some electrical phenomena which appear when the mica is split in a high vacuum. In anticipation of the results relating to Scotch tape, Obreimoff notes,
If split in darkness, mica becomes slightly luminescent (triboluminescense). This is due to electric discharges between the mica surfaces through the air. If we split them under an air pressure of 1.0-0.1 mm. mercury the glow spreads to all the air in the vessel and is similar to the glow of a Geissler tube. In a high vacuum ( mm. mercury) the glass of the vessel fluoresces like an X-ray bulb. The light is feeble and can be observed only after the eye has rested about 3 minutes in darkness.
This fluorescing of the glass of the vessel suggested the presence of x-rays, and it only appears when a vacuum is present, just like the tape experiments to be described.
I can’t resist pointing out how one actually splits a piece of mica which is isolated in a high vacuum! I take the liberty of reproducing the figure of the experimental apparatus from the paper:
A glass wedge is placed under the strip of mica being separated, and a glass hammer, attached to a glass-encased iron block, is adjacent to it. An electromagnet is used to manipulate the block from the outside and smack it against the glass wedge.
In 1939, N.E. Harvey wrote a ‘discussion’ of the luminescence of adhesive tape. Harvey gives a rough explanation of the process of luminescence:
It is apparent that these phenomena have a decidedly electrical flavor. A sheet of collodion stripped from glass or ebonite has a high negative charge, leaving the glass and ebonite positive. It is attracted to the glass with considerable force and sticks to the hand and other objects. The sign of the charge is easily determined by pith ball experiments.
The explanation of such luminescence appears to be this: whenever two surfaces are separated from each other the capacity diminishes and the voltage rises until a discharge takes place, exciting the surrounding gas to luminesce. It is not possible to prove that mica sheets or tire tape, surgeons’ tape or Scotch tape are oppositely charged as a whole when pulled apart, but there are no doubt local positive and negative regions developed, the discharge between them giving rise to luminescence.
That a discharge does actually take place can be readily shown by stripping surgeons’ tape or Scotch tape in an atmosphere of 2 to 4 cm Hg pressure of Neon gas. Then the luminescence is reddish instead of yellowish.
A rough diagram of the hypothesized process is shown below:
As the tape (or mica) is separated, local regions of separated + and – charge are created. When these regions are separated far enough, the electric force pulls the electrons from the negative side to the positive side. Along the way, though they collide with (and excite) gas molecules, which then produce fluorescence. The color of fluorescence depends on the type of gas molecules present.
How can we get x-rays from such a process? If we evacuate the chamber in which the separation is taking place, the electrons are now free to travel the entire distance from one surface to another. With a large enough potential difference between the surfaces, they act essentially as a natural cathode ray tube! Electrons travel from the cathode (- surface) to the anode (+ surface), and when they arrive they are slowed down by Bremsstrahlung radiation:
This is the sequence of events suggested by the authors of the recent Nature article. But, as we’ve noted, triboluminescence has been observed from adhesive tape before, and x-ray emission from triboluminescence has been observed; so what is new about the current crop of experiments?
There are several new aspects. First, no modern studies of x-ray emission in the peeling of Scotch tape had been performed, and it is striking that such an inexpensive and mundane material could have such properties. Second, they demonstrated that a significant amount of x-rays are produced in the process. This was illustrated in an x-ray image produced via a collection of 20s images:
Finally, the authors performed a detailed study of the emission properties of sticky tape, and noted a striking correlation between the emission of x-rays and so-called ‘stick-slip’ events in the unspooling of the tape. The tape was unwound using a motor for which the force of unwinding could be measured, at a peel speed of 3 cm/s. It was noted that the emission is correlated very strongly with a sharp drop in the unwinding force; presumably this is a moment when a large section of tape suddenly comes loose from the spool.
X-rays with a peak photon energy of 15 keV were observed, and radio frequency emissions (much lower energy) were observed, also correlated with stick-slip.
This x-ray triboluminescence already looks to have possible application as an inexpensive, portable x-ray imaging system.
Though the explanation given above for the phenomenon seems reasonable, a detailed understanding of triboluminescence in this case (and in general) is still not available, in large part due to the complicated nature of the systems involved. Hopefully these results will shed some illumination on the phenomenon!
* N.E. Harvey, “The luminescence of adhesive tape,” Science 89 (1939), 460-461.
** J.W. Obreimoff, “The splitting strength of mica,” Proc. R. Soc. A 127 (1930), 290-297.
*** V.V. Karasev, N.A. Krotova and B.W. Deryagin, “Study of electronic emission during the stripping of a layer of high polymer from glass in a vacuum,” Dokl. Akad. Nauk. SSR 88 (1953), 777-780.
Carlos G. Camara, Juan V. Escobar, Jonathan R. Hird, Seth J. Putterman (2008). Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape Nature, 455 (7216), 1089-1092 DOI: 10.1038/nature07378