Mechanoluminescence
Luminescence due to mechanical action
What causes the flashes of light when opening a self-seal envelope?
This phenomenon has been around for a long time (try Google for 'flashes self-seal envelope'). The type of envelope is shown below: it's the kind that has a rubbery adhesive. The flashes are often explained as an example of triboluminescence: however, the tribo prefix implies friction*, so it is better to class this as mechanoluminescence, i.e., luminescence due to mechanical action.
*From ancient Greek τρίβω (tríbō) “to rub”.
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I want to try this myself!
If you want to try it yourself, get yourself a white envelope (see reason below). Slice open the top open as shown, find a dark room and wait a few minutes to get adapted to the dark, then peel away the strip. You will see blue flashes at the join between the strips, as illustrated above using a long exposure on a compact digital camera [Canon S90, f/2, ISO 3200, 2.5 s exposure].
What colour are the flashes?
Our group was the first to measure the emission spectrum for this phenomenon. You can see the spectrum below. There are a set of small peaks in the UV from 300-400 nm, and these are due to emissions from excited nitrogen molecules (which are in the air). The average human would not be able to see the nitrogen emission, as our eyes become very insensitive below 380 nm. More importantly there is a much bigger peak at about 435 nm (deep blue), which is due to fluorescence from the paper, and would be very easy to see. The fluorescence spectrum of the paper, excited by UV at 300 nm, is shown as the dashed line, and it corresponds exactly to the mechanoluminescence.
What causes the blue that we see?
The main peak at is due to optical brightening agents in the paper: these are added during manufacture to make the paper appear very white. The same type of molecules are used in washing detergents for clothes. If we repeat the experiment with a brown envelope, there is no peak at 435 nm (graph below). Indeed the effect is more difficult to see by eye.
What is the mechanism for the mechanoluminescence?
By attaching electrodes to the envelope, we were able to measure the build-up of electric charge when the adhesive and paper are separated. The electric discharge causes the emission from nitrogen gas in the air, much like atmospheric lightning. This discharge also excites the optical brighteners in the paper. Remarkably we find that the adhesive side is always negatively charged relative to the paper, and we believe this is due to transfer of ions during the drying of the adhesive. More details can be found in the further reading [1].
