While astrophysicists, particle physicists, and cosmologists attempt to figure out the secrets of the universe, there are still many smaller mysteries that need solving.
A few such mysteries can be found in ordinary scotch tape, or at least when you unfurl it for use. When you unroll scotch tape, you may have noticed a couple of things. First is the high-pitched screeching noise, and the second, if you happen to be wrapping presents in the dark, is that it can produce blue light (as well as X-rays, though it would be difficult to notice that during normal use).
The blue light, despite being the weirder of the two mysteries, has been explained for some time. The odd appearance of light is down to "triboluminescence", the same phenomenon that makes mints light up like lightning after being hit by a hammer.
"Triboluminescence is the emission of spontaneous light by mechanical force. This mechanical force is typically activated as a result of frictional contact such as rubbing or scratching," a review on the topic explains. "Another form of luminescence occurs when the mechanical force is sufficient to break material bonds resulting in fracture. This luminescence is referred to as fractoluminescence, fracture stress-mechanoluminescence, or fracture triboluminescence."
According to that team, this is what is responsible for the light you see when you pull the tape apart.
"The explanation of such luminescences 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," a 1939 paper on the luminescence of adhesive tape explains.
"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 [0.8 to 1.6 inches] Hg pressure of neon gas. Then the luminescence is reddish instead of yellowish. Red luminescence also occurs when two strips of mica are pulled apart or when collodion or ambroid or rubber cement is stripped from glass in a low-pressure neon atmosphere. When black tire tape is stripped in neon, the reddish luminescence is not marked."
So, what about the sound? In a new paper, scientists placed scotch tape on a thick glass plate and recorded with microphones and a high-speed camera to investigate what was going on.
It was already known, by physicists and anyone attempting to use tape, that as you unroll it, it slips and sticks in a jerky, non-smooth motion. Even when the action appears smooth, on a microscopic level, there is still some slipping and jerking going on. And as this occurs, tiny cracks appear in the adhesive layer of the tape, perpendicular to the direction of unrolling (or spreading out to the side of the tape). Analyzing the sound and video footage, the team found that it is these cracks that are responsible for that awful screaming noise. Specifically, it is because these cracks move at supersonic speed, or faster than the speed of sound in air.
"The screeching sound of peeling tape consists of a train of weak shocks that are generated when the transverse fracture bands, in the slip phase, reach the edge of the tape," the team explains in their paper, adding that the camera showed that a shockwave was produced when the crack reached the air.
"A partial vacuum is produced between the tape and the solid when the crack opens," they added. "The crack moves too fast for this void to be filled immediately, even though air is sucked in from the direction perpendicular to the crack. The void therefore moves with the crack until it reaches the end of the tape and collapses onto the stationary air outside."