There are some things in life that many people just don't think to question. Water wets things. Gravity sucks. And cellophane tape screams when you peel it.
Unlike the intricacies of gravity, though, the screaming of tape has now been explained. A team of physicists led by Er Qiang Li of the University of Science and Technology of China used ultra-high-speed cameras and sensitive microphones to record what really happens as ordinary transparent Scotch tape peels away from glass.
The answer is surprisingly technical: The screech is a train of tiny shockwaves, which burst forth when supersonic fractures racing through the tape's adhesive layer reach its edges.
Yep. Your garden-variety self-adhesive tape emits teeny tiny sonic 'booms'.
Actually, adhesive tape's noisy protestations have been under investigation for decades. In 2010, a team of physicists observed elastic waves traveling up the detached part of the tape and speculated that the screech emanated from them. Then a 2014 paper linked the sound to fractures in the tape, yet stopped short of discerning the exact mechanism.
Li and his colleagues wanted to get to the bottom of the puzzle. They designed an experiment to observe in detail what happens when a 19-millimeter-wide (0.75-inch) strip of Scotch tape is peeled from glass.
Now, when you peel tape, it doesn't just lift away in one smooth motion, but in a jerky, chaotic pattern physicists call "stick-slip". This stick-slip behavior has been studied for decades.
As you peel, the adhesive continues to cling stubbornly to the surface for a fraction of a second. That's the stick part. When the pulling force finally overcomes the adhesive bond, it suddenly gives way. That's the slip. This process repeats over and over as you unpeel the tape.
During each slip phase, however, something dramatic is happening inside the adhesive on a microscopic scale. It doesn't peel evenly across its entire width; rather, it tears in narrow bands that race sideways across the tape, from one edge to the other.
These are called transverse fractures, and Li and his colleagues found they're the key to why tape screams.
The team recorded peeling tape using two microphones and two high-speed cameras, one pointing at the underside of the tape from below the glass, and another above the experiment using a schlieren imaging system to capture disturbances in the surrounding air.
They found that what makes the fractures so unusual is their speed. They recorded fracture speeds ranging from about 250 to 600 meters per second (560 to 1,340 miles per hour). For comparison, the speed of sound in air at room temperature is about 342 meters per second. That means some of the fractures are moving across the adhesive layer at speeds approaching twice the speed of sound.
Because the fractures travel so quickly, they leave behind a tiny gap between the tape and the glass, a fleeting pocket of partial vacuum. Air can't rush in fast enough to fill it as it forms. The pocket travels with the crack until it reaches the edge of the tape, whereupon air rushes in, and the pocket suddenly collapses.
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The abrupt collapse of the cavity is what launched a weak shockwave into the air. These isolated shocks move at slightly more than the speed of sound – 355 meters per second – more of a sonic whisper than a boom, but driven by similar supersonic mechanisms.
Finally, by comparing the arrival time of the sound at two microphones placed on opposite sides of the tape, the researchers confirmed that each shock originates at the edge rather than along the length of the crack.
"The elastic waves traveling in the detached tape could also produce some sound," the researchers conclude, "but our imaging results showed clearly that the train of weak shocks overpowers any such contributions."
So there you have it. You don't have to be a Mach pilot to break the sound barrier. Just find a roll of tape and go nuts.
The research has been published in Physical Review E.