Researchers in France have revealed the microscopic secrets of the physics of sticky tape.
“It’s a beautiful system, I wouldn’t have believed that you have such a rich dynamic,” says Stéphane Santucci from the École Normale Supérieure, lead author of a paper published in the journal Physical Review Letters.
Santucci became fascinated by sticky tape while moving house. He noticed that as the tape is peeled away from the roll, it does not come smoothly, but sticks for a moment and then progresses jerkily, sticking again a centimetre or two further along.
Santucci set PhD student Vincent de Zotti to work unrolling tape at an even speed in front of a super high-speed camera, filming at up to a million frames per second.
To their surprise, within the centimetre-scale stick-slip pattern there was a smaller scale version of the same behaviour going on: each slip was made up of a series of jerky micro-slips, each less than two tenths of a millimetre long, separated by sticky pauses of around a tenth of a millisecond.
And in each microslip event the tape did not lift evenly. One edge lifted first, initiating a pulse of lifting motion that travelled across the tape to the other edge, at times at speeds of nearly a kilometre per second.
“The presence of both microscopic and macroscopic mechanical instabilities is puzzling,” Santucci says. “We are trying to understand how the two instabilities are entangled.”
It was already known that the larger scale slips were related to the stretchiness of the tape, but to model the behaviour of the micro-slips the team had to include the energy required to bend it. During each micro-stick the energy went into deforming the tape, and in each micro-slip it was released as kinetic energy.
The peeling is further complicated by the glue, which, although apparently pliable, behaves like a brittle glassy solid when tape is peeled at high speed. The cameras showed that the tape left behind deposits of glue at each sticking point, which can affect the stickiness of the tape as well as its transparency.
Thermal imaging showed the temperature at each sticking point was also shooting up a couple of degrees, another threat to reliable glue behaviour.
Santucci and his colleagues are now working towards ways to stabilise the dynamics, which will be a welcome boon for industrial situations where, for example, the screech of tape peeling at high speeds can reach deafening noise levels.
The answers await a more complete understanding of the dynamics, however. The simplistic solutions that the model suggests, such as peeling the tape much more slowly, or using stiffer tape, could increase costs and decrease efficiency.
“To really find a way to control and tame the instability, we are not there yet,” Santucci says.