EXPERIMENTS with extensive imaging of the seemingly smooth act of peeling tape from a surface suggest that it is far from simple. It involves a multiple start-stop jerky process in the microscale. The results of these high-speed camera experiments, carried out by Stephane Santucci of the Ecole Normale Superieure in Lyon, France, and his colleagues, have been published in the latest issue of “Physical Review Letters”. The observations have also led to a modelling of the process and the formulation of a microscopic theory.
Adhesive tapes are used in thousands of applications, but researchers still do not fully understand how the peeling process operates at the microscopic level. The experiments by Santucci’s group, and the theory developed, explain how the energy provided by the peeling force is converted to kinetic energy in the moving tape. Characterising this effect could lead to a reduction in the noise produced by peeling tape (a serious concern for industry, where the sound can be deafening) as well and to improvements of future adhesive tapes.
Anyone who has peeled a tape knows that it tends to start and stop every few millimetres, a phenomenon that is called macroscopic stick-slip. But in 2010, researchers discovered that within each millimetre-scale slip, there are many so-called micro-stick-slip events separated by a few hundred micrometres or less. In fact, even when the tape rolls out steadily, it actually comprises several start-stops on the microscopic scale that occur at high frequency.
In 2015, Santucci and his team found that these micro stick-slips were controlled by a release of bending energy in the tape near the separation front, the line dividing the attached tape from the separated tape. But the mechanism driving the microslips was still unclear. In their latest research, the team revisited their previous technique, in which a motor peels one layer of Scotch tape off another layer that is stuck down on a transparent surface. Several parameters associated with the act, such as the peeling angle and velocity and the tape’s bending stiffness, were also varied. A high-speed video camera mounted on a microscope recorded the peeling.
It was found that the longer the duration of the “stick”, the more the distance covered during the following slip. “The more you wait, the longer the jumps should be,” says Santucci. “During this time you’re storing elastic energy in the bending of the ribbon.” But the relationship between these two quantities was not a simple linear one. The slip distance was proportional to the cube root of the time between slips. The researchers have also proposed a theoretical model for the micro-stick-slip process and have compared it with their data. The model seems to fit the observational data well.
The researchers draw parallels between the motion of the tape separation front and the propagation of cracks in materials because both create new surfaces as they progress. This analogy makes sense, says Jay Fineberg, a soft matter physicist at the Hebrew University of Jerusalem. “I’m continually surprised that a diverse range of seemingly unrelated questions in physics turn out to be quite related to rapid fracture processes,” he says. “These problems range from peeling problems to the basic physics of friction and earthquake propagation.” He suggests that the latest research is “another piece to this rather diverse puzzle” of fracture.