Sticky tape peels in fits and starts

When you peel a piece of adhesive tape away from its roll, the motion is neither smooth nor steady at macroscopic or microscopic scales. The tape will stubbornly stick, then it will finally give, with a length of tape peeling all at once, and then it will stick again—a motion aptly called stick–slip. The unintuitive relationship between the duration of a stick and the distance of the subsequent slip has now been characterized at microscopic scales, in experiments by Stéphane Santucci of the École Normale Supérieure in Lyon, France, and his colleagues.

The researchers attached two layers of acrylic film tape, such as you might use to wrap a gift, to a transparent plate. As shown from a side view in the image and video, the top layer was peeled by a motor at a constant velocity and an adjustable angle. Using a high-speed camera with a macrolens to monitor the position of the peel front, the team found that the slip distance, often in the hundreds of microns, scaled as the cube root of the duration of the stick regardless of the tape’s peel angle or bending modulus. Those microscopic slips combined into a macroscopic slip at the millimeter scale.
A look at the local energy explains the stick–slip dynamic. The increasing elastic bending energy in the stick phase turns into kinetic energy in the slip phase. By recasting the radius of curvature and velocity in terms of the slip distance and stick period, Santucci and colleagues derived an equation that matched their experimental observations. One aspect not yet understood is the motion of the peel front as observed by the formation of kinks in the tape. The tape starts to peel at one side with the separation spreading as a wave moving transverse to the direction of the peel, similar to the motion of dislocations in crystalline materials. Why that is the case remains unexplained. (V. De Zotti et al., Phys. Rev. Lett. 122, 068005, 2019 .)