  Insects, pick-and-place manufacturing, engineered adhesives, and soft robots
employ soft materials to stick to surfaces even in the presence of roughness.
Experiments show that the force required for making contact is lower than for
releasing it, a phenomenon known as the adhesion hysteresis. The common
explanation for this hysteresis is either contact aging or viscoelasticity.
Here, we show that adhesion hysteresis emerges even for perfectly elastic
contacts and in the absence of contact aging and viscoelasticity because of
surface roughness. We present a crack-perturbation model and experimental
observations that reveal discrete jumps of the contact perimeter. These
stick-slip instabilities are triggered by local differences in fracture energy
between roughness peaks and valleys. Pinning of the contact perimeter retards
both its advancement when coming into contact and its retraction when pulling
away. Our model quantitatively reproduces the hysteresis observed in
experiments and allows us to derive analytical predictions for its magnitude,
accounting for realistic rough geometries across orders of magnitude in length
scale. Our results explain why adhesion hysteresis is ubiquitous and reveal why
soft pads in nature and engineering are efficient in adhering even to surfaces
with significant roughness.
