  Viscoelasticity is well known to cause significant hysteresis of crack
closure and opening when an elastomer is brought in and out of contact with a
flat, rigid, adhesive counterface. A separate origin of adhesive hysteresis is
small-scale, elastic multistability. Here, we study a system in which both
mechanisms act concurrently. Specifically, we compare the simulated and
experimentally measured time evolution of the interfacial force and the real
contact area between a soft elastomer and a rigid, flat punch, to which
small-scale, single-sinusoidal roughness is added. To this end, we further the
Green's function molecular dynamics method and extend recently developed
imaging techniques to elucidate the rate- and preload-dependence of the
pull-off process. Our results reveal that hysteresis is much enhanced when the
saddle points of the topography come into contact, which, however, is impeded
by viscoelastic forces and may require sufficiently large preloads. A similar
coaction of viscous- and multistability effects is expected to occur in
macroscopic polymer contacts and to be relevant, e.g., for pressure-sensitive
adhesives and modern adhesive gripping devices.
