Pull-off dynamics of mushroom-shaped adhesive structures

Dynamic adhesion characteristics of synthetic adhesives have attracted massive attention recently. Specific to the mushroom-shaped adhesive structures with outperformed adhesion properties, a clear understanding of the pull-off dynamics, especially the role of retraction velocity, has not been addressed yet. In this paper, based on a custom-built adhesion test apparatus allowing in-situ high-speed measurement of the interface failure, we conducted detachment tests on hundreds-micrometer-scale mushroom-shaped adhesive structures with different cap thicknesses at a retraction velocity range spanning 4 orders of magnitude. It is found that the crack propagation mode for a thin or thick cap remains the same at different retraction velocities, whereas for an intermediate cap the transition from the edge-crack mode to the center-crack mode is observable. Notably, for center-crack mode, the crack area at pull-off remains relatively constant at different velocities. The variation of the pull-off forces with velocity exhibits a scaling law at high velocity regardless of the propagation mode. Dynamic detachment models are developed by considering the rate-dependent work of adhesion to demonstrate the critical-crack-dimension invariance at different velocities and the scaling law of pull-off force with the velocity with the same scaling exponent for center- and edge-crack mode. The theoretical scaling agrees well with experiments. Furthermore, finite element analysis of the viscoelastic detachment demonstrates the stress redistribution against retraction velocities. A prominent feature is the increasing length of the cohesive zone at pull-off with the increasing velocity, indicating a potential trend of a transition to a long-range adhesive interaction. At a sufficiently large velocity, the stress spike at the crack tip disappears and a theoretical strength is almost obtained at the region beneath the stalk.