Adhesion-Induced Instability Regulates Contact Mechanics of Soft Thin Elastic Films.

Adhesive contact of soft materials plays an essential role in flexible electronics, soft robots, and biological systems. Classical contact mechanics theories assume that the contact region between a spherical indenter and a flat surface is circular. In this paper, however, we show that fingering instability and inner cavitation may appear and disappear during the loading-unloading process when a soft thin elastic film is indented with a spherical indenter. We further demonstrate that this adhesion-induced instability can enhance the resistance to deformation and thus significantly increase contact force. Finally, we find an important dimensionless number, defined as the ratio of adhesion energy to elastic energy, and thus reveal that the competition between adhesion energy and elastic energy yields the critical condition for adhesion-induced instability. Thus, our findings bridge the gap between contact mechanics and interfacial instabilities of soft materials and may have many important implications for the applications of soft materials.