Modeling interfacial instability patterns during debonding a rigid spherical indenter from thin elastic films

Adhesive contact of soft materials has many significant applications, including the fabrication of microelectronic systems, transfer of flexible electronic devices, and antifouling coatings. Many experiments found that cavitation and fingering instabilities will appear at the adhesive interface between rigid indenters and soft films. However, most of these studies focus on flat indenters. Thus, for the adhesive of spherical indenters and soft films, how to modulate the instability patterns and how interfacial instabilities impact the mechanical responses of the film are still poorly understood. Here, we study the detachment of a spherical indenter from a thin elastic film by using 2D and 3D finite-element simulations. Consistently with previous experimental observations of flat indenters, we find that the critical pull-off force increases exponentially with the confinement ratio when the interface separation is stable. But the interface separation becomes unstable when the confinement ratio is bigger than a certain critical value, causing a reduction in the pull-off force. Moreover, we can also study the effects of parameters difficult to handle experimentally and show that the initial instability pattern can be either cavitation or fingerings depending on the indentation depth, indenter radius, and adhesion energy density between the indenter and the film. Finally, we find that allowing the film to slip freely in the lateral direction can significantly suppress the fingering instabilities. Together, our work identifies the key factors that control interfacial instability patterns and the methods that impede interfacial instabilities, which will provide meaningful guidance for regulating the interfacial adhesion of soft materials.