Contact Formation and Viscoelastic Detachment in Non-Circular Soft Adhesive Contacts

Abstract: Adhesive contact measurements on soft polymers are commonly interpreted using Johnson-Kendall-Roberts (JKR) theory, which is formulated for circular contacts. Here, we examine contact formation and detachment in non-circular soft adhesive contacts using PDMS crossed-cylinder experiments. The crossing angle was varied from 30 to 90 degrees, producing contacts from highly elongated ellipses to nearly circular geometries while keeping the material pair fixed. During loading, the contact aspect ratio b/a rapidly approached an angle-dependent plateau, indicating approximately self-similar growth. This motivates use of the area-equivalent radius c=sqrt(a*b) and geometric-mean curvature radius Reff=sqrt(R1*R2). The loading branches follow a JKR-type linearization and yield a nearly angle- and preload-independent work of adhesion, W_load=24 mJ/m^2. Johnson-Greenwood elliptical-contact fits give comparable values. In contrast, unloading and pull-off are strongly history dependent. The unloading branches require a substantially larger effective separation energy, W_unload,eff, which increases with preload and decreasing crossing angle. A reduced viscoelastic model based on the same area-equivalent description captures the principal unloading response over 50-80 degrees using a single shared parameter set across angles and preloads. These results show that contact formation is governed primarily by area-equivalent scaling, whereas detachment is governed by geometry- and history-dependent dissipative separation.