Contact formation and viscoelastic detachment in non-circular soft adhesive contacts.

Adhesive contact measurements on soft polymers are commonly interpreted using Johnson-Kendall-Roberts (JKR) theory, which is formulated for circular contacts. Non-circular contacts generally require more complex elliptical-contact descriptions, making it unclear when a scalar measure of contact size is sufficient. Here, we examine contact formation and detachment in non-circular soft adhesive contacts using PDMS crossed-cylinder experiments. The cylinder crossing angle was varied from 30° to 90°, producing contacts ranging from highly elongated ellipses to nearly circular geometries while keeping the material pair fixed. During loading, the contact aspect ratio b/a initially evolved but rapidly approached an angle-dependent plateau, indicating approximately self-similar growth. This motivates, but does not by itself prove, an area-equivalent representation based on the radius together with the geometric-mean curvature radius . With these substitutions, the loading branches follow a JKR-type linearization and yield a nearly angle- and preload-independent loading work of adhesion, Wload = 24 ± 1 mJ m-2. Comparable values obtained from Johnson-Greenwood elliptical-contact fits provide independent support that this result is not an artifact of neglecting ellipticity at the level of the global c(P) response. In contrast, unloading and pull-off are strongly history dependent. The unloading branches can still be represented by an effective JKR-like relation using c, but only with a substantially larger effective separation energy, Wunload,eff, which increases with preload and with decreasing crossing angle. A reduced viscoelastic model based on the same area-equivalent description rationalizes the delayed contact recession and the principal unloading response over 50°-80° at the tested displacement rate. These results show that contact formation in elliptical soft adhesive contacts is governed primarily by area-equivalent scaling, whereas detachment is governed by geometry- and history-dependent dissipative separation. The framework provides a practical route for analyzing non-circular soft contacts using simple global measures while retaining the essential distinction between contact formation and viscoelastic detachment.