Determining the elasto-adhesion length by void collapse in ultra-soft materials

The elasto-adhesion length (l_EA) is a materials property that describes the size scale where the relative importance of adhesion and elastic forces transitions for defined structures and functions. Conventional approaches for determining l_EA require independent measurements of the critical energy release rate (G_c) and Young's modulus (E). Herein, we introduce a new method to obtain l_EA, only relying upon visual inspection. This method relies upon the introduction of a controlled void within a soft material, for example an ultra-soft hydrogel as studied here. The geometry of the void within the hydrogel is tuned to control the relative strength of adhesion and elastic energy, and a relationship between the observed equilibrium configuration of the void and l_EA is derived. We validate this new method with independent force-based contact adhesion tests. This method allows for the easy measurement of l_EA for difficult-to-handle soft materials and can be amended for various chemistries and materials.