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

The elasto-adhesion length () 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 require independent measurements of the critical energy release rate () and Young's modulus (). Herein, we introduce a new method to obtain , 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 is derived. We validate this new method with independent force-based contact adhesion tests. This method allows for the easy measurement of for difficult-to-handle soft materials and can be amended for various chemistries and materials.