Effect of solid surface tension on crack nucleation in soft gels

Solid surface tension has been shown to affect the deformation, adhesion, and crack propagation in soft solids. However, the effect of surface tension on crack nucleation, in particular, has been largely ignored. We study the effect of surface tension on the critical force to puncture (P_c ) a soft acrylic copolymer gel with spherically-tipped indenters of tip radii comparable to or smaller than the characteristic length scale Y_s/E. Previously, we have shown that the size of a fracture process zone beneath an indenter tip is governed by the tip radius, R, and is critical in determining the failure regime, i.e. whether it is limited by a critical stress or a critical energy. In an energy-limited regime, P_c scales linearly with R. However, this scaling relation is derived assuming that the resistance to deformation and failure during puncture is mainly contributed by the elasticity of the gel, while surface tension was not taken into account in the analysis. Our results lead us to identify the characteristic material length scale, Y_s /E and have demonstrated a new and interesting way in which soft materials resist failure at micron length scales.