Puncture of Polymer Gels at Small Size Scales

Understanding failure processes of polymer gels is critical for numerous applications, ranging from adhesives to protective materials. We discuss the failure processes associated with the deep indentation and puncture of soft gels with an axisymmetric probe. We commonly observe that the first critical transition, associated with puncture, occurs at nominal stresses that can be as much as 100 to 1000 times the elastic modulus. To understand how soft gels can sustain such stress levels prior to initial failure, we have studied puncture as a function of size scale, velocity, and material network structure. Spherically-tipped indenters of radii, R=0.4-66 um were used to characterize puncture at length scales well above the network mesh size (nm), on the same order of magnitude as the elasto-capillary length (um), and significantly below the elasto-fracture length (mm). Critical energy release rate was found to be in agreement with the predicted scaling from the classical Lake-Thomas model modified for gel fracture via the failure mechanism of chain pull-out and plastic yielding of micelles. These experiments and proposed relationships provide new insight into how gels fail and how design paradigms may be shifted to more effectively engineer with soft gels.