Influence of Geometric Confinement and Interfacial Forces on Puncture

Puncture is ubiquitous in nature, serving functions ranging from predation and protection to foraging. Inspired by these natural mechanisms, extensive research has explored the puncture probe shapes and strategies found in biological systems and has applied these insights to advance human technologies such as drug delivery, protective equipment, and material characterization techniques. Regarding these techniques, puncture testing has proven effective for characterizing both low-strain and fracture mechanical descriptors. However, prior studies have largely overlooked the influence of the substrate itself. Our work investigates how substrate boundary conditions affect puncture mechanics in ultra-soft gels. By confining gels in rigid wells of varying height and radius, we uncover how these parameters influence measured elastic and fracture behavior during puncture. We find that radial and height confinement reduce puncture energy by restricting deformation pathways, while samples with high-curvature menisci exhibit increased puncture resistance due to pre-stress from interfacial tension. These confinement effects reveal how established contact mechanics theory deviates for substrates with intrinsic negative curvature. These advances help us to decouple the materials contributions from geometry contributions in puncture-based mechanical property measurements, opening up quantitative pathways for high-throughput mechanical testing platforms.