Brittle Hydraulic Fracture In Transparent Heterogeneous Hydrogel

Hydraulic fractures occur miles underground, below complex, layered, heterogeneous rocks, making direct measurements of their dynamics or structure extremely difficult. As such, these fractures are typically studied at the surface within blocks of glass, PMMA, or rocks that are hydraulically broken with air or fluid. We have developed a system to study hydraulic fractures within hydrogels, where we can take advantage of their highly tunable rheology, optical transparency, and 2-3 orders of magnitude lower fracture speeds and breakdown pressures compared to PMMA. Using a combination of fast camera photography and laser sheet microscopy, we can study the morphology and dynamics of hydraulic fractures at extremely high spatiotemporal fidelity. Furthermore, the hydrogels can be polymerized with transparent mechanical heterogeneities, whose distribution can be characterized through comparable experiments with silica beads. This allows the study of the effect of increasingly dense mechanical heterogeneity on fracture properties, where we observe the emergence of different fracture morphologies.