  Fracture propagation is highly sensitive to the conditions at the crack tip.
In heterogeneous materials, microscale obstacles can cause propagation
instabilities. Macroscopic heterogeneities modify the stress field over scales
larger than the tip region. Here, we experimentally investigate the propagation
of fluid-driven fractures through multilayered materials. We focus on analyzing
fracture profiles formed upon injection of a low-viscosity fluid into a
two-layer hydrogel block. Experimental observations highlight the influence of
the originating layer on fracture dynamics. Fractures that form in the softer
layer are confined, with no penetration in the stiffer layer. Conversely,
fractures initiated within the stiffer layer experience rapid fluid transfer
into the softer layer when reaching the interface. We report the propagation
dynamics and show that they are controlled by the toughness contrast between
neighboring layers, which drives fluid flow. We model the coupling between
elastic deformation, material toughness, and volume conservation. After a short
transient regime, scaling arguments capture the dependence of the fracture
geometry on material properties, injection parameters, and time. These results
show that stiffness contrast can accelerate fracture propagation and
demonstrate the importance of macroscopic scale heterogeneities on fracture
dynamics. These results have implications for climate mitigation strategies
involving the storage of heat and carbon dioxide in stratified underground rock
formations.
