A poroelastic signature of the dry/wet state of a crack tip propagating steadily in a physical hydrogel

We report detailed measurements of the rate-dependent fracture energy Γ of poroelastic gelatin gels for two distinct boundary conditions on the crack faces. When the crack tip is in contact with a reservoir of solvent, Γ increases linearly with the crack velocity V. When the tip is exposed to air, Γ(V) exhibits a marked departure from linearity at low enough velocities. We show that viscous dissipation associated with poroelastic flow in the tip vicinity plays a minor role in this phenomenon. We interpret the previously overlooked, peculiar behavior of dry cracks as resulting from the rate-dependent hydration state of the gelatin chains in the cohesive zone, where they are pulled-out. At large velocities, the cohesive zone is fully dry and Γ is larger than in the wet tip case. At velocities low enough for solvent to flow from the bulk of the gel, partial rehydration occurs and reduces the difference between the dry and wet fracture energies.