Tearing a hydrogel of complex rheology

Tough hydrogels of many chemical compositions are being discovered, and are opening new applications in medicine and engineering. To aid this rapid and worldwide development, it is urgent to study these hydrogels at the interface between mechanics and chemistry. A tough hydrogel often deforms inelastically over a large volume of the sample used in a fracture experiment. The rheology of the hydrogel depends on chemistry, and is usually complex, which complicates the crack behavior. This paper studies a hydrogel that has two interpenetrating networks: a polyacrylamide network of covalent crosslinks, and an alginate network of ionic (calcium) crosslinks. When the hydrogel is stretched, the polyacrylamide network remains intact, but the alginate network partially unzips. We tear a thin layer of the hydrogel at speed v and measure the energy release rate G. The v–G curve depends on the thickness of the hydrogel for thin hydrogels, and is independent of the thickness of the hydrogel for thick hydrogels. The energy release rate approaches a threshold, below which the tear speed vanishes. The threshold depends on the concentration of calcium. The threshold may also depend on the thickness when the thickness is comparable to the size of inelastic zone. The threshold determined by slow tear differs from the threshold determined by cyclic fatigue. We discuss these experimental findings in terms of the mechanics of tear and the chemistry of the hydrogel.