Brittle or ductile? Wear reveals the nature of hydrogel damage

The more you learn about hydrogels, the weirder they become. They take their shape from water, but act as a brittle solid when you try to break them in your hands. However, they are also quite stretchy. These contradictions are sorted out a bit in Shab’s new paper “Brittle or Ductile? Abrasive Wear of Polyacrylamide Hydrogels Reveals Load-Dependent Wear Mechanisms,” published inĀ Tribology Letters recently. Very light pressure used to wear a polyacrylamide hydrogel with an abrasive probe results in wear that looks very much like a brittle material: debris that is worn off and gone. When the wearing probe is pushed harder, more of the surface interacts with it, and it appears to wear more like a ductile material in which the debris is scraped out, but sticks around near the edges. Now that we know these conditions, we can ask more questions about how fractures initiate from the surface, and how much energy is needed to create them?