Jiho discovered this model during his modeling of hydrogel slip interfaces as thin-film complex fluid shear. In complex fluid rheology, distinct components like a solid and fluid affect the flow character of the entire volume — and their history and state also drive the flow behavior. So this paper connects those two ideas, and proposes the specific form of the flow stresses to accurately predict the friction. This adds a dimension to the rate-and-state model, allowing it to capture both shear-weakening and shear-strengthening behaviors in the same model rather than two separate models.
So do we know exactly which mechanism is changing with time in a hydrated gel interface? No, but we have some ideas that water is moving around in the interface, and into the hydrated gel itself, as the aluminum ring slides over it. Read our paper in Extreme Mechanics Letters to learn all the details. Free for 50 days starting 2020-10-07: