We describe the resistance to slip of an interface as friction, and the origin of that friction can vary widely: roughness, plasticity, lubrication, temperature, etc. Some of those originating mechanisms can also change with time: asperities flattening, lubricating migration, or other things. The way that people have tried to quantify those changing mechanisms is to identify a state variable, or a quantitative descriptor, that contributes to changing friction with time. Friction also has a driving speed, or rate, which is the macroscale relative speed between the moving bodies. The interaction with the changing state and the input rate gave rise to the “rate-and-state friction model.” It shows how the history and state can be used to predict friction.

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:

https://doi.org/10.1016/j.eml.2020.101013