Signature of Transition between Granular Solid and Fluid: Rate-Dependent Stick Slips in Steady Shearing

Dynamical force measurements for contacting soft surfaces upon steady sliding: Fixed-depth tribology

The tribology between surfaces can have a profound impact on the response of a mechanical system, such as how granular particles are driven to flow. In this work, we perform experiments that time resolve the tangential and normal components of the force between two semicylindrical polydimethylsiloxane samples immersed in fluid, as they slide against each other in a range of controlled speeds. The time-averaged friction force shows a nonmonotonic dependence on the sliding speed over four decades, which is consistent with the paradigmatic Stribeck diagram and three dynamical regimes associated with it. Our specially designed fixed-depth setup allows us to study the fluctuation of force that exhibits strong stick-slip patterns in one of the regimes. Data from repetitive experiments reveal that both the "onset speed" for the stick-slip patterns and its spatial location along the sample change gradually during the course of our experiments, indicating changes on the sample surfaces. In addition, we conduct counterpart experiments by using spherical samples rubbing against each other, to make a direct connection of the interparticle tribology to the granular flow reported in our previous work [J.-C. Tsai et al., Phys. Rev. Lett. 126, 128001 (2021)10.1103/PhysRevLett.126.128001].

Effects of polydispersity on the plastic behaviors of dense two-dimensional granular systems under shear

We study particle-scale motion in sheared highly polydisperse amorphous materials, in which the largest particles are as much as ten times the size of the smallest. We find strikingly different behavior from the more commonly studied amorphous systems with low polydispersity. In particular, an analysis of the nonaffine motion of particles reveals qualitative differences between large and small particles: The smaller particles have dramatically more nonaffine motion, which is induced by the presence of the large particles. We characterize how the nonaffine motion changes from the low- to high-polydispersity regimes. We further demonstrate a quantitative way to distinguish between "large" and "small" particles in systems with broad distributions of particle sizes. A macroscopic consequence of the nonaffine motion is a decrease in the energy dissipation rate for highly polydisperse samples, which is due both to a geometric consequence of the changing jamming conditions for higher polydispersity and to the changing character of nonaffine motion.