Intruder friction effects on granular impact dynamics
© 2018 American Physical Society. There is considerable recent interest in intruders impacting into granular materials. Many studies focus on a collisional model where the drag force acting on an intruder varies as the square of the intruder speed. However, it is unclear how intruder friction affects granular impact dynamics. Here, we experimentally study impacts into quasi-two-dimensional beds of photoelastic granular beds of three circular intruders of similar size and mass, but with varying friction coefficients associated with the intruder edges (smooth, "sandy," and gear). We compare typical measures of the dynamics for the three intruders, including impact depth and speed vs time. We show that the smooth and sandy intruders share similar impact dynamics, while the gear intruder displays smaller impact depth, speed, and impact time. We attribute the differences between the gear intruder's dynamics and those of the other two to differences in the collision-generated force networks associated with the grain-scale roughness of the gear intruder. For the smooth and sandy intruders, the force networks align close to the normal direction of the intruder boundaries. For the gear intruder, the grain-scale geometric roughness leads to force chains that are closer to vertical, rather than in the coarse-grained normal direction to the intruder edge. This leads to a stronger drag force for the gear intruder. Hence, in the range that we have explored, the granular impact dynamics are highly sensitive to grain-scale roughness of the intruder and relatively insensitive to microscale roughness that is associated with the conventional friction coefficient.
Zheng, H; Wang, D; Chen, DZ; Wang, M; Behringer, RP
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