Abstract
Granular packings exhibit significant changes in rheological and structural properties when the rotational symmetry of spherical or circular particles is broken. Here, we report on experiments exploring the differences in dynamics of a grain-scale intruder driven through a packing of either disks or pentagons, where the presence of edges and vertices on grains introduces the possibility of rotational constraints at edge-edge contacts. We observe that the intruder’s stick-slip dynamics are comparable between the disk packing near the frictional jamming fraction and the pentagonal packing at significantly lower packing fractions. We connect this stark contrast in packing fraction with the average speed and rotation fields of grains during slip events, finding that rotation of pentagons is limited and the flow of pentagonal grains is largely confined in front of the intruder, whereas disks rotate more on average and circulate around the intruder to fill the open channel behind it. Our results indicate that grain-scale rotation constraints significantly modify collective motion of grains on mesoscopic scales and correspondingly enhance resistance to penetration of a local intruder.
Highlights
1 Introduction even at low packing fractions relative to a packing composed of disk grains
Simulations and experiments have characterized the dynamics of local intruding 2 Experimental methods rods or grains as a function of parameters including basal friction [2, 3], packing fraction [2,3,4], drive speed [4, 5], 2.1 2D granular system with spring driven intruder drive force [6, 7], and driving spring constant for studies of stick-slip dynamics [1,2,3, 8]. In these works and many studies of granular materials in general, the model granular medium is composed of disks or spheres, idealized shapes rarely found in natural granular materials
Higher φ leads to strong enough jamming that the packing buckles and particles pop out of the monolayer. These are significantly different packing fractions, the intruder exhibits qualitatively similar stick-slip dynamics, characterized by sequences of sticking periods in which the intruder is nominally stationary and slip events in which the intruder slips through the medium, for the two different particle shapes. (For disks, a packing fraction near φ = 0.65 yields intermittent flow, in which the intruder moves through the system with only occasional clogging events [2, 3].)
Summary
The granular medium is illuminated from below by a white light LED panel, with the light first passing through a diffuser and circular polarizer. The angle θ of the intruder is measured 50 times per second, and the intruder’s angular velocity θis computed by a finite difference of the θ(t) time series. Sample time series of intruder velocity and force as measured by the load cells are shown in Fig. 2 as a function of the cumulative drive angle of the stepper motor, which is the cumulative time scaled by ω
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