How particle shape affects granular segregation in industrial and geophysical flows

Significance

Granular materials like cereal, pharmaceuticals, sand, and concrete commonly organize such that grains segregate according to size rather than uniformly mixing. For example, in a jar of nuts, the largest ones are commonly found at the top. Here, we use computer simulations to explore how grain shape controls this phenomenon in industrial and natural settings. We find that even small differences in shape can substantially change the amount and style of segregation, with different effects depending on whether the system is wet or dry. This study demonstrates the importance of grain shape in different systems ranging from food and medicine production to geophysical hazards and processes such as landslides, river erosion, and debris flows on Earth and other celestial bodies.

Abstract

Industrial and environmental granular flows commonly exhibit a phenomenon known as “granular segregation,” in which grains separate according to physical characteristics (size, shape, density), interfering with industrial applications (cement mixing, medicine, and food production) and fundamentally altering the behavior of geophysical flows (landslides, debris flows, pyroclastic flows, riverbeds). While size-induced segregation has been well studied, the role of grain shape has not. Here we conduct numerical experiments to investigate how grain shape affects granular segregation in dry and wet flows. To isolate the former, we compare dry, bidisperse mixtures of spheres alone with mixtures of spheres and cubes in a rotating drum. Results show that while segregation level generally increases with particle size ratio, the presence of cubes decreases segregation levels compared to cases with only spheres. Further, we find differences in the segregation level depending on which shape makes up each size class, reflecting differences in mobility when smaller grains are cubic or spherical. We find similar dynamics in simulations of a shear-driven coupled fluid-granular flow (e.g., a simulated riverbed), demonstrating that this phenomenon is not unique to rotating drums; however, in contrast to the dry system, we find that the segregation level increases in the presence of cubic grains, and fluid drag effects can qualitatively change segregation trends. Our findings demonstrate competing shape-induced segregation patterns in wet and dry flows that are independent from grain size controls, with implications for many industrial and geophysical processes.