Shear-driven mixing of segregated granular materials

Abstract

As granular materials flow and settle, interactions among particles of different sizes or properties drive mixing and segregation, producing dynamics that shape systems ranging from silos to asteroids. A hallmark of polydisperse granular flows is shear-driven size segregation, in which larger grains tend to rise above smaller ones through particle-scale rearrangements. Despite substantial progress in modeling granular flow and segregation, the complementary process of granular mixing remains less well understood. Here, we investigate the evolution of initially segregated dense granular materials driven out of equilibrium by imposed shear. We ask: what controls the extent and rate of mixing and restratification in a sheared bidisperse granular flow? Addressing this question is essential for understanding how external forcing disrupts or reinforces particle-size organization, and for optimizing processes that require controlled mixing. Using theoretical analysis and numerical simulations, we develop a framework that quantifies the degree of mixing and segregation dynamics of dense bidisperse granular flows. Our results identify the controlling roles of particle-size ratio and the Péclet number, clarify the conditions under which segregated states persist or are transiently homogenized, and provide a basis for improved prediction and control of granular mixtures in natural and industrial settings.

16 pages, 5 figures, Theme Issue RSTA: Sand, silos and asteroids: clustering challenges in granular materials research