Particle Sorting in Scree Slopes: Characterization and Interpretation From the Micromechanical Perspective

Abstract Particle sorting is one of the most interesting phenomena observed in the formation of scree slopes. We present a study of this phenomenon through physical model tests and numerical simulations at the grain scale, with a focus on the link between the degree of sorting and particle attributes, as well as the underlying mechanisms. By characterizing the degree of sorting with the graphic segregation index, we show that the degree of sorting increases as the difference in particle size and shape between the component materials of the deposited mixture increases. We propose two factors, the relative size factor ( SF r ) and relative shape factor ( OR r ), and show that the degree of sorting increases with the absolute values of these two factors (| SF r | and | OR r |). By monitoring the motions of tracer particles in the numerical simulations, we found that particle migration proceeds in a periodical mode of "collision/friction ⇌ regaining ⇌ collision/friction." Also, we performed energy analyses, revealing that relatively large and rounded particles tend to have a lower rate of kinetic energy dissipation, migrate at a higher velocity over a longer period of time, and have a larger migration distance. In addition, we define a relative kinetic energy dissipation rate | D k_r | and find that it is positively correlated with | SF r | and | OR r |; the discrepancy in particle size and shape between the component materials is responsible for the difference in their kinetic energy dissipation rates, and it is the latter that determines their migration distance difference and sorting features.