Categorization of Brazil nut effect and its reverse under less-convective conditions for microgravity geology

Abstract

Due to its important role in the sorting of particles on microgravity bodies by size, Brazil nut effect (BNE) is a major subject of study for understanding the evolution of planetesimals. Recent studies have revealed that the mechanism for the BNE on microgravity bodies is the percolation of particles or void-filling, rather than granular convection. This study also considers the mechanism for the BNE under 'less-convective' conditions and introduces three categories of behaviour for particles that mainly depend on the dimensionless acceleration of vibration Γ (ratio of maximum acceleration to gravitational acceleration), using a simplified analytical model. The conditions for Γ proposed by the model for each category are verified by both numerical simulations and laboratory experiments. 'Less-convective' conditions are realized by reducing the friction force between particles and the wall. We found three distinct behaviours of the particles when Γ > 1: the (i) 'slow BNE', (ii) 'fast BNE', and (iii) 'fluid motion' (the reverse BNE may be induced), and the thresholds for Γ correspond well with those proposed by the simplemodel. We also applied this categorization to low-gravity environments and found that the categorization scales with gravity level. These results imply that laboratory experiments can provide knowledge of granular mobility on the surface of microgravity bodies.