Why do fine particles tend to collect near the center of a pile during mixing?

In the context of sample preparation for chemical analysis, the tendency for fine particles to collect near the center of a pile during mixing is a phenomenon related to the physical properties of the material, specifically particle size, shape, and density.1

When a material consists of a broad range of particle sizes and shapes, these physical characteristics influence how the particles behave during handling. While fine particles often migrate toward the center of a pile, rounded particles tend to collect at the pile’s edge. This segregation is a challenge that must be circumvented to ensure that the laboratory sample remains representative during the mixing process. Effective mixing operations must account for these tendencies, as well as external influences such as air turbulence or electrostatic and magnetic fields, to prevent bias in the final test sample.1