Why can very finely ground samples be difficult to mix properly?

In analytical chemistry, achieving a homogeneous mixture is a critical step in sample preparation. While reducing particle size is often necessary to reach the liberation size of an analyte, very finely ground material can become difficult to mix adequately for several reasons.1

The effectiveness of a mixing operation is influenced by the physical characteristics of the particles, including their size, shape, and density, as well as external factors such as air turbulence and electrostatic or magnetic fields. When a sample contains a broad range of particle sizes and shapes, fine particles tend to segregate by collecting in the center of a pile, while rounded particles migrate toward the edges. Furthermore, excessive grinding can lead to the absorption of atmospheric moisture or other gases, which may further complicate the ability to achieve a uniform distribution of the material.1