Ordered powder mixing

UNTIL recently, the only accepted theory of powder mixing was based on a statistical randomisation of non-cohesive particles 1 . The introduction of the concept of ordered mixing 2 enables a logical theory to be established for the mixing of small cohesive particles to a considerable degree of homogeneity. The concept of ordered mixing has attracted considerable support 3–5 . A basic principle of ordered mixing is that fine particles will adhere, especially to larger particles. The adhesional forces involved may be electrostatic or surface tensional. For a homogeneous system to be formed, the weight of fine particles adhering to unit surface area of the larger particles should be constant. We report here such a condition in the mixing of a fluid energy milled closely sized fraction (2.6 µm) of salicylic acid with a multi-sized distribution of sucrose crystals, in the ratio 1 : 1,000 by weight.