In order that powders may be mixed, the powder particles need to move relative to each other. There are three main mechanisms by which powder mixing occurs: namely, convection, shear and diffusion.

Mechanisms of Powder Mixing

Convective Mixing

Convective mixing arises when there is the transfer of relatively large groups of particles from one part of the powder bed to another, e.g. as might occur when a mixer blade or paddle moves through the mix. This type of mixing contributes mainly to the macroscopic mixing of powder mixtures and tends to produce a large degree of mixing fairly quickly. Mixing does not, however, occur within the group of particles moving together as a unit, and thus in order to achieve a random mix, an extended mixing time is required.

Shear Mixing

Shear mixing occurs when a ‘layer’ of material f lows over another ‘layer’, resulting in the layers moving at different speeds and therefore mixing at the layer interface. This might occur when the removal of a mass by convective mixing creates an unstable shear/slip plane which causes the powder bed to collapse, or in high-shear or tumbling mixers, where the action of the mixer induces velocity gradients within the powder bed and hence ‘shearing’ of one layer over another.

Diffusive Mixing

In order to achieve a true random mix, movement of individual particles is required. This occurs with diffusive mixing. When a powder bed is forced to move or flow, it will ‘dilate’, i.e. the volume occupied by the bed will increase. This arises because the powder particles become less tightly packed and there is an increase in the air spaces or voids between them. Under these circumstances there is the potential for the powder particles to pass through the void spaces created either under gravitational forces (e.g. in a tumbling mixer) or by forced movement (e.g. in a fluidized bed). Mixing of individual particles in this way is referred to as diffusive mixing.

All three mixing mechanisms are likely to occur in a mixing operation. Which mechanism predominates and the extent to which each occurs will depend on the mixer type, mixing process conditions (mixer load, speed, etc.), particle characteristics and flowability of the components of the powder.

Reference:

• Aulton, M. E., & Taylor, K. (2018). Aulton’s Pharmaceutics: The Design and Manufacture of Medicines, 5th ed. Elsevier.