Dispersion Theory

Dispersion is a process of mixing by the breaking apart of solid particles into a bulk liquid using a high speed, rotating saw-tooth blade, or other special impeller design, and subsequent mixing thereof. The blade or impeller produces high shear forces that break apart the particles.

In any dispersion process it is important to maximise cycling and minimise turbulence in order to achieve rapid and homogeneous dispersion.

In dispersion processing, the major part of the effective work is developed in a region of a few centimetres distance from the impeller edge, where the major part of the applied energy is utilised. The balance of energy serves to force streaks of the treated material towards the walls of the tank to produce an upward flow near the wall. When the rising streaks of treated matter reach the upper level of the batch, they flow inwards and towards the rotating shaft, then travel downwards to the impeller and are thus subjected to a succession of treatments in the acceleration region of the vanes.
The phenomenon of material movement in a circulatory mode from the walls of the tank, towards the impeller shaft, is termed CYCLING.

When, however, the circulation effect produced by the rotation of the impeller – particularly in very fluid materials – dominates, then the entire batch rotates and this action is termed TURBULENCE.

In fact, both are inseparably linked – it is essential that the material rising from the batch to the surface should never effect a complete circle of the tank. The material should always reach the shaft before it is completely rotated. If this occurs, then we say there is CYCLING, but if it does not occur, then there is TURBULENCE.

Good cycling will result in high efficiency dispersion, whereas turbulence will result in very poor efficiency and the desired outcome may not even be achieved.

The selection and function of impellers is of paramount importance in achieving the critical level of cycling referred to above. We refer the reader to our analysis of the highly efficient Varishear® Mixer elsewhere on this site under the section titled “Varishear Super Productivity”, where the selection, design and adaptation of impellers to various fluids and the achievement of efficient dispersion is discussed in some depth.

These guidelines and notes are presented as general guides only and no warranty is implied or provided.

® Varishear is a registered trademark of Jones Industrial Mixers.