Abstract
The dispersion of solids in liquids at high concentration, the suspension of solids in viscous liquids and the preparation of emulsion with a high internal phase ratio are typical problems involving a significant change of viscosities over the course of the process accompanied by the development of strong non-Newtonian properties. Industrial examples include the preparation of paints and coatings, the manufacturing of food products, and suspension polymerization to name a few. Classical mixing technologies to handle these problems range from high solidity open turbines to close-clearance impellers. The main drawback is that they are not necessarily efficient over the whole range of viscosity and/or rheology encountered in the process, especially when working in the laminar regime. For instance, open impellers perform well at low viscosity but generate caverns and segregation at higher viscosities especially in the case of shear-thinning fluids. On the other hand, close-clearance impellers are very poor mixers at low viscosity but outperform all other technologies (except perhaps static mixers) at higher viscosities. In industry, several alternatives are available that are capable of dealing with physically evolving media undergoing for instance a significant change of viscosity, namely planetary mixers and coaxial mixers. Another possibility is the use of single or dual eccentric impellers operated at steady state or in a dynamic fashion. These mixing technologies are most often empirically designed, and there is a very limited scientific knowledge available to optimise their operation.
