Mixing and rheology explained

A number of stirrers, homogenisers and process mixers are available, but which equipment can be used best for a specific formulation development? Stirrers providing lamellar flow or turbulent flow have different geometries. When is high shear needed and when is low shear needed? What role does the homogeniser play? The emulsification is the outcome of two processes: breaking of the oil phase into small drops and the stabilisation of the formed drops. The initial particle size distribution of an emulsion is the result of both steps. The particle size distribution plays a critical role for the emulsion stability, but how can the particle size distribution be determined and which particle size will provide a stable emulsion? The viscosity of the emulsion and the rheological properties are dependent on the particle size distribution. Emulsions can be fluid like milk and viscous like toothpaste. Both emulsion types should be easily applicable to skin. Mixing of complex mixtures seems easier than it really is. However, with just some background information of the effect good mixing equipment can provide, the mixing process becomes a sensible tool rather than a threat and it leads to stable formulations.

 Cosmetic formulations are complex mixtures of ingredients. All kinds, from single phase to multiple phase mixtures, can be found. The physical appearance of single ingredients for cosmetic formulations ranges from solid powders and pellets, via high viscous pastes to high and low viscous liquids. Chemically, the ingredients are water and oil soluble or insoluble in both water and oil, and must be dispersed. Some formulations require heat and some can be mixed in a cold process. The different cosmetic formulations all require a homogeneous mixture at the end of the production process, but the process method to reach a homogeneous mixture can differ greatly. For a small scale laboratory production, a device consisting of at least a power package, a shaft and an impeller to provide agitation and a vessel for the ingredients to be mixed is needed.1 For intensive mixing, a rotor/stator system is standard equipment and functions as a homogeniser. When scale-up production becomes a need at a later stage of a product’s development, process mixers are an interesting choice that are often sourced from the same supplier as the larger production equipment. The conditions inside a process mixer of the same kind but different size are much closer than a laboratory stirrer for a 1 kg batch and a 1,000 kg process mixer.

Stirrer geometries and flow properties1,2,3