FLUID MIXING OF COSMETIC FORMULATIONS 341 . ::'• . :.:.. . .. -:- •:•.•: ..• -= •. • .. . . . '.•q - .:•:.: -? -• ........ ....... : .... . -. • ..• • ... Figure 9.•Streak photo make by passing plane of light through tank and photographing flow of minute suspended particles. Fluid is water. FourAn. diameter •rbine operating in 12-in. diameter tank. It should be pointed out here that the absolute value of the •/H ratio is not so important as relative values, which are related to the D/T ratio. Thus, most mixing processes are carried out by considering the power level required in the systen•, and the ratio of impeller size to tank size re- quired for a particular impeller. Additional information on mixing theory is given in the article by Rushton and Oldshue (5). Process Requirements Every mixing process has its own requirement for level of power, flow to fluid shear ratio and direction of fluid flow. In many mixing processes, the direction of fluid flow is not critical and the important mixing criterion is the ratio of flow to fluid shear. In these processes research and field experience indicate that once the fluid flow to fluid shear is satisfied, all types of impellers, regardless of shape or design, give approximately the same process result. It does mean that different designs of impellers will have different optimum D/T ratios to produce the required fluid flow to fluid shear ratio. If the direction of flow is important, such as solid suspension, differences

34:2 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS will be observed in axial and radial flow impellers and depending upon the particnlar requirements of the process, selection of various other impeller shapes may be made. The key is to know what fluid regime is required in the process, and select the proper impeller type and speed and power level to satisfy that requirement. LIQUID-LIquID CONTACTING There are two types of emulsions encountered in mixing processes. The first type is an unstable emulsion, such as is desired when contacting one phase with another to carry out an extraction process. This may be for neutralization, purification or extraction. After extraction, the emul- sion is allowed to separate. In these cases, it is normally desired to put in a power level high enough to accomplish the process result, but not too high to produce unduly long settling times. This imposes a rigorous design requirement. The settling qualities of a given emulsion are usually not predictable except by labora- tory tests. The other type of emulsion is a permanent eminlsion in which it is desired to disperse one phase in the other, and to maintain a stable emulsion. In this case, mixing is a key factor, but of even more importance many times, is the formulation of the material itself. If we place two liquids in contact with each other, there will be no tendency for a dispersion to be produced. If we place a mixer in this tank, and produce a dispersion, the system always has a tendency to revert to the lowest interfacial area, which means returning to a separated condi- tion. If the mixer has produced a small particle size, and a greater inter- facial area, and the properties of the formulation are such that there is no marked tendency to separate, then we have what is termed a stable emul- sion. The term stable is a relative one, and depends upon the period of observation of the emulsion. If we desire a given emulsion, there are two requisites. One is that the system be inherently stable so that when the desired particle size is pro- duced, there is no tendency to return to separated state. Secondly, it is essential to have a mixing device which will produce the desired particle size and interfacial area. If the formulation is incorrect, then no amount of dispersion will produce a stable emulsion, since the dispersion will always tend to return to a stable condition. If the formulation is correct, then it is necessary to determine what degree of fluid shear is required in the system to produce the required degree of emulsion. We can classify mixers into their relative requirements for flow and fluid shear. Turbines and propellers are capable of providing high pumping capacity in relatively low fluid shear. However, by making