340 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (a) Standard flat blade turbine. (b) Paddle. Figure 8.--Radial flow impellers. At constant power input to the mixing tank, [.•fH] p o: [D/T]8/3 (3) Thus, a large impeller running at a slow speed gives a large level of flow to a low level of fluid shear, while a small impeller at high speed gives a high level of fluid shear to a low level of fluid flow.

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