334 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 2.--Unbaffled flow pattern, turbine type mixer. over-all fluid flow characteristics. A baffled tank has the following charac- teristics: 1. Top-to-bottom turnover with complete intermixing of all fluid streams throughout the entire vessel. 2. The absence of a vortex through which air is drawn down into the fluid. 3. The absence of a swirling flow pattern as contrasted to top-to- bottom turnover. In most mixing cases, a baffled flow pattern is desirable. There are times, however, when it is desirable to draw down powders from the surface or introduce gas from the surface, then an unbaffled flow pattern is useful.

FLUID MIXING OF COSMETIC FORMULATIONS ...... .. 335 Figure 3.--Baffled flow pattern, turbine type mixer. Unbaffled flow patterns are often unstable and cause severe fluid forces which act on the impeller and the shaft and are erratic in scale-up per- formance. Unbaffled flow patterns (Fig. 2) are used only when process- ing conditions require and special considerations must often be taken for these applications. A baffled flow pattern may be produced in several different ways. In a vertical cylindrical tank the most common way is the use of four baffles, each one-twelfth the tank diameter in width, placed equally around the tank (Fig. 3). This baffle width may often be modified when dealing with non-Newtonian fluids. With top-entering propeller type mixers, up to about three horsepower capacity, the mixer can be placed in an angular