GAS BUBBLE FORMATION 325 Figure 1. Aeration from vortex formation is deep enough to touch the impeller, the surrounding air may be sucked in to form bubbles. Except when it is desired to deliberately aerate the fluid or to draw the floating material from the fluid surface (e.g., to wet the dry pigments placed on the fluid surface), the formation of vortex is generally unde- sirable as it will reduce the mixing efficiency. A reduction in the mixer rpm may eliminate the vortex but it will also reduce the intensity of mixing and shear force required to obtain a fine emulsion. In a jacketed kettle, it may also reduce the rate of heat transfer and result in poor cooling or heating of the batch. One way to suppress the vortex and, at the same time, increase the mixing efficiency is to use baffles (2). In a cylindrical tank, for example, four vertical baffles, each one-tenth tank diameter in width, placed equally around the tank will serve such a purpose. The baffles reduce the tangential velocity component but increase the radial and vertical flow. The effect is that the fluid does not merely rotate around the tank axis

326 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 2. Elimination of vortex by using spatula but is mixed uniformly. The photograph in Fig. 2 was taken at the same mixer rpm as Fig. 1 but a spatula was placed on one side of the beaker to serve as a baffle. The absence of vortex and bubbles clearly indicates the effect of the baffle. Another effective way to suppress a vortex when using a propeller mixer is to tilt the mixer and place it off-center. This will create a dif- ference in the velocity with which the fluid impinges on the tank wall and a vortex formation is thus prevented without using baffles. Some high-speed mixers are provided with built-in baffles to mini- mize the chance of surface turbulence and aeration. However, it is gen- erally better to place the head ooe a high-speed mixer as near the bottom ooe the process kettle as possible to avoid air entrapment. In the cosmetic industry, an anchor-type mixer is often used to pro- cess high-consistency creams. Although such a mixer is normally op- erated at a relatively low rpm, air entrapment is still possible if the mixer tips are not completely covered by the product. As illustrated in Fig. 3,