324 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS or fluid make-up can sometimes encourage microbial growth or undesir- able oxidation. The pinholes in a lipstick or make-up stick can create poor appearance and weaken the stick. Excessive incorporation of air in clear gel-type products can be unsightly or create an under-fill ap- pearance upon standing on the shelf. Air bubbles in cosmetics are usually introduced during the manu- facturing processes. If the viscosity of the product is low, the incor- porated air can escape into the atmosphere in a short time and the product will become bubble-free. If, on the other hand, the product is very viscous, the rate of escape may be very low. From the theological viewpoint, if the product has an appreciable yield value, the entrained bubbles may not escape at all and the product can remain aerated during the entire shelf life. Control of gas bubbles is thus particularly im- portant in plastic or thixotropic products. However, since there are many ways by which gas bubbles can be incorporated during numerous manufacturing steps, control of gas bubbles in finished products is not always easy. The purpose of this paper is to examine in detail the common sources and mechanisms of bubble incorporation into cosmetic prepara- tions during various manufacturing processes, and to suggest preventive methods. DISCUSSION External Entrainment The mechanisms of bubble formation often encountered in cosmetic preparations may be divided into two main categories: bubble incor- poration due to the entrainment of the external gas and bnbble forma.- tion due to the internal generation of gases. There are many variations but four important operations which are frequently the sources of ex- ternal entrainment of air will be discussed. Mixing Operations Broadly speaking, mixing covers agitation, blending, homogenizing, and milling it is probably the most common source of air incorporation into products such as facial creams, hand lotions, and liquid make-ups. Propeller or turbine mixers are widely used to process flowable liquids or emulsions. Depending on the mixer speed, impeller size, and loca- tion, a vortex such as the one illustrated in Fig. 1 can form. If the vortex

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