334 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS are designed so that the jets do not hit the receiving fluid vertically, but rather they are to flow along the wall of the bottle being filled (Fig. 8). Among those illustrated, (B) and (C) are usually the better types from the aeration viewpoint. The type (D) with multiple pinhole orifices can sometimes introduce more air than a vertical nozzle because of the high jetting velocity. (A) (B) (C) (D} Figure 8. Various types of filling nozzles (A) Vertical nozzle (B) J-shaped nozzle (C) Double-orifice nozzle (D) Pinhole nozzle Incorporation of Powdered Material In cosmetic processing, as in the preparation of a liquid eyeliner or fiuid make-up, it is often necessary to add powdered materials such as pigments or zinc stearate into a liquid. During such addition, the powders can carry into the liquid bulk an appreciable amount of air which may remain in the product. This is a very common problem but not always easy to solve. Often a slow and uniform addition or a sprinkling of the powders rather dumping them into the batch will ease the problem. If the batch under- goes viscosity changes during the manufacturing process, the powdered material should be added when the batch is at its lowest viscosity (e.g., while the batch is hot) so that any entrained air bubbles can be freed.

GAS BUBBLE FORMATION 335 If a very large quantity of powdered material must be incorporated in a batch and if the resulting mixture has a very high consistency, the use of a vacuum mixing kettle may be necessary. For example, a tooth- paste is most commonly made in a vacuum mixing kettle or it must be passed through vacuum deaerating equipment afterwards to remove the entrapped air bubbles. A molten lipstick bulk is sometimes processed in an evacuated kettle to free entrapped air bubbles. The use of such a vacuum kettle for a lipstick would be unnecessary if the formula can be revised to lower the yield value of the bulk at its molten state. In some applications, the presence of air is undesirable only because it contains oxygen. In such instances, nitrogen or other inert gases can be used to purge the air from the enclosed system. One possible way of mixing a batch of a fluid without air entrainment is to recirculate the fluid with a pump. An in-line mixer or homogenizer can be added in the line to achieve effective mixing or dispersing. To be effective, however, care must be taken to make sure that the circulation is not localized. Such a system is quite ideal for many applications but the consistency of the batch cannot be too high. Internal Generation of Bubbles All of the mechanisms discussed so far involve entrainment of the external air, i.e., the air which was not originally present in the liquid bulk. However, in addition to these mechanisms, it is also possible to form bubbles through internal generation of gases. Chemical Reaction As in a fermentation process, many gases can be generated through chemical reactions. If a viscous product undergoes a chemical reaction during the manufacturing operation or shelf life resulting in a liberation of a gas, the product can obviously contain gas bubbles. Although there are many chemical reactions which can lead to the liberation of gases, most cosmetic ingredients are relatively inert and this type of generation is relatively uncommon. Physical Change In the absence of a chemical reaction, gas bubbles can sometimes form as a result of a physical change. For example, if the change in en- vironment is such as to produce a decrease in the solubility of the dis- solved air in a product, the excess air can be liberated as air bubbles.