328 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS nizers should be turned on at high speeds during this stage. After the cooling water is turned on, the temperature of the batch will drop and the viscosity as well as the yield value of the emulsion will increase greatly. Since any entrapped air will have difficulty in escaping after this period, further mixing should be done with extreme care to avoid permanent aeration. If possible, the thickeners used to increase the viscosity of the emulsion should not be added at the beginning but should be added after emulsification to allow the entrained air bubbles to escape (3). Pumping Considerable air can be entrained sometimes when a cosmetic preparation is pumped from one kettle to another. A common source of aeration in this type of operation is a leakage in some section of the line. This can be due to a faulty gasket, a poor connection, or pinholes in the line. The entire line should be inspected for the source of leak- age. Since air incorporation is most likely to take place in the suction side of the pump, it is generally advisable to connect the pump as closely to the originating tank as possible to reduce the number of connections in the suction side and thus reduce the chance of air entrainment. Jetting and Pouring Operations The jetting operation discussed here is a discharging of a fluid through a nozzle, tube, or hose, into atmosphere or through atmosphere into another fluid. In this sense pouring may be considered as a special kind of jetting. These operations are very common in cosmetic process- ing as in the filling of a hand lotion or liquid shampoo into bottles, transferring of a batch of cream from a kettle to a storage tank through a rubber hose, or pouring of a drum of liquid into a batch in a process tank as illustrated in Fig. 4. These jetting or pouring operations can often be the source of the troublesome air bubble entrainment. In some cases the solution may be fairly simple. For example, the problem of air entrainment in transferring into a storage tank (Fig. 4, B) can be corrected by pumping the material through the inlet located near the tank bottom. A more careful pouring or use of a deflective plate to force the fluid to fall along the kettle wall can also minimize the prob- lem of pouring in Fig. 4, C. However, sometimes the problem is much more complex and requires thorough analysis before an intelligent solu- tion can be prescribed.

GAS BUBBLE FORMATION 329 (A) (B) (c) Figure 4. Air entrainment by jetting and pouring To explain the basic mechanism of air entrainment by jets, Fig. 5 illustrates the discharge of a fluid through a small nozzle into a ves- sel containing the same fluid. The jet in (A) is perfectly smooth and there is no air in the receiving fluid. The jet in (B) is discharged at a much higher rate and the jet surface is irregular and the surrounding air is trapped by the jet surface and carried into the bulk. The jet in (A) is evidently in a laminar flow and (B) is in a turbulent flow. DV• R e - • (A) (B) LAMINAR TURBULENT Figure 5. Laminar and turbulent jets