Mechanisms and Control of Gas Bubble Formation in Cosmetics

Tong Joe Lin

332 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS o o- • - __ Figure 6. Laminar bubblc cntrainment Figure 7. Gas film formation

GAS BUBBLE FORMATION 333 served this type of entrainment from a perfectly smooth, viscous jet at a Reynolds number as low as 10. Furthermore, a study on such laminar jets indicated that bubble entrainment could occur only when the linear velocity of the jet exceeded a critical value termed "minimum entrain- ment velocity." By keeping the jet velocity below this value, laminar jet entrainment by the film breakup mechanism can be eliminated. The minimum entrainment velocity is a [unction of the jet diameter as well as the physical properties of the jetting fluid. For Newtonian jets having a uniform velocity profile, the following correlation was found by the author (5): We = 10 Re ø'74 or L • J where We = Weber number Re = Reynolds number D = jet diameter at the point where jet meets the receiving fluid Ve = minimmn entrainment velocity p = density of the liquid 3' = surface tension of the liquid • = viscosity of the liquid By solving the equation for minimum entrainment velocity, the fol- lowing equation is obtained: 6.22 •yo.794 •)0.206 •00.206 •0.587 From this equation, it can be seen that a fluid with low surface tension and high viscosity will have a low minimum entrainment velo- city and will be likely to trap air by this mechanism. A solution to this problem is then to use a nozzle and filling rate such that the linear jet velocity is always below the minimum entrainment velocity. In some instances, a reduction of jet velocity may not be possible or desirable and it may be necessary to solve the problem by redesigning the filling nozzle. Ideally, the filling nozzle should be submerged into the receiving fluid at all times to avoid air entrainment however, this is not always possible in modern filling machines. Some filling nozzles

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GAS BUBBLE FORMATION 331 constant, the linear flow rate, V, will be reduced to one-fourth. There- fore, the Reynolds number for the larger jet will be only one-half of the smaller jet and this might bring the jet to the laminar flow region to avoid turbulent bubble entrainment. The above calculation assumes no change in the fluid viscosity as the jet nozzle is enlarged. This will be true only with a Newtonian fluid, the viscosity of which is independent of the rate of shear. Many cos- metic preparations are non-Newtonian and particularly emulsion prod- ucts are generally pseudoplastic or thixotropic, i.e., shear-thinning. Shear-thinning means that the viscosity decreases with increasing rate of shear. Because of the high linear velocity, the shear stress on the fluid while it flows through the nozzle is much greater in the smaller nozzle than in the larger nozzle. Therefore, if the fluid is shear-thinning, the viscosity of the fluid in the smaller jet may be much smaller than the viscosity of the same fluid flowing through a larger nozzle. Nat- ura]ly, this will make the Reynolds number even greater in the smaller jet and increase the chance of aeration when the filling material is thixotropic. For these reasons, filling nozzles with very small discharg- ing holes should be avoided. In addition to the bubble problem, dis- charging of a thixotropic emulsion through very small openings can sometimes cause a permanent viscosity breakdown. At times, the turbulent bubble entrainment problem can be solved by varying the filling temperature. Many cosmetic creams are filled at an elevated temperature for practical or aesthetic reasons. Since the viscosity of an emulsion is usually low at a high temperature, reduction of filling temperature may increase the fluid viscosity and hence reduce the Reynolds number at which the product is filled. The mechanism of bubble formation by jet discussed above involves turbulent entrainment. However, depending on the physical proper- ties of the fluid, the discharging rate, and the geometry of the nozzle, air can be entrained even when the jet is perfectly smooth and in laminar flow (4). The author investigated the bubble entrainment by such laminar jets using high-speed photography and, as illustrated in Fig. 6, this is due to the formation of a very thin film of gas which breaks away to form air bubbles in the product. Figure 7 is a photograph taken by the author showing formation and breakup of such a cylindrical air film. This type of bubble entrainment by a laminar jet is quite common in the filling of viscous cosmetic preparations. The author has ob-

332 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS o o- • - __ Figure 6. Laminar bubblc cntrainment Figure 7. Gas film formation

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330 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In fluid mechanics, transition from a laminar flow to turbulent flow is related to a dimensionless parameter, Reynolds number defined as follows: where Re = Reynolds number D = diameter of the jet V = linear velocity of lhe jet 0 = density of the fluid /• = viscosity of the fluid When the nozzle is perfectly smooth and sufficiently long the transi- tion from a laminar flow to turbulent flow normally occurs at a Reynolds number above 1000. However, if the nozzle is not smooth, straight, or too short, the jet may be in a turbulent flow at a lower Reynolds num- ber. The turbulent jets can often be the cause of bubble problems in cosmetic processing. For example, if a hot lipstick is tooured from a kettle into a mold through a valve with a short exit pipe, the product may become aerated. This is because, in flowing through the valve and elbow, the velocity distribution in the pipe becomes irregular causing uneven stream sur- face which traps the sun-ounding air as the liquid plunges into the mold. By lengthening the exit with a smooth pipe, it is possible to smooth the jet stream and reduce the chance of air entrapment by turbu- lence. The length o[ the tube required to smooth the flow of a jet is dependent on the jet velocity as well as the physical properties of the fluid. Generally, the lower the viscosity, the longer the pipe length should be to achieve a smooth laminar flow. A similar troublesome bubble entrainment problem can occur in the filling of cosmetic preparations using a straight circular nozzle. The modern filling machines generally operate at a very high speed and the discharged fluid can easily be in turbulent flow depending on the nozzle design and fluid properties. Often the lengthening of the filling nozzle alone will not solve the problem completely. Reduction in the filling rate is a possible solntion but it will affect the production rate. One possible way to minimize the turbulent en- trainment is to use a filling nozzle with a larger dimneter. For example, 1)y doubling the jet diameter, D, while keeping the volnmetric flow rate

GAS BUBBLE FORMATION 331 constant, the linear flow rate, V, will be reduced to one-fourth. There- fore, the Reynolds number for the larger jet will be only one-half of the smaller jet and this might bring the jet to the laminar flow region to avoid turbulent bubble entrainment. The above calculation assumes no change in the fluid viscosity as the jet nozzle is enlarged. This will be true only with a Newtonian fluid, the viscosity of which is independent of the rate of shear. Many cos- metic preparations are non-Newtonian and particularly emulsion prod- ucts are generally pseudoplastic or thixotropic, i.e., shear-thinning. Shear-thinning means that the viscosity decreases with increasing rate of shear. Because of the high linear velocity, the shear stress on the fluid while it flows through the nozzle is much greater in the smaller nozzle than in the larger nozzle. Therefore, if the fluid is shear-thinning, the viscosity of the fluid in the smaller jet may be much smaller than the viscosity of the same fluid flowing through a larger nozzle. Nat- ura]ly, this will make the Reynolds number even greater in the smaller jet and increase the chance of aeration when the filling material is thixotropic. For these reasons, filling nozzles with very small discharg- ing holes should be avoided. In addition to the bubble problem, dis- charging of a thixotropic emulsion through very small openings can sometimes cause a permanent viscosity breakdown. At times, the turbulent bubble entrainment problem can be solved by varying the filling temperature. Many cosmetic creams are filled at an elevated temperature for practical or aesthetic reasons. Since the viscosity of an emulsion is usually low at a high temperature, reduction of filling temperature may increase the fluid viscosity and hence reduce the Reynolds number at which the product is filled. The mechanism of bubble formation by jet discussed above involves turbulent entrainment. However, depending on the physical proper- ties of the fluid, the discharging rate, and the geometry of the nozzle, air can be entrained even when the jet is perfectly smooth and in laminar flow (4). The author investigated the bubble entrainment by such laminar jets using high-speed photography and, as illustrated in Fig. 6, this is due to the formation of a very thin film of gas which breaks away to form air bubbles in the product. Figure 7 is a photograph taken by the author showing formation and breakup of such a cylindrical air film. This type of bubble entrainment by a laminar jet is quite common in the filling of viscous cosmetic preparations. The author has ob-

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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.

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