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Ind. Eng. Chem. Res. 1998, 37, 2052-2058
Rheology of Liquid Membranes Rajinder Pal† Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
The rheology of a primary water-in-oil (W/O) emulsion and a multiple water-in-oil-in-water (W/ O/W) emulsion (liquid membrane) was investigated using a controlled-stress rheometer. The dispersed-phase (water) concentration of the primary W/O emulsion was 68% by volume. The primary W/O emulsion behaves like a highly shear-thinning fluid. Upon aging, there occurs a dramatic decrease in the zero-shear viscosity and storage modulus of the primary W/O emulsion over the first few hours of storage. The multiple W/O/W emulsion was prepared with a primary W/O emulsion concentration of 73 vol %. The fresh multiple W/O/W emulsion behaves like a shear-thinning non-Newtonian fluid. Upon aging, the rheological parameters (viscosity, storage, and loss moduli) of the multiple W/O/W emulsion initially increase with an increase in storage time and then decrease with a further increase in storage time. These observations are explained in terms of droplet size changes that occur during aging. The multiple W/O/W emulsion also exhibits shear-thickening (dilatancy) under certain conditions. Introduction In the literature, the terms “liquid membrane (LM)” and “multiple emulsion (ME)” are used interchangeably (Davis, 1981; Garti and Aserin, 1996). Multiple emulsions or liquid membranes are complex two-phase oil/ water systems having a variety of applications, especially in wastewater treatment and pharmaceutical sciences. There may exist at least two types of multiple emulsions, namely, water-in-oil-in-water (abbreviated W/O/W) and oil-in-water-in-oil (abbreviated O/W/O) multiple emulsions. In the case of a W/O/W multiple emulsion, the oil droplets contain fine water droplets, and the oil droplets themselves are dispersed in a continuous water phase. The O/W/O multiple emulsion, on the other hand, consists of tiny oil droplets entrapped within large water droplets, which, in turn, are dispersed in a continuous oil phase. Multiple emulsions are referred to as liquid membranes because the liquid film which separates the internal liquid phase from the external liquid phase acts as a thin semipermeable membrane between internal and external phases. Mass transfer of species can take place between internal and external phases by diffusion through this liquid film or membrane. Multiple emulsions are generally prepared using a two-step procedure. In the first step, the primary emulsion is prepared. For the preparation of a W/O/W multiple emulsion, the primary emulsion is an ordinary water-in-oil (W/O) emulsion which is prepared using water and a low-HLB (hydrophilic-lipophilic balance) surfactant solution in oil. The primary emulsion in the case of an O/W/O multiple emulsion is an ordinary oilin-water (O/W) emulsion which is prepared using an oil and an aqueous solution of a high-HLB surfactant. In the second step, the primary emulsion (W/O or O/W) is reemulsified in either an aqueous solution of a highHLB surfactant to produce a W/O/W multiple emulsion or an oil containing a low-HLB surfactant to produce an O/W/O multiple emulsion. The second emulsification step is carried out in a low shear device so as to avoid † Phone: (519) 885-1211, ext. 2985. Fax: (519) 746-4979. E-mail:
[email protected].
the rupturing of the multiple droplets. However, some of the internal phase of the multiple droplets is unavoidably lost to the external phase during the emulsification process (Florence and Whitehill, 1982). The key factors affecting the formation of the multiple emulsion are the chemical nature of various components, the concentration of the surfactants used in both steps of emulsification, the volume fraction of the primary emulsion in the whole multiple emulsion, and the mixing conditions (Matsumoto et al., 1976; Florence and Whitehill, 1982). Multiple emulsions/liquid membranes have a number of useful applications in the fields of wastewater treatment and pharmacology. For example, one can easily remove phenolic or other toxic materials from wastewater by formulating a suitable W/O/W multiple emulsion that contains sodium hydroxide in the inner aqueous phase (Davis, 1981). The phenol is extracted from the external aqueous phase to the internal aqueous phase by diffusion through the oil film. In the internal aqueous phase, the phenol is converted to the phenolate ion which is unable to diffuse out of the internal droplet. Multiple emulsions have also been used for the removal of toxins from body fluids (Frankenfeld et al., 1976). According to Frankenfeld et al. (1976), multiple emulsions could also be used for the emergency treatment of drug overdose. To show the feasibility of their proposal, Frenkenfeld et al. (1976) carried out in vitro studies to remove acidic drugs (such as barbiturates and salicylates) from the aqueous phase. An appropriate W/O/W multiple emulsion system was formulated. The drug was contained in the external aqueous phase. The un-ionized drug permeated from the aqueous donor phase (external phase) into the inner aqueous phase. The internal phase contained sodium hydroxide to convert the un-ionized drug to an oil-insoluble anion. The studies carried out by Frankenfeld et al. (1976) clearly demonstrate that liquid membranes are capable of rapid uptake of drugs from various donor systems. The applications of multiple emulsions are not limited to wastewater treatment and pharmacology. Multiple emulsions have also been formulated as foods (Dickinson and Stainsby, 1988; Owusu et al., 1992; Dickinson
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et al., 1991, 1994), cosmetics (Florence and Whitehill, 1982), and wax polish (Mackles, 1968). The understanding of the rheological behavior of multiple emulsions is important in the formulation, handling, mixing, processing, storage, and pipeline transportation of such systems. Furthermore, rheological studies can provide useful information on the stability and internal microstructure of multiple emulsions. While there is a substantial amount of literature published on the rheology of simple emulsions (Sherman, 1983; Pal and Rhodes, 1989; Pal, 1996, 1997), little attention has been given to multiple emulsions. One factor that complicates the rheological behavior of multiple emulsions is the osmotic pressure gradient between the internal and external phases (Matsumoto and Kohda, 1980). For example, in the case of W/O/W multiple emulsions, the osmotic pressure gradient between the internal and external aqueous phases may result in a time-dependent change in the volume of the internal aqueous phase because of the migration of water through the oil film. A change in the volume of the internal aqueous phase implies a change in the total dispersed-phase concentration of the multiple emulsion. This, of course, results in a change in the rheological properties as the rheological properties are strongly dependent upon the total dispersed-phase concentration of the emulsion. Matsumoto and Kohda (1980) investigated the changes in viscosity on aging for a series of dilute W/O/W multiple emulsions. The volume fraction of the primary water-in-oil (W/O) emulsion in the freshly prepared W/O/W emulsions was either 0.05 or 0.10. As the emulsions were dilute, they exhibited Newtonian behavior. The change in the viscosity on aging was found to be strongly influenced by the composition of the internal aqueous phase. When glucose or sodium chloride was present in the internal aqueous phase, the viscosity of the multiple W/O/W emulsion increased initially upon aging and then decreased with a further increase in aging time. The initial increase in viscosity upon aging was due to swelling of emulsion droplets, caused by the migration of water from the external aqueous phase to the internal aqueous phase through the oil film. The swelling of the emulsion droplets ultimately led to the rupturing of the oil film between the internal and external aqueous phases. Consequently, the viscosity decreased with increasing aging time. When glucose was introduced into the external aqueous phase, instead of the internal aqueous phase, it was impossible to prepare stable W/O/W multiple emulsions, especially at high glucose concentrations; the droplets of the multiple emulsion disappeared rather rapidly due to fast diffusion of water from the internal aqueous phase to the external aqueous phase. In this paper, we report new results on the rheology of concentrated multiple W/O/W emulsions. Both steady and oscillatory behaviors of multiple emulsions are investigated. The effects of aging on the rheology of primary W/O emulsion and multiple W/O/W emulsion are determined. Experimental Work Materials. The emulsions were prepared using a light mineral oil supplied by Drug Trading Co. Ltd. The viscosity and density of oil at 22 °C were 30 mPa‚s and 0.85 g/mL, respectively. The water used throughout the experiments was deionized. The surfactant used for the
preparation of primary W/O emulsion was Emsorb 2500, a commercially available surfactant manufactured by Henkel Corp., Cincinnati, OH. The chemical name of Emsorb 2500 is sorbitan monooleate. It is oil-soluble and has a low HLB value of 4.6. To disperse the primary W/O emulsion into the aqueous phase and hence form the multiple W/O/W emulsion, a high-HLB surfactant was incorporated into the aqueous phase. The surfactant used was Triton X-100, a commercially available nonionic surfactant manufactured by Union Carbide Chemicals & Plastics Technology Corp., Danbury, CT. Triton X-100 is an octylphenol ethoxylate with an average of 9-10 molecules of ethylene oxide. It is water-soluble and has a high HLB value of 13.5. Procedure. The primary W/O emulsion was first prepared by shearing together the known amounts of the surfactant/oil solution (30.3% by weight of Emsorb 2500; the viscosity of this surfactant solution was 66.3 mPa‚s at 22 °C) and deionized water in a variable-speed homogenizer (Gifford-Wood model 1-L). The mixture was sheared for about 10 min at a high speed. The primary W/O emulsion was then re-emulsified in an aqueous surfactant solution (1.06% by weight of Triton X-100) so as to prepare the W/O/W multiple emulsion. To avoid rupture of the multiple emulsion (hence, release of the inner water droplets), the second emulsification process was carried out at a slow homogenizer speed for about 5 min. The dispersed-phase concentration of the primary W/O emulsion (that is, water concentration) was kept as 68 vol %. The volume fraction of the primary W/O emulsion in the whole multiple emulsion was kept as 0.73. All the rheological measurements were carried out at 22 °C in a Bohlin controlled-stress rheometer (Bohlin CS-50) using a cone-and-plate measuring system. The cone diameter was 40 mm, and the plate diameter was 60 mm. The cone angle was 4°, and the gap at the cone tip was 150 µm. The steady-shear data, that is, viscosity versus shearstress data, were collected in the direction of increasing shear stress over a wide range of shear stress. The oscillatory measurements, that is, storage and loss moduli measurements, were carried out over a frequency range of 0.01-10 Hz at a low shear stress of 0.1 Pa. The information regarding the droplets (size and structure) was obtained by taking photomicrographs. The photomicrographs were taken with a Zeiss optical microscope equipped with a camera. The emulsion samples were diluted (generally with the same continuous phase) before taking the photomicrographs. Results and Discussion Rheological Behavior of the Primary W/O Emulsion. Figure 1 shows the photomicrographs of the primary W/O emulsion. The water droplets are quite small, ranging in diameter from about 2 to 3 µm. The viscosity versus shear-stress data for the nearly-fresh primary W/O emulsion are shown in Figure 2. The emulsion is non-Newtonian in that the viscosity decreases with an increase in shear stress. It is interesting to note that the fresh primary W/O emulsion exhibits a slip effect at intermediate values of shear stress. A sudden break in the flow curve at intermediate values of shear stress, where an unexpected Newtonian plateau in the viscosity is seen, is caused by slip flow (Barnes, 1995). Such a slip flow is well-known and
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Figure 3. Effect of aging on the rheology of the primary waterin-oil (W/O) emulsion.
Figure 1. Photomicrographs of the primary water-in-oil (W/O) emulsion.
Figure 2. Viscosity versus shear stress for the nearly-fresh primary water-in-oil (W/O) emulsion.
has been observed by many researchers in the area of suspension and emulsion rheology (Dzuy and Boger, 1983; Pal, 1990; Buscall et al., 1993; Gregory and Mayers, 1993; Barnes, 1995). The effect of aging on the rheology of the primary W/O emulsion is shown in Figure 3. There occurs a dramatic decrease in zero-shear viscosity over the first few hours of storage. The high-shear viscosity decreases only slightly with aging time. Figure 4 shows the effect of aging on the storage modulus of the primary W/O emulsion. The storage modulus, like the viscosity, decreases rapidly over the first 24 h of storage and then tends to level off. The rapid decrease in zero-shear viscosity and storage modulus of the primary W/O emulsion with aging could be due to two possible mechanisms: (1) The first possible mechanism is the coarsening of water droplets. It is well-known that an increase in the average droplet size of a concentrated emulsion may cause a dramatic
Figure 4. Effect of aging on the storage modulus versus frequency data for the primary water-in-oil (W/O) emulsion.
decrease in the rheological parameters, such as zeroshear viscosity and storage modulus (Pal, 1996). Recently, Aronson and Petko (1993) also observed that the yield stress of a concentrated water-in-oil emulsion drops significantly over the first 24 h of storage because of the coarsening of droplets. The coarsening of water droplets in the W/O emulsion occurs due to either Ostwald ripening (whereby water diffuses from small to large droplets) or coalescence of droplets caused by film rupture. Aronson and Petko (1993) further observed that the presence of electrolytes in the aqueous phase reduces the coarsening of droplets dramatically; it appears that the electrolytes enhance the stability of the W/O emulsion by increasing the resistance of the water droplets to coalescence. (2) The second possible mechanism leading to the rapid decrease in zero-shear viscosity and storage modulus of the emulsion with aging is the relaxation of the flocculated microstructure of water droplets with aging time. Note that a high degree of shear thinning (see Figure 2) and a nearly flat storage modulus (see Figure 4) for the fresh W/O emulsion clearly suggest that this emulsion is highly flocculated. It seems that the dispersed water droplets flocculate to form a three-dimensional network structure. However, it is not clear which of the two mechanisms just discussed is dominant in the present W/O emulsion. No attempt was made to monitor the droplet size of the primary W/O emulsion with aging time. Rheological Behavior of the Multiple W/O/W Emulsion. The rheology of the multiple W/O/W emulsion at φprimary ) 0.73 was monitored as a function of the storage time. Figure 5 shows the flow curves (viscosity versus shear-stress plots) for the multiple emulsion as a function of time. To avoid overcrowding
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Figure 5. Viscosity versus shear stress for the multiple waterin-oil-in-water (W/O/W) emulsion, as a function of storage time.
Figure 7. Effect of aging on the storage modulus versus frequency data for the multiple W/O/W emulsion. The storage modulus data for the highly aged multiple emulsion (age ) 19.13 days) are compared with the fresh multiple emulsion separately in part b. Figure 6. Shear rate versus shear stress for the aged multiple W/O/W emulsion (age ) 6.1 h).
of the data, not all the data collected after different storage times are shown in the figure. For example, the rheological data collected after storage times of 2.5, 6.7, 52.5, and 103 h are not shown. The fresh multiple W/O/W emulsion behaves like a non-Newtonian shear-thinning fluid; that is, viscosity decreases with an increase in shear stress (or shear rate). Upon aging, the flow curve of the multiple emulsion initially shifts upward in the direction of higher viscosities. However, with a further increase in the storage time, the flow curve shifts in the downward direction, indicating a decrease in viscosity. In other words, the viscosity first increases, goes through a maximum, and then decreases with a further increase in storage time. An interesting point to note is that the multiple emulsion after storage times of 2.5, 3.4, 6.1, and 6.7 h exhibits shear thickening (dilatancy) at high values of shear stress; that is, the viscosity of the emulsion shows a sharp increase with an increase in shear stress. The critical shear stress at which onset of shear thickening occurs is approximately 45 Pa. The shear-thickening effect is absent in the case of the aged multiple emulsion with storage times of 20.1 h and greater. It appears that the fresh emulsion also does not exhibit the shear-thickening effect. However, it is difficult to draw a definite conclusion as the data for the fresh emulsion were collected only up to a shear stress of 60 Pa. It is important to point out that the shear-thickening effect observed in rotary viscometers can sometimes be due to secondary flows and turbulence at high shear rates. However, this is not the case here. For example, Figure 6 shows the plot of shear rate versus shear stress for the aged multiple emulsion (age ) 6.1 h) that
Figure 8. Effect of aging on the loss modulus versus frequency data for the multiple W/O/W emulsion. The loss modulus data for the highly aged multiple emulsion (age ) 19.13 days) are compared with the fresh multiple emulsion separately in part b.
exhibits the shear-thickening effect (refer to Figure 5). Clearly, the shear rate drops significantly at the onset of the shear-thickening phenomenon. Because the shear rate decreases with an increase in shear stress in the shear-thickening zone, secondary flows and turbulence are expected to be absent.
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Figure 9. Photomicrographs of (a) the fresh multiple water-in-oil-in-water (W/O/W) emulsion, (b) the aged multiple water-in-oil-inwater (W/O/W) emulsion (t ) 27.8 h), and (c) the aged multiple water-in-oil-in-water (W/O/W) emulsion (t ) 20 days).
The storage and loss moduli data for the multiple W/O/W emulsion as a function of frequency are shown in Figures 7 and 8. The data were collected at a low
shear stress of 0.1 Pa. The moduli, like viscosity, initially increase with storage time and then decrease with a further increase in storage time.
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Figure 10. Droplet size of the multiple water-in-oil-in-water (W/ O/W) emulsion: (a) droplet size distribution; (b) Sauter mean diameter as a function of storage time.
Discussion The observations made regarding the multiple W/O/W emulsion in the preceding section can be summarized as follows: (a) the viscosity of the multiple emulsion initially increases with an increase in storage time and then decreases with a further increase in storage time; (b) the aged multiple emulsion with storage time (age) less than 20.1 h exhibits shear thickening (dilatancy); (c) the storage and loss moduli of the multiple emulsion, like viscosity, initially increase with an increase in storage time and then decrease with a further increase in storage time. Some of the flow behavior characteristics of the multiple emulsion can be explained in terms of the droplet size. Parts a-c of Figure 9 show photomicrographs of the multiple W/O/W emulsion taken after different storage times. The multiple droplets consist of a large number of small water droplets entrapped within the large oil droplets. Furthermore, not all the multiple droplets are of spherical shape. The droplet size distributions of the fresh and aged multiple emulsions are shown in Figure 10a. The droplet size generally decreases with an increase in storage time. This can be seen more clearly in Figure 10b where the Sauter mean diameter is plotted as a function of storage time; the Sauter mean diameter decreases significantly with storage time. The decrease in droplet size of the multiple emulsion could be due to two possible mechanisms: (1) The water molecules from the internal aqueous droplets may permeate through the oil phase to the external aqueous phase because of osmotic pressure difference. This will obviously result in the shrinkage of the internal aqueous droplets as well as the multiple droplets. (2) The internal aqueous droplets may be expelled from the multiple droplets to the external aqueous phase due to rupture of the oil layer (Florence and Whitehill, 1982). The decrease in droplet size of the multiple emulsion can have two opposing effects on the rheological param-
eters (viscosity, storage modulus) of emulsion: (a) The viscosity and storage modulus may increase because of the decrease in droplet size. At a given dispersed-phase concentration, finer emulsions are well-known to exhibit higher viscosities and storage moduli than the coarse emulsions (Pal, 1996). (b) The viscosity and storage modulus may decrease because of the decrease in the dispersed-phase concentration. Note that the effective dispersed-phase concentration of the emulsion decreases when water passes from the internal aqueous phase to the external aqueous phase. The observed increase in the rheological parameters initially upon aging is likely due to dominance of the former effect, whereas the decrease in the rheological parameters observed with further aging may be due to the latter effect. The increase in viscosity and storage modulus initially upon aging could also be due to solubilization of water (from the external aqueous phase of the multiple W/O/W emulsion) in the oil phase (Matsumoto and Kohda, 1980). The solubilization of water from the external aqueous phase of the multiple W/O/W emulsion will cause swelling of the oil films which separate the internal and external aqueous phases; consequently, an increase in viscosity and storage modulus is expected. Note that the oil phase of the primary W/O emulsion contains a large amount of surfactant (30.3% by weight of Emsorb 2500). Therefore, it is reasonable to presume that a large number of reverse micelles of the surfactant are present in the oil phase. When a multiple W/O/W emulsion is prepared, the oil/water interface is expanded. The expansion of the oil/water interface is expected to enhance the solubilization of water in the reverse micelles present in the oil phase. However, it is important to point out that the solubilization of water in the oil layer should result in an increase in the droplet size of the multiple emulsion, whereas Figure 10 shows a decrease in the droplet size during aging. Unfortunately, the droplet size for the multiple emulsion was not monitored continuously during the aging process. As shown in Figure 10, the droplet size data were measured for only four aging times (fresh emulsion, 27.8 h, 96.8 h, and 20 days). It is quite possible that the size of the multiple emulsion droplets initially increased during the first few hours of aging and then decreased later with further aging. The shear-thickening effect observed in the present emulsions is likely due to the transition from a twodimensional layered arrangement of particles at low shear stress to a random three-dimensional arrangement at a critical value of shear stress. This explanation was offered by Hoffman (1972, 1974, 1982) and others (Barnes, 1989) to explain shear thickening in suspensions. It is interesting to note that the shearthickening phenomenon has never been observed in simple emulsions (Barnes, 1989). According to the published literature, the shear-thickening effect depends strongly on the shape and rigidity of the dispersed particles. To observe shear thickening in dispersions, it seems that the particles should be more or less rigid (Barnes, 1989). Also, more anisotropic particles tend to produce shear-thickening more readily (Beazley, 1980). The dispersed particles of the present multiple W/O/W emulsions do seem to possess the shape and rigidity favorable for shear thickening. The viscosity of the dispersed particles of the multiple emulsion is pretty high. Note that a primary water-in-oil emulsion of high viscosity (see Figure 3) was used to produce the
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dispersed particles of the multiple emulsion. Furthermore, the dispersed particles of the multiple emulsion are anisotropic, as can be seen from the photomicrographs (Figure 9a-c). Conclusions On the basis of the experimental results and the data analysis, the following conclusions can be reached: (1) The primary W/O emulsion at a dispersed-phase (water) concentration of 68 vol % is highly non-Newtonian in that the viscosity decreases substantially with an increase in shear stress. (2) There occurs a dramatic decrease in the zero-shear viscosity of the primary W/O emulsion over the first few hours of storage. The high shear viscosity decreases only slightly with aging time. (3) The storage modulus of the primary W/O emulsion decreases rapidly over the first 24 h of storage and then tends to level off with further aging. (4) The fresh multiple W/O/W emulsion at a primary W/O emulsion concentration of 73 vol % behaves like a shear-thinning non-Newtonian fluid. (5) The viscosity of the multiple W/O/W emulsion initially increases with an increase in storage time and then decreases with a further increase in storage time. The storage and loss moduli exhibit a similar behavior. (6) The aged multiple W/O/W emulsion after storage times of 2.5, 3.4, 6.1, and 6.7 h exhibits shear thickening (dilatancy) at high values of shear stress. The shearthickening effect disappears when the age of the emulsion exceeds 6.7 h. The exact mechanism as to why the shear-thickening effect appears and disappears upon aging is not clear at present. Acknowledgment Financial support from the Natural Sciences and Engineering Research Council of Canada is gratefully appreciated. Literature Cited Aronson, M. P.; Petko, M. F. Highly concentrated water-in-oil emulsions: Influence of electrolyte on their properties and stability. J. Colloid Interface Sci. 1993, 159, 134. Barnes, H. A. Shear-thickening (dilatancy) in suspensions of nonaggregating solid particles dispersed in Newtonian liquids. J. Rheol. 1989, 33, 329. Barnes, H. A. A review of the slip (wall depletion) of polymer solutions, emulsions and particle suspensions in viscometers: its cause, character, and cure. J. non-Newtonian Fluid Mech. 1995, 56, 221. Beazley, K. In Rheometry: Industrial Applications; Walters, K., Ed.; Research Studies Press: Chichester, U.K., 1980. Buscall, R.; McGowan, I. J.; Morton-Jones, A. J. The rheology of concentrated dispersions of weakly attracting colloidal particles with and without wall slip. J. Rheol. 1993, 37, 621.
Davis, S. S. Liquid membranes and multiple emulsions. Chem. Ind. 1981, 3, 683. Dickinson, E.; Stainsby, G. Emulsion stability. In Advances in food emulsions and foams; Dickinson, E., Stainsby, G., Eds.; Elsevier: London, 1988; pp 1-25. Dickinson, E.; Evison, J.; Owusu, R. K. Preparation of fine protein stabilized water-in-oil-in-water emulsions. Food Hydrocolloids 1991, 5, 481. Dickinson, E.; Evison, J.; Gramshaw, J. W.; Schwope, D. Flavor release from a protein-stabilized water-in-oil-in-water emulsion. Food Hydrocolloids 1994, 8, 63. Dzuy, N. Q.; Boger, D. V. Yield stress measurement for concentrated suspensions. J. Rheol. 1983, 27, 321. Florence, A. T.; Whitehill, D. The formulation and stability of multiple emulsions. Int. J. Pharm. 1982, 11, 277. Frankenfeld, J. W.; Fuller, G. C.; Rhodes, C. T. Potential use of liquid membranes for emergency treatment of drug overdose. Drug Dev. Commun. 1976, 2, 405. Garti, N.; Aserin, A. Double emulsions stabilized by macromolecular surfactants. Adv. Colloid Interface Sci. 1996, 65, 37. Gregory, T.; Mayers, S. A note on slippage during the study of the rheological behaviour of paste inks. Surf. Coatings Int. (JOCCA) 1993, 76, 82. Hoffman, R. L. Discontinuous and dilatant viscosity behaviour in concentrated suspensions. I: Observation of a flow instability. Trans. Soc. Rheol. 1972, 16, 155. Hoffman, R. L. Discontinuous and dilatant viscosity behaviour in concentrated suspensions. II: Theory and experimental tests. J. Colloid Interface Sci. 1974, 46, 491. Hoffman, R. L. Discontinuous and dilatant viscosity behaviour in concentrated suspensions. III: Necessary conditions for their occurrence in viscometric flows. Adv. Colloid Interface Sci. 1982, 17, 161. Mackles, L. Wax composition and method for making the same. U.S. patent 3,395,028, 1968. Matsumoto, S.; Kohda, M. The Viscosity of W/O/W Emulsions. J. Colloid Interface Sci. 1980, 73, 13. Matsumoto, S.; Kita, Y.; Yonezawa, D. An attempt at preparing water-in-oil-in-water multiple-phase emulsions. J. Colloid Interface Sci. 1976, 57, 353. Owusu, R. K.; Qinhong, Z.; Dickinson, E. Control release of L-triptophan and Vitamin B2 from model water/oil/water multiple emulsions. Food Hydrocolloids 1992, 6, 443. Pal, R. Rheology of highly flocculated oil-in-water emulsions. Chem. Eng. Commun. 1990, 98, 211. Pal, R. Effect of droplet size on the rheology of emulsions. AIChE J. 1996, 42, 3181. Pal, R. Viscosity and storage/loss moduli for mixtures of fine and coarse emulsions. Chem. Eng. J. 1997, 67, 37. Pal, R.; Rhodes, E. Viscosity/concentration relationships for emulsions. J. Rheol. 1989, 33, 1021. Sherman, P. Rheological properties of emulsions. In Encyclopedia of emulsion technology; Becher, P., Ed.; Marcel Dekker: New York, 1983; Vol. 1, pp 405-437.
Received for review November 25, 1997 Revised manuscript received March 3, 1998 Accepted March 5, 1998 IE970860X
