Volume fractions, emulsions, effect

The HLB system has made it possible to organize a great deal of rather messy information and to plan fairly efficient systematic approaches to the optimiza-tion of emulsion preparation. If pursued too far, however, the system tends to lose itself in complexities [74]. It is not surprising that HLB numbers are not really additive their effective value depends on what particular oil phase is involved and the emulsion depends on volume fraction. Finally, the host of physical characteristics needed to describe an emulsion cannot be encapsulated by a single HLB number (note Ref. 75). 

The interaction between the dispersed-phase elements at high volume fractions has an impact on breakup and aggregation, which is not well understood. For example, Elemans et al. (1997) found that when closely spaced stationary threads break by the growth of capillary instabilities, the disturbances on adjacent threads are half a wavelength out of phase (Fig. 43), and the rate of growth of the instability is smaller. Such interaction effects may have practical applications, for example, in the formation of monodisperse emulsions (Mason and Bibette, 1996). 

H.M. Princen, M.P Aronson, and J.C. Moser Highly Concentrated Emulsions. II Real Systems. The Effect of Film Thickness and Contact Angle on the Volume Fraction in Creamed Emulsions. J. CoUoid Interface Sci. 75, 246 (1980). 

In a subsequent theoretical analysis, Princen [26] initially used a model of infinitely long cylindrical drops to relate the geometric and thermodynamic properties of monodisperse HIPEs to the volume fraction of the dispersed phase. Thus the analysis could be restricted to a two-dimensional cross-section of the emulsion. Two principle emulsion parameters were considered the film thickness between adjacent drops (h) and the contact angle (0) [27-29]. The effects of these variables on the volume fraction, , both in the presence and absence of a compressive force on the emulsion, were considered. The results indicated that if both h and 0 are kept at zero, the maximum volume fraction () of the uncompressed emulsion is 0.9069, which is equivalent to = 0.7405 in real emulsions with spherical droplets (cf. Lissant s work). If 0 is zero (or constant) and h is increased, the maximum value of decreases on the other hand, increasing 0 with zero or constant h causes to increase above the value 0.9069, again at zero compression. This implies that, in the presence of an appreciable contact angle, without any applied compressive force, values of <(> in excess of the maximum value for undeformed droplets can occur. Thus, the dispersed phase... 

Pons et al. have studied the effects of temperature, volume fraction, oil-to-surfactant ratio and salt concentration of the aqueous phase of w/o HIPEs on a number of rheological properties. The yield stress [10] was found to increase with increasing NaCl concentration, at room temperature. This was attributed to an increase in rigidity of films between adjacent droplets. For salt-free emulsions, the yield stress increases with increasing temperature, due to the increase in interfacial tension. However, for emulsions containing salt, the yield stress more or less reaches a plateau at higher temperatures, after addition of only 1.5% NaCl. 

An important quantity, which characterizes a macroemulsion, is the volume fraction of the disperse phase 4>a (inner phase volume fraction). Intuitively one would assume that the volume fraction should be significantly below 50%. In reality much higher volume fractions are reached. If the inner phase consists of spherical drops all of the same size, then the maximal volume fraction is that of closed packed spheres (fa = 0.74). It is possible to prepare macroemulsions with even higher volume fractions volume fractions of more than 99% have been achieved. Such emulsions are also called high internal phase emulsions (HIPE). Two effects can occur. First, the droplet size distribution is usually inhomogeneous, so that small drops fill the free volume between large drops (see Fig. 12.9). Second, the drops can deform, so that in the end only a thin film of the continuous phase remains between neighboring droplets. 

Colloidal interactions between emulsion droplets play a primary role in determining emulsion rheology. If attractions predominate over repulsive forces, flocculation can occur, which leads to an increase in the effective volume fraction of the dispersed phase and thus increases viscosity (McCle-ments, 1999). Clustering of milk fat globules due to cold agglutination increases the effective volume fraction of the milk fat globules, thereby increasing viscosity (Prentice, 1992). 

Consider first the effect of a dispersed phase, of volume fraction continuous phase of viscosity D0 and dispersed particles (droplets) which do not attract. At low volume fractions the Einstein equation should apply to a suspension of solid particles at constant temperature,... 

Figure 5. Schematic diagram showing the effect of changing the volume fraction of second phase on the apparent viscosity at a fixed rate of shear of a two-phase emulsion. The different dotted lines refer to different viscosities of the pure phases A and B, The solid line suggests the viscosity that may be displayed by a system in which both the viscosities of the pure phases and the relative proportions of phases are changing continuously, as in a pyrolysis run.

Table 1. Effect of the HLB of the surfactant on the formation and stability of concentrated emulsions. The concentrated emulsion contains styrene and water as the two phases and the volume fraction of the dispersed phase is 0.9. The concentrated emulsion was prepared at room temperature and its stability test was conducted by heating the emulsion at 50 °C for 3 h and 24 h, respectively...

Effects of Solids Size. The effect of solids size on the viscosity of the emulsion-solids mixtures is shown in Figure 19 for synthetic OAV emulsions. The oil concentration (solids-free basis) is 60% by volume, and the solids used are silica sand. The comparison is made at shear stresses of 6 and 14 Pa. The viscosity is expressed as the relative viscosity (t7ows/ 7ow)t lhat is, the viscosity of the emulsion-solids mixture divided by the viscosity of the solids-free emulsion. At low solids volume fraction (<0.1), solids size has little effect. 

Effect of Emulsion Characteristics. As discussed in Chapter 4, the rheology of emulsions is affected by several factors, including the dis-persed-phase volume fraction, droplet size distribution, viscosity of the continuous and dispersed phases, and the nature and amount of emulsifying surfactant present. All of these parameters would be expected to have some effect on flow behavior of the emulsion in porous media. However, the relationship between bulk rheological properties of an emulsion and its flow behavior in porous media is feeble at best because, in most cases, the volume... 

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