Emulsions viscosity ratio

In Fig. 21.43, the dependence of the oil drop size distribution on the ALR is shown for a constant plant oil emulsion viscosity ratio. The viscosity of the plant oil emulsion was 0.013 Pa s, which equals a viscosity ratio of 4. It may be seen that the oil drop size decreased with increasing ALR, which is in line with previous findings. This is caused by increased stress acting on the feed due to the higher gas velocity [9]. 

In order to also check for the two-phase spraying flows, the impact of the emulsion viscosity ratio A, X503/X50,3,Nozzle and X9o,3/v9o,3,Nozzie of SE and DE were also plotted as a function of the modified dimensionless We number We Drop/, for both INMIX and EXMIX nozzles and two different flow rate conditions as shown in the Fig. 23.21 exemplarily for the maximum secondary drop diameters... 

The model system used by Mabille et al. [149, 150] was a set of monodisperse dilute (2.5 wt% of dispersed oil) emulsions of identical composition, whose mean size ranged from 4 p.m to 11 p.m. A sudden shear of 500 s was applied by means of a strain-controlled rheometer for durations ranging from 1 to 1500 s. All the resulting emulsions were also monodisperse. At such low oil droplet fraction, the emulsion viscosity was mainly determined by that of the continuous phase (it was checked that the droplet size had no effect on the emulsion viscosity). The viscosity ratio p = t]a/t]c = 0.4 and the interfacial tension yi t = 6 mN/m remained constant. 

Figure 1.20. Effect of the viscosity ratio p on the emulsion polydispersity P. The dashed line represents the limit between polydisperse and monodisperse emulsions. (Adapted from [149]).

The preparation of a ferrofluid emulsions is quite similar to that described for double emulsions. The starting material is a ferrofluid oil made of small iron oxide grains (Fe203) of typical size equal to 10 nm, dispersed in oil in the presence of an oil-soluble surfactant. The preparation of ferrofluid oils was initially described in a US patent [169]. Once fabricated, the ferrofluid oil is emulsifled in a water phase containing a hydrophilic surfactant. The viscosity ratio between the dispersed and continuous phases is adjusted to lie in the range in which monodisperse fragmentation occurs (0.01-2). The emulsification leads to direct emulsions with a typical diameter around 200 nm and a very narrow size distribution, as can be observed in Fig. 1.33. 

Figure 3.6 shows that, for a given viscosity ratio, ry2/Vi> between the dispersed (rj2) and continuous (t/j) phases, reducing the interfacia] tension increases the Weber number, lowering the energy needed to cause droplet break-up. As discussed in Section 11.2.1, this relationship can be used to predict whether an emulsion will be easier to form if it is a water-in-oil emulsion rather than an oil-in-water emulsion. 

The aim of this first section is to describe the rupturing mechanisms and the mechanical conditions that have to be fulfilled to obtain monodisperse emulsions. A simple strategy consists of submitting monodisperse and dilute emulsions to a controlled shear step and of following the kinetic evolution of the droplet diameter. It will be demonstrated that the observed behavior can be generalized to more concentrated systems. The most relevant parameters that govern the final size will be listed. The final drop size is mainly determined by the amplitude of the applied stress and is only slightly affected by the viscosity ratio p. This last parameter influences the distribution width and appears to be relevant to control the final monodispersity. 

As a conclusion, if the viscosity ratio p between the internal and external phases lies between 0.01 and 2, the shear applied on a polydisperse emulsion made of large drops leads to a monodisperse one with a mean diameter governed by the stress. This fragmentation occurs through elongation of the drops and the development of a Rayleigh instability with a characteristic time of the order of one second. The obtained monodispersity probably results from the fact that the Rayleigh instability develops under shear for a critical diameter of the deformed drops. 

Let us first consider an inverted W/O emulsion made of 10% of 0.1 M NaCl large droplets dispersed in sorbitan monooleate (Span 80), a liquid surfactant which also acts as the dispersing continuous phase. At this low droplet volume fraction, the rheological properties of the premixed emulsion is essentially determined by the continuous medium. The rheological behavior of the oil phase can be described as follows it exhibits a Newtonian behavior with a viscosity of 1 Pa s up to 1000 s 1 and a pronounced shear thinning behavior above this threshold value. Between 1000 s 1 and 3000 s1, although the stress is approximately unchanged, the viscosity ratio is increased by a factor of 4. 

For a viscosity ratio M of order unity or less, r is the relaxation time of the droplet shape and of the resulting viscoelastic stress in the dispersion. Thus, for rjs I cP, F 10 dyn/cm, a (xm, we obtain t 10 sec, and the stress in the emulsion relaxes almost instantly after cessation of flow. However, when the fluid medium is more viscous and the droplets are bigger, say, rjg 100 P, a 10 jxm, with the same F, we obtain T 0.01 sec. Although this latter value of r corresponds to rather fast relaxation, it can produce appreciable normal stress differences at high shear rates thus (Choi and Schowalter 1975)... 

Figure 9.26 Master curve of reduced shear viscosity versus reduced shear rate for oil-in-water emulsions with (j) = 0.80. The droplet radii are in the range a = dn/ 4-12/um, interfacial tensions r — 0.9-5.9 dyn/cm, and viscosity ratios M — 8-25 the ratio r/d i2 is in the range 90-300 Pa. (From Otsubo and Prud homme 1994, reprinted with permission from Steinkopff Publishers.)...

The concentric cylinder viscometers are supplied with different inner and outer cylinders such that various gap widths can be formed. For rheological measurements of emulsions and suspensions, care must be taken to ensure a gap width of at least 20 times the suspended particle size in order to avoid wall effects. Moreover, experiments should be conducted with different gap widths to ensure the absence of any wall slip that is usually encountered in emulsion viscosity measurements (J6). However, uniformity of shear rate can be achieved only when the ratio of the gap width to the inner cylinder radius is small. 

Theoretical aspects of emulsion formation in porous media were addressed by Raghavan and Marsden (51-53). They considered the stability of immiscible liquids in porous media under the action of viscous and surface forces and concluded that interfacial tension and viscosity ratio of the immiscible liquids played a dominant role in the emulsification of these liquids in porous media. A mechanism was proposed whereby the disruption of the bulk interface between the two liquids led to the initial formation of the dispersed phase. The analysis is based on the classical Raleigh-Taylor and Kelvin-Helmholz instabilities. 

Fiori and Farouq Ali (73) proposed the emulsion flooding of heavy-oil reservoirs as a secondary recovery technique. This process is of interest for Saskatchewan heavy-oil reservoirs, where primary recovery is typically 2-8%. Water-flooding in these fields produces only an additional 2-5% of the original oil in place because of the highly viscous nature of the oil. In laboratory experiments, a water-in-oil emulsion of the produced oil is created by using a sodium hydroxide solution. The viscous emulsion formed is injected into the reservoir. Its high viscosity provides a more favorable mobility ratio and results in improved sweep of the reservoir. Important parameters include emulsion stability and control of emulsion viscosity. 

Emulsion Pipeline Operations. Prediction of pipeline pressure gradients is required for operation of any pipeline system. Pressure gradients for a transport emulsion flowing in commercial-size pipelines may be estimated via standard techniques because chemically stabilized emulsions exhibit rheological behavior that is nearly Newtonian. The emulsion viscosity must be known to implement these methods. The best way to determine emulsion viscosity for an application is to prepare an emulsion batch conforming to planned specifications and directly measure the pipe viscosity in a pipe loop of at least 1-in. inside diameter. Care must be taken to use the same brine composition, surfactant concentration, droplet size distribution, brine-crude-oil ratio, and temperature as are expected in the field application. In practice, a pilot-plant run may not be feasible, or there may be some disparity between pipe-loop test conditions and anticipated commercial pipeline conditions. In these cases, adjustments may be applied to the best available viscosity data using adjustment factors described later to compensate for disparities in operating parameters between the measurement conditions and the pipeline conditions. 

Emulsion Viscosity Phase ratio V rVjj Droplet diameter Emulsion preparation Aging... 

Figure 15. Effect of phase ratio Vj/Vj on emulsion viscosity Tjem and splitting efficiency 7js Vj...total water content I>splitted .obtained continuous water phase after splitting.

Emulsions are characterized in terms of dispersed / continuous phase, phase volume ratio, droplet size distribution, viscosity, and stability. The dispersed phase is present in the form of microscopic droplets which are surrounded by the continuous phase both water-in-oil (w/o) and oil-inwater (o/w) emulsions can be formed. The typical size range for dispersed droplets which are classified as emulsions is from 0.25 to 25 p (6). Particles larger than 25 p indicate incomplete emulsification and/or impending breakage of the emulsion. Phase volume ratio is the volume fraction of the emulsion occupied by the internal (dispersed) phase, expressed as a percent or decimal number. Emulsion viscosity is determined by the viscosity of the continuous phase (solvent and surfactants), the phase volume ratio, and the particle size (6). Stroeve and Varanasi (7) have shown that emulsion viscosity is a critical factor in LM stability. Stability of... 

Figure 33 shows the apparent viscosity variation with shear rate for 44 fim glass beads suspended in a 50% bitumen-in-water emulsion at 25 °C. The viscosity ratio of the dispersed liquid phase (bitumen) to the... 

Liquid droplets cannot be treated the same as solid particles in their codispersed systems. This behavior has been indicated by equation 66 or 68, in which the Einstein constant increases with increasing viscosity ratio of the dispersed phase to the continuous phase. As is shown by Yan et al. (195, 197, 198), liquid droplet and solid particle effects are additive only when the solid concentration is low, say s < 0.05, and when both solid particles and liquid droplets have comparable sizes. However, when the particle-to-droplet size ratio is large, the particles and the droplets become additive (192) for a wider solid concentration range (Figures 34 and 35). The apparent viscosity of the system may be added in terms of the two distinct model systems pure emulsion characterized by solid-free dispersed phase volume fraction and pure suspension characterized by the volume fraction of the solids. The additive rule for the ternary systems is similar to the rule for bimodal solid particle suspensions due to Farris (139) ... 

If the rheology of suspensions and emulsions is difficult to describe theoretically and to determine experimentally, in the case of polymer blends these difficulties reach another order of complication. It suffices to note that in blends both phases are viscoelastic, the viscosity ratio varies over a wide range, and morphology can be very complex. To understand the rheological behavior of blends, it is useful to refer to simpler systems that can offer important insight. The following systems (Table 7.2) are commonly considered and will be treated in the following discussion. 

Figure 7.8. Intrinsic viscosity of emulsion vs. the viscosity ratio (defined in the Figure) [Oldroyd, 1953, 1955].

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