Emulsion bulk viscosity properties

Rheology. Bulk Viscosity Properties. The rheological properties of an emulsion are very important. High viscosity may be the reason that an emulsion is troublesome, a resistance to flow that must be dealt with, or a desirable property for which an emulsion is formulated. The simplest description applies to Newtonian behavior in laminar flow. The viscosity, r], is given in terms of the shear stress, t, and shear rate, 7, by ... 

Interfacial Viscosity, The foregoing discussion of rheology has dealt with the bulk viscosity properties. A closely related and very important property is the interfacial viscosity, which can be thought of as the two-dimensional equivalent of bulk viscosity, operative in the oil-water interfacial region. As droplets in an emulsion approach each other, the thinning of... 

In addition to bulk viscosity properties, a closely related and very important property is the interfacial viscosity, which can be thought of as the two-dimensional equivalent of bulk viscosity, operative in the oil-water interfacial region. As droplets in an emulsion approach each other the thinning of the films between the drops, and their resistance to rupture, are thought to be of great importance to the ultimate stability of the emulsion. Thus, a high interfacial viscosity can promote emulsion stability by retarding the rate of droplet coalescence, as discussed in later sections. Further details on the principles, measurement, and applications to emulsion stability of interfacial viscosity are reviewed by Malhotra and Wasan [32]. 

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... 

Barnes (6) and Tadros and Vincent (7) demonslrated die importance of a number of factors on emulsion properties and stability, among them the relative volume of flic dispersed phase, i.e., the volume fraction, and the average size of the droplet, flic bulk viscosity of each phase, and also flic nature and concentration of the emulsifier. The latter must be of vital importance as there are no stable emulsions or foams known wifliout the presence of surface-active compounds. Sometimes fliis becomes not immediately visible in some systems as stabilizers may be inherent in many natural emulsions or foams. 

Polymeric additives may aid in emulsion formation as a result of surface-active properties but are usually more important as stabihzers. Their action may result from steric or electrostatic interactions, from changes in the interfacial viscosity or elasticity, or from changes in the bulk viscosity of the system. In many if not most cases, the function of polymeric stabihzers is a combination of several actions (Fig.ll.lc). 

Perhaps the most important and striking features of high internal phase emulsions are their rheological properties. Their viscosities are high, relative to the bulk liquid phases, and they are characterised by a yield stress, which is the shear stress required to induce flow. At stress values below the yield stress, HIPEs behave as viscoelastic solids above the yield stress, they are shear-thinning liquids, i.e. the viscosity varies inversely with shear rate. In other words, HIPEs (and high gas-fraction foams) behave as non-Newtonian fluids. 

It is generally accepted that the soft-core RMs contain amounts of water equal to or less than hydration of water of the polar part of the surfactant molecules, whereas in microemulsions the water properties are close to those of the bulk water (Fendler, 1984). At relatively small water to surfactant ratios (Wo < 5), all water molecules are tightly bound to the surfactant headgroups at the soft-core reverse micelles. These water molecules have high viscosities, low mobilities, polarities which are similar to hydrocarbons, and altered pHs. The solubilization properties of these two systems should clearly be different (El Seoud, 1984). The advantage of the RMs is their thermodynamic stability and the very small scale of the microstructure 1 to 20 nm. The radii of the emulsion droplets are typically 100 nm (Fendler, 1984 El Seoud, 1984). 

Characterization of such emulsions therefore often involves three phases the water phase, the oil phase, and the solids. Complete characterization of an emulsion could therefore involve detailed chemical and physical analysis of all of the emulsion components, as well as any bulk properties that might be of interest (viscosity, density, etc.). This level of detail is clearly beyond the scope of this discussion. For the purposes of this chapter, emulsion characterization will be defined as the quantification of the phases present, the determination of the nature and size distribution of... 

After all, the stability and size distribution of this phase determine most bulk emulsion properties. Fixed proportions of oil, water, and solids can be combined in various ways to produce emulsions having different size distributions of the dispersed phase, given only small differences in emulsifier or ion additions to the water or oil phases. These physical differences can lead to significantly different viscosity and stability in emulsions with nominally identical bulk composition. 

Unlike microemulsion-mediated particle synthesis, the macroemulsion method is relatively simple and mature. The main reasons behind this are that the properties of the final products are rather easily controlled by (a) the water phase properties like viscosity and (b) the engineering aspects mentioned in Section 6.2.1. The process, thus, is stabilized for bulk production of powders. However, some new developments on the process have been reported in recent times. One of them is the emulsion combustion method described in Chapter 5 [172, 173]. In this process, atomization and firing (800"-850 C) of the emulsion leads to the formation of dry and crystalline powders that can be directly collected in a bag filter. This is also a facile procedure for obtaining hollow particles, useful for thermal insulation and various other purposes [178]. 

Solvent systems suitable for coimtercurrent distribution must always form two discrete liquid phases, and a finally divided suspension in one phase in the other must separate quickly into two bulk-liquid layers. These properties must be retained in the presence of appreciable amounts of sample. The two phases should, therefore, differ in density, neither should have a high viscosity, while the interfacial tension should not promote the formation of stable emulsions. Formation of emulsions, which separate only with difficulty with real samples, is the most serious practical problem. Solvent systems should be selected based on their selectivity for the separation. In addition, distribution ratios can be optimized by changing the phase ratio or distribution properties of the phases by using additives (e.g., pH, complexing agents, salts, polymers, etc.). 

This exclusion of polymers from the interior of the vesicles results in an osmotic compression of the water layers and a decrease in the water layer thickness and lamellar phase volume. This effect allows the control of bulk properties such as viscosity and also provides a probe of water layer dimensions in lamellar dispersions. The lamellar surfactant system used in this study is the sodium dodecyl sulfate (SDS)/dodecanol (Ci20H)/water system that has been used to prepare submicron diameter emulsions (miniemulsions) from monomers for emulsion polymerization (5) and for the preparation of artificial latexes by direct emulsification of polymer solutions such as ethyl cellulose (4). This surfactant system forms lamellar dispersions (vesicles) in water at very low surfactant concentrations (< 13 mM). 

In Chapter 17, we discuss rheological properties, in particular viscosity and elasticity, of colloidal systems. These properties are at the basis of quality characteristics such as strength, pliancy, fluidity, texture, and other mechanical properties of various materials and products. In addition to bulk rheology, rheological features of interfaces are discussed. Interfacial rheological behavior is crucial for the existence of deformable dispersed particles in emulsions and foams. Emulsions and foams, notably their formation and stabilization, are considered in more detail in Chapter 18. 

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