Dense emulsions

As reported in this chapter, the microscopic origin of both compressibility and elasticity of dense emulsions is rather well understood. Emulsions have elastic properties arising from either surface tension or surface elasticity and plasticity. Some protein-stabilized emulsions obey the same phenomenology as solid-stabilized emulsions they exhibit substantially higher osmotic resistances and higher shear moduli than surfactant-stabilized emulsions [38 0]. Moreover, they are strongly resistant to water evaporation. Proteins possess the ability to form... 

Using the usual assumptions, we obtain the following dimensionless unsteady-state material and energy balance equations for the dense (emulsion) phase of the bubbling fluidized bed ... 

The dense emulsion phase is perfectly mixed and is at incipient fluidization conditions with constant voidage. 

Dense emulsion-phase monomer mass-balances Here the mass-balance equation is... 

The rheological properties of a dense emulsion with close-packed droplets depends on whether or not the droplets are small enough to be agitated significantly by Brownian motion. If not, because of the high packing density of the droplets, the emulsions should be elastic and have a finite elastic modulus at low frequencies. For liquids with viscosities near that of water, Brownian behavior should dominate for particle radii less than or equal to 1 fim, while non-Brownian behavior occurs when a > 10 m. 

Princen and Kiss (1986) carried out step-strain measurements on emulsions of paraffin oil droplets of mean radius 10 pm, polydisperse in size, in water containing 20% commercial surfactant as a stabilizer. They showed that the modulus G of these dense emulsions can be represented by the simple formula... 

Measurement of the viscoelastic properties of foams and dense emulsions is also complicated by slip at the rheometer surfaces (Yoshimura and Prud homme 1988). The liquid in an aqueous foam lubricates flat rheometer fixtures, reducing the strain imposed on the bulk foam. This lubrication is a desirable feature of some foam products such as shaving cream, but it complicates rheological studies. The use of roughened surfaces, such as sandpaper bonded to the rheometer fixtures, seems to be an effective countermeasure (Khan et al. 1988). ... 

The earliest analysis of the deformation properties of a liquid foam is that of Princen (1983), who modeled two-dimensional foams and dense emulsions at rest by an array of regular hexagons (see Fig. 9-32a). While Princen s model was limited to hexagons with a particular orientation relative to the imposed shearing flow, this restriction was lifted in the work of Khan and Armstrong (1986) and Kraynik and Hansen (1986). Polydisperse cell sizes have also been considered (Weaire et al. 1986 Khan and Armstrong 1987 Weaire and Fu 1988 Kraynik et al. 1991 Okuzono et al. 1993), as well as wet foams with... 

This prediction is analogous to the empirical expression for dense emulsions, Eq. (9-49), in the limit 0 —1. If the foam is made irregular, with the average area per foam cell held constant, the modulus is predicted to be less than that of Eq. (9-581 by about 14% (Weaire and Fu 1988). For the regular tetrakaidecahedron, Reinelt averaged Go over all initial foam orientations and obtained... 

Liu et al. (1996) have reported high levels of viscous dissipation in dense emulsions under high-frequency oscillatory shearing deformations. Their data for the complex modulus G can be represented empirically by the expression... 

Golomb D, Pennell S, Ryan D, Barry E, and Swett P (2007) Ocean sequestration of carbon dioxide Modeling the deep ocean release of a dense emulsion of liquid C02-in-water stabilized by pulverized limestone... 

A cell may be viewed as a very dense emulsion of macromolecules in water, embedded in a semipermeable lipoproteic membrane. Water is expected to be adsorbed on the polar sites of the macromolecules. Since in biological cells there are approximately three water molecules only per polar site, it seems that in biosystems all water must be viewed in an adsorbed, rather than liquid, phase. This picture has been able to account for a lot of seemingly uncorrelated phenomena the Na+/K+ ratio [5,6], anomalous viscosity [6], and inert-gas anaesthesia [7],... 

A further level of detail is introduced by considering the make-up of the bed - a mixture of bubbles that contain essentially no particles and a dense emulsion phase consisting of solids and gas in intimate contact (from the two phase theory of fluidization - see Chapter 7). The overall bed density is thus a function of the density of the particulate solids, the proportion of sohds and gas in the emulsion phase and the proportion of the bed occupied by bubbles. 

Emulsions are metastable colloids made out of two immiscible fluids, one being dispersed in the other, in the presence of surface active agents. The droplet volume fiaction may vary from zero to almost one dense emulsions are sometimes called biliquid foams since their structure is very similar to the cellular structure of air-liquid foams for which the continuous phase is very minor. From dilute to highly concentrated, emulsions exhibit very different internal dynamics and mechanical properties. When diluted, droplets are agitated by Brownian motion and behave as viscous Newtonian fluids, whereas when more concentrated, namely above the random close packing volume fraction which is 64% for monodisperse droplets, the internal dynamics are severely restricted and they behave as viscoelastic solids. Simple direct emulsions are composed of oil droplets dispersed in water while inverse emulsions are composed of water droplets dispersed in an oil continuous phase. In fact, emulsions are in principle made out of two immiscible phases for which the interfacial tension is therefore non-zero, and may involve other hydrophilic-like or lipophilic-like fluids in the presence of suitable surface active species, each phase being possibly comprised of numerous components. Sometimes, simple emulsions may also contain smaller droplets of the continuous phase dispersed within each droplet of the dispersed phase. Such systems are called double emulsions or multiple emulsions. ... 

Droplet Growth in Dense Emulsions Undergoing Coalescence... 

We have so far established a reasonable description of the coalescence induced destruction of dense emulsions, by considering a thermally activated rupture of the thin film (governed by the Arrhenius equation), and by considering a mean field growth equation involving only one frequency so that R(t) is given by ... 

The destruction of dense emulsions is a rich domain. A large variety of behavior is observed. At the macroscopic scale, the system may exhibit a demixtion, as represented by Figure 8.21, or an homogeneous destruction (the emulsion remains macroscopically homogeneous at any time). At the colloidal scale, the droplet size distribution may change from almost bimodal to polydisperse or very monodis-perse. In addition, when the continuous phase spinodally decomposes, one phase may cause severe destruction, as described in Section 8.4. 

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