Elasticity of Concentrated Emulsions

T. D. Dimitrova and F. Leal Calderon Bulk Elasticity of Concentrated Protein-Stabilized Emulsions. Langmuir 17, 3235 (2001). 

Low internal phase emulsions typically result when high shear conditions are used for emulsification, while low shear mixing can lead to high internal phase, or concentrated, emulsions [435]. There are several conditions needed to form a concentrated emulsion. Low shear mixing is required while the internal phase is slowly added to the continuous phase, and the surfactants used to create the emulsion need to be able to form elastic films [435—438]. The formation of concentrated emulsions has also been linked to surfactant-oil phase interactions [436] and therefore the oil-water interfacial tension and the potential for surfactant-surfactant interactions [439]. 

Soft glasses are known to exhibit remarkable nonlinear shear rheology. They are yield-stress fluids that respond either like an elastic solid when the applied stress is zero or below the yield stress, or a like a viscoelastic fluid when a stress greater than the yield value of the material is applied [185]. Above their yield stresses, soft glasses are shear thinning fluids and very often the shear stress increases with the shear rate raised to the one-half power. This is well documented for the case of concentrated emulsions [102, 182, 186], microgel suspensions [31], and multilamellar... 

Compared to the quasistatic elastic and yield behavior of concentrated emulsions and foams, the rate-dependent viscous properties are even more eomplex and relatively unexplored. Formally, die shear stress, r, may be expressed as a function of flic shear rate, y, as... 

Fig. 4. Effect of sodium dodecylsulfonate concentration on fast elastic deformation 9 (I), and stability of concentrated emulsions T (II).

It confirms that the elasticity of concentrated multiple emulsions is governed by the internal droplets solely. 

Figure 2.8 Mason s plot of the rescaled elastic modulus of concentrated emulsions as a function of the volume fraction inverse dodecane in water emulsion stabilized with Span 80 mnltiple emulsion WIOIW stabilized by Span 80 and modified polyacrylic acid 10C12. The straight line is a linear fit to the multiple emulsions data.

H.M. Princen Rheology of Foams and Highly Concentrated Emulsions I. Elastic Properties and Yield Stress of a Cyhndrical Model System. J. Colloid Interface Sci. 91, 160 (1983). 

In concentrated emulsions and foams the thin liquid films that separate the droplets or bubbles from each other are very important in determining the overall stability of the dispersion. In order to be able to withstand deformations without rupturing, a thin liquid film must be somewhat elastic. The surface chemical explanation for thin film elasticity comes from Marangoni and Gibbs (see Ref. [199]). When a surfactant-stabilized film undergoes sudden expansion, then immediately the expanded... 

Quasi-elastic light scattering is an excellent technique for studying the formation and stability of submicrometer emulsions. Improvements in the methods of quasi-elastic light scattering data acquisition and analysis that enable full particle-size distribution studies of sub-micrometer emulsion systems are discussed. Using several oil/water emulsion systems as examples, we demonstrate the ability of these techniques to determine the effect of emulsifier concentration on the particle-size distribution produced by an inversion method of emulsification. Some of the benefits of obtaining the full distribution are also discussed. 

Disperse phase volume fraction. The viscosity of food emulsions tends to increase with increased disperse phase volume fraction (Figure 10). The viscosity increases relatively slowly, with 4> at low droplet concentrations, but increases steeply when the droplets become closely packed together. At higher droplet concentrations, the particle network formed has predominantly elastic characteristics. 

There has been considerable work on tlie elastic properties of soft particle dispersions composed of compressed emulsions and microgels. The elastic properties of compressed emulsions have been explored experimentally and tlieoretically [79,124-129]. Concentrated microgel suspensions [87, 121,130] and multilamellar vesicles [77,82,131,132] have also been studied extensively. The elasticity of these different systems exhibit interesting analogies that will be analyzed in this section. 

Proteins are considered to adsorb at the interface, partially unfold and interact to form molecular "gel-like" networks (Wilde et ak, 2004). Collapse of emulsions or foams involves the stretching of the interface and the elasticity of the protein structure is supposed to oppose this effect. For mobile surfactants or lipids stretching of the interface will lead to concentration gradients and rapid diffusion of the molecules to restore the status quo (Wilde et al., 2004). A source of instability for most foods is the presence of both proteins and small mobile molecules at the interface. The incompatibility of the two mechanisms means that mixed interfaces are less stable than interfaces containing pure protein or pure surfactant or lipid (Wilde et al., 2004). If there is sufficient surfactant or lipid present then they will eventually displace the protein. It is the structures formed during the battle for control of the interface that gives rise to instability. 

FIG. 17 Elastic modulus—frequency sweeps at various dodecane volume fractions indicated above. Concentrated emulsions stabilized by 6(2C12)Na (M = 42,000 and Mw = 125,000 g/mol) at a concentration of 2%. (From Ref. 202.)... 

Princen HM. Rheology of foams and highly concentrated emulsions. I. Elastic properties and yield stress of a cylindrical model system. J Colloid Interface Sci 1983 91 160-175. 

Elastic deformation also reveals itself in foams and concentrated emulsions. The shear stress in this case is determined by an increase in the interfacial area due to the deformation of the system. Mechanical properties of solidified foams and other solid-like cellular structures are governed by the degree of dispersion, type of backbone structure and a combination of mechanical characteristics of dispersed phase and dispersion medium. 

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