Viscoelastic emulsion

The current state OF THE ART of various aspects of macro- and microemulsions is reflected in this volume. The symposium upon which this volume is based was organized in six sessions emphasizing major areas of research. Major topics discussed include a review of macro- and microemulsions, enhanced oil recovery, reactions in microemulsions, multiple emulsions, viscoelastic properties of surfactant solutions, liquid crystalline phases in emulsions and thin films, photochemical reactions, and kinetics of microemulsions. 

A linear viscoelastic constitutive model of dilute emulsion viscoelastic properties was proposed by Oldroyd [111, 112]. The model considered low deformation of monodispersed drops of one Newtonian liquid in another, with an interphase. Choi and Schowalter [113] extended their cell model to dilute emulsions with Newtonian matrix and viscoelastic drops under infinitesimally small oscillatory deformation. Oldroyd s model was modified by Palierne [126, 127] for dilute viscoelastic hquids emulsions with polydispersed spherical drops (thus, subject to small deformations) with constant interfacial tension coefficient, Vu, at concentrations below that where the drop-drop interactions start complicating the flow field, that is, < 0.1 ... 

Vinckier, I., Minale, M., Mewis, J., and Moldenaers, P. (1999) Rheology of semi-dilute emulsions viscoelastic effects caused by the interfadal tension. Colloids Swf., 150, 217-228. 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

The surface characteristics of these species are determined by the particulates and stress transfer across the membrane will tend to be low, reducing internal circulation within the drop. The structure of the interface surrounding the drop plays a significant role in determining the characteristics of the droplet behaviour. We can begin our consideration of emulsion systems by looking at the role of this layer in determining linear viscoelastic properties. This was undertaken by... 

E. Dickinson and I. Chen Viscoelastic Properties of Protein-Stabilized Emulsions Effect of Protein-Surfactant Interactions. I. Agric. Food Chem. 46, 91 (1998). 

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. 

Figure 3.5 Demonstration of correlation between the stickiness of protein-coated droplet pair encounters in shear flow (left ordinate axis) and viscoelasticity of concentrated emulsions (right ordinate axis) with the strength of protein-protein attraction as indicated by the second virial coefficient A2 determined from static light scattering , percentage capture efficiency (0%) A, complex shear modulus (G ) for emulsions stabilized by asl-casein or (3-casein (pH = 5.5, ionic strength in the range 0.01-0.2 M).

Hence, from the previously described light-scattering study of caseinate self-assembly in solution, we can postulate that heating/cooling not only alters the nature and strength of the physical (hydrophobic) interactions between emulsion droplets covered by caseinate. It most likely also transforms the nanoscale structural characteristics of the protein network in the bulk and at the interface, thereby affecting the viscoelastic and microstructural properties of the emulsions. 

Chen, J., Dickinson, E. (1998) Viscoelastic properties of protein-stabilized emulsions effect of protein-surfactant interactions. Journal of Agricultural and Food Chemistry, 46, 91-97. 

Evidence for the flocculation of emulsion droplets is commonly derived from a combination of rheological and creaming stability experiments. For instance, a marked increase in both the viscoelasticity of emulsions of moderately high oil volume fraction (40 vol%) and the rapid serum... 

Moschakis, T., Murray, B.S., Dickinson, E. (2005). Microstructural evolution of viscoelastic emulsions stabilized by sodium caseinate and xanthan gum. Journal of Colloid and Interface Science, 284, 714-728. 

The viscoelastic samples to be tested by this method may be in different forms. The simplest to work with is a soft or liquid-like viscoelastic material such as mayonnaise or other food emulsions. These are easy samples to work with terms of sample loading. More solid-like samples such as cheese or food gels are more difficult to load onto the instrument in a consistent matter. The degree of compression of soft samples should ideally be controlled using a normal force measure or force rebalance system. Slippage is also a concern and roughened plates or even adhesives may be needed if slip is an issue. As this protocol is a general one, it is assumed that the sample is already loaded on the rheometer and has achieved equilibrium in terms of temperature and viscoelastic stmcture (time-dependent behavior). 

A. Guerrero, P. Partal, and C. Gallegos, Linear viscoelastic properties of sucrose ester-stabilized oil-in-water emulsions, J. Rheol., 42 (1998) 1375-1388. 

Palierne JF (1990) Linear rheology of viscoelastic emulsions with interfacial-tension. Rheol Acta 29 204-214... 

Two additional stabilizing influences will be summarized next that of viscoelastic films and that of solid-particle films. In general, where electrical surface charge is an important determinant of stability, it is easier to formulate a very stable O/W emulsion than a W/O emulsion because the electric double layer thickness is much greater in water than in oil. (This is sometimes incorrectly stated in terms of greater charge being present on droplets in an O/W emulsion.) However, there are ways to effectively stabilize W/O emulsions. 

There have now been attempts to determine rheological properties on the nanoscale. The nano-rheological properties (surface viscoelasticity) of emulsion droplets have been estimated through modelling based on data from atomic force microscope measurements [371]. 

Concentrated emulsions can exhibit viscoelasticity, as can gelled foams and some suspensions. Compared with the previous equations presented, additional coefficients (including primary and secondary normal stress coefficients) are needed to characterize the rheology of viscoelastic fluids [376,382]. 

The structure of the interfacial layers in food colloids can be quite complex as these are usually comprised of mixtures of a variety of surfactants and all are probably at least partly adsorbed at interfaces which even individually, can form complex adsorption layers. The layers can be viscoelastic. Phospholipids form multi-lamellar structures at the interface and proteins, such as casein, can adsorb in a variety of conformations [78]. Lecithins not only adsorb also at interfaces, but can affect the conformations of adsorbed casein. The situation in food emulsions can be complicated further by the additional presence of solid particles. For example, the fat droplets in homogenized milk are surrounded by a membrane that contains phospholipid, protein and semi-solid casein micelles [78,816], Similarly, the oil droplets in mayonnaise are partly coated with granular particles formed from the phospho and lipo-protein components of egg yolk [78]. Finally, the phospholipids can also interact with proteins and lecithins to form independent vesicles [78], thus creating an additional dispersed phase. 

The high molar mass species reside mostly in the aqueous phase with a number of peptide groups residing in the oil/water interface [293]. Although these latter surfactants are less effective at reducing interfacial tension, they can form a viscoelastic membrane-like film around oil droplets or air bubbles. These tend to be used in the preparation of, for example, O/W emulsions. These trends are by no means exclusive, mixtures are the norm and competitive adsorption is prevalent. Caseinate, one of the most commonly used surfactants in the food industry, is itself a mixture of interacting proteins of varying surface activity [814],... 

Steric stabilization appears to be the dominant stabilizing force in most food colloids [78,824], Casein-coated emulsion droplets provide an example. The presence of protein in an adsorption layer can also contribute viscoelasticity and provide a barrier to coalescence. 

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