Bulk rheology of emulsions

As discussed in the introduction, the bulk rheology of emulsion systems can be investigated using steady-state (shear stress as a function of shear rate), constant stress, and oscillatory techniques. These methods are the same as those described for interfacial rheology. In this section, some results on various emulsion systems will be described to illustrate the use of rheological measurements in investigating the interaction between emulsion droplets. First, the viscosity-volume fraction relationship for o/w and w/o emulsions... 

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... 

The chapter next dealt with the rheology of emulsions stabilized by polymeric surfactants. The factors affecting the rheology of emulsions were briefly discussed. This was followed by a section on interfacial rheology and its correlation with emulsion stability. The bulk rheology of oil-in-water and water-in-oil emulsions stabilized by polymeric surfactants was described. Both steady-state and viscoelastic investigations were described. These emulsions show a transition from predominantly viscous to predominantly elastic response as the frequency of oscillation exceeds a critical value. This allows one to obtain... 

Measurement of the droplet size distribution, rates of flocculation, Ostwald ripening and coalescence are also described in Chapter 6. These methods should be applied for agrochemical emulsions to ensure their long-term physical stability. In addition, the bulk rheology of the system should be investigated after storage at various temperatures. 

These systems are widely used in many food products. A section will be devoted to the interfedal phenomena in food colloids, in particular their dynamic properties and the competitive adsorption of the various components at the interlace. The interaction between proteins eind polysaccharides in food colloids will be briefly described. This is followed by a section on the interaction between polysaccharides and surfactants. A short review will be given on surfactant association structures, microemulsions and emulsions in food [3]. Finally, the effect of food surfactants on the interfacial and bulk rheology of food emulsions will be briefly described. The formation of aggregation networks and the application of fractal concepts is then considered. This is followed by a section on applications of rheology in studying food texture and mouth feel. 

This chapter outlines emulsion characterization techniques ranging from those commonly found infield environments to those in use in research laboratories. Techniques used in the determination of bulk emulsion properties, or simply the relative amount of oil, water, and solids present, are discussed, as well as those characterization methods that measure the size distribution of the dispersed phase, rheological behavior, and emulsion stability. A particular emphasis is placed on optical and scanning electron microscopy as methods of emulsion characterization. Most of the common and many of the less frequently used emulsion characterization techniques are outlined, along with their particular advantages and disadvantages. 

The answer is yes. In fact, in many colloidal systems containing large liquid-fluid Interfaces, such as foams and emulsions, there is abundemt evidence for the action of surface rheology in addition to the omnipresent rheology of the adjoining bulk phases. Let us give three illustrations. 

Some approaches analyzed directly flic influence of flic stabilizing adsorption layers and concluded that diere is a dependence of the stability of an emulsion on flic interfacial concentration and the sum of inter-molecular interactions (8—10). Murdoch and Leng (11) pointed out the role of bulk and interfacial rheological parameters to describe these processes. This concept was further treated by several authors (12—14). A very comprehensive approach was given by Wasan and co-workers (15,16) who considered the surface shear and dilational rheology, and also some hy-drody-namic parameters in their analysis of emulsion films. 

Wasan and coworkers (63,65,174) extended techniques for studying film rheology of the foam lamella to studies of crude-oil emulsion lamella. Using a capillary balance technique and light interferometry, the film thinning of foams was studied with and without chemical demulsifiers, with solvent properties changed, etc. (182). They confirmed that there were two contributions to emulsion stability - a struc-tiual component that originates from the nature of the bulk phase, and an adsorbed-layer contribution to film stability (170). This will be covered in another chapter in this series. 

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. 

Bulk physicochemical properties of emulsions 5.2.6.1 Emulsion rheology... 

Surfactants play a major role in the rheology of food emulsions. Both interfacial and bulk rheologies have to be considered and these will be summarized below. 

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