Rheological behavior of emulsions

Here, as usual, h and R denote the film thickness and radius, and R is the curvature radius of the spherical part of the particle surface. The dependence of the dimensionless drag coefficient, fy, on the distance h for different values of the ratio R/R is illustrated in Figure 5.49. The increase of R/R and the decrease of hlR may lead to an increase of the drag force,/j, by an order of magnitude. That is the reason the film between a deformed particle and a wall can be responsible for the major part of the energy dissipation. Moreover, the formation of doublets and flocks of droplets separated by liquid films seems to be of major importance for the rheological behavior of emulsions. 

The stability of any emulsion is largely due to the nature of the film that is formed between two approaching droplets. Coalescence of drops in any emulsion system is a dynamic process. The rheological behavior of emulsions depends on the response of the thin liquid films and the plateau borders during shear and dilation. In real emulsions, the size and distribution of the drops is generally poly disperse. Hence,... 

Rogers [49] gave an overview of the means for controlling the rheological behavior of emulsions. He described the effect of the emulsifier type, of the emulsifier concentration,... 

Dubey, B. N., Duxenneuner, M. R., Kuechenmeister, C., Windhab, E. J. (2011). Infiuences of rheological behavior of emulsions on the spraying process. In Proceedings of 24th Annual Conference on Liquid Atomization and Spray Systems. 

The Bingham Fluid. The Bingham fluid is an empirical model that represents the rheological behavior of materials that exhibit a no flow region below certain yield stresses, tv, such as polymer emulsions and slurries. Since the material flows like a Newtonian liquid above the yield stress, the Bingham model can be represented by... 

Let us first consider an inverted W/O emulsion made of 10% of 0.1 M NaCl large droplets dispersed in sorbitan monooleate (Span 80), a liquid surfactant which also acts as the dispersing continuous phase. At this low droplet volume fraction, the rheological properties of the premixed emulsion is essentially determined by the continuous medium. The rheological behavior of the oil phase can be described as follows it exhibits a Newtonian behavior with a viscosity of 1 Pa s up to 1000 s 1 and a pronounced shear thinning behavior above this threshold value. Between 1000 s 1 and 3000 s1, although the stress is approximately unchanged, the viscosity ratio is increased by a factor of 4. 

Other relevant studies on physical properties of emulsions are those of McClements et al. (1993), Dickinson and Pawlowsky (1996), Dickinson et al. (1996), and Demetriades et al. (1997). The application of nuclear magnetic resonance (NMR) technique (Simoneau et al., 1993) and ultrasonic spectroscopy (Demetriades et al., 1996) to study the stability of emulsions were discussed. The rheological behavior of salad dressings and mayonnaises that are emulsions will be discussed in Chapter 5. 

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 rheological behavior of an emulsion can be Newtonian or non-Newtonian depending upon its composition. At low to moderate values of dis-... 

Vasiljevic D, Vuleta G, Dakovic LJ, Primorac M. Influence of emulsifier concentration on the rheological behavior of w/o/w multiple emulsions. Pharmazie 1994 49 933—934. 

An overview of the lithographic process is presented with particular emphasis on the role that surface and colloid chemistry plays. Recent research has shown the importance of these chemistries in fountain solution, ink and plate interactions and the effect of these interactions on the dynamic behavior of the lithographic ink on press. Data on the rheological behavior of preformed fountain solution/ink emulsions is presented along with an evaluation of prints made with the inks on an actual press run. The importance of pre-testing the emulsification behavior of printing inks in predicting their printability is demonstrated. 

Pons R, Solans C, Tadros ThF. Rheological behavior of highly concentrated oil in water (O/W) emulsions. Langmuir 1995 11 1966—1971. 

If the rheology of suspensions and emulsions is difficult to describe theoretically and to determine experimentally, in the case of polymer blends these difficulties reach another order of complication. It suffices to note that in blends both phases are viscoelastic, the viscosity ratio varies over a wide range, and morphology can be very complex. To understand the rheological behavior of blends, it is useful to refer to simpler systems that can offer important insight. The following systems (Table 7.2) are commonly considered and will be treated in the following discussion. 

Effects of addition of a compatibilizing block copolymer, poly(styrene-b-methyl methacrylate), P(S-b-MMA) on the rheological behavior of an immiscible blend of PS with SAN were studied by dynamic mechanical spectroscopy [Gleisner et al., 1994]. Upon addition of the compatibilizer, the average diameter of PS particles decreased from d = 400 to 120 nm. The data were analyzed using weighted relaxation-time spectra. A modified emulsion model, originally proposed by Choi and Schowalter [1975], made it possible to correlate the particle size and the interfacial tension coefficient with the compatibilizer concentration. It was reported that the particle size reduction and the reduction of occur at different block-copolymer concentrations. 

Gladwell, N., Rahalkar, R. R., and Richmond, P. (1985a). Rheological behavior of soya oil water emulsion Dependence upon oil concentration. /. Food Sci. 50, 440-443. 

The physical properties that influence rheolc ical behavior are internal phase content size, shape, and panicle size distribution viscosity and rheological behavior of the continuous phase and temperature. For the case of emulsions two additional parameters, droplet deformabilily and viscosity of the dispersed phase, should also be considered. 

The viscosity and rheological behavior of the continuous phase also modifies the behavior of emulsions and suspensions. In fact. Eqs. [50 -[53] establish that the larger the viscosity of the continuous phase (T) ), the larger the viscosity of the su.spension diJ. Furthermore, if the continuous phase is strongly viscoela,stic. die swelling and rod climbing may appear (26). Shear thickening has also been observed in concentrated emulsions of very viscous internal phase (27). 

The behavior of emulsions as a particular type of disperse system is controlled by many factors. There is a large number of properties of the corresponding liquid/ liquid interface which can be determined by well-established methods, such as dynamic surface tensions, adsorbed amount, exchange of matter across the interface, and dilational and shear rheology. Although first models exist, a general view... 

The rheological behavior of W/O/W emulsion studied with a cone-and-plate viscometer has shown a negative thixotropic flow pattern, mostly under low shear rate. Upon raising the shear rate or the shear time an increase in the shear stress was observed, which induced phase inversion... 

Note that the occurrence of nonNewtonian behavior means that the viscosity concept is no longer valid and that it has to be replaced by the concept of apparent viscosity. It is found that the rheological behavior of many concentrated emulsions may be rendered, at least approximately, by a power law variation in which the shear stress T is proportional to the th power of the shear rate y ... 

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