Viscosity effects, emulsion

Hand lotions, of the emulsion type, are also, in most cases, oil in water emulsions. A true emulsifying agent is usually used to form the emulsion and for cleansing properties. Vegetable gums, like tragacanth or Irish moss extract, are added, not only to stabilize the emulsion but to increase the emollient properties, because of the added viscosity effect. 

The model system used by Mabille et al. [149, 150] was a set of monodisperse dilute (2.5 wt% of dispersed oil) emulsions of identical composition, whose mean size ranged from 4 p.m to 11 p.m. A sudden shear of 500 s was applied by means of a strain-controlled rheometer for durations ranging from 1 to 1500 s. All the resulting emulsions were also monodisperse. At such low oil droplet fraction, the emulsion viscosity was mainly determined by that of the continuous phase (it was checked that the droplet size had no effect on the emulsion viscosity). The viscosity ratio p = t]a/t]c = 0.4 and the interfacial tension yi t = 6 mN/m remained constant. 

This work shows that high shear rates are required before viscous effects make a significant contribution to the shear stress at low rates of shear the effects are minimal. However, Princen claims that, experimentally, this does not apply. Shear stress was observed to increase at moderate rates of shear [64]. This difference was attributed to the use of the dubious model of all continuous phase liquid being present in the thin films between the cells, with Plateau borders of no, or negligible, liquid content [65]. The opposite is more realistic i.e. most of the liquid continuous phase is confined to the Plateau borders. Princen used this model to determine the viscous contribution to the overall foam or emulsion viscosity, for extensional strain up to the elastic limit. The results indicate that significant contributions to the effective viscosity were observed at moderate strain, and that the foam viscosity could be several orders of magnitude higher than the continuous phase viscosity. 

The presence of a thermodynamically incompatible polysaccharide in the aqueous phase can enhance the effective protein emulsifying capacity. The greater surface activity of the protein in the mixed biopolymer system facilitates the creation of smaller emulsion droplets, i.e., an increase in total surface area of the freshly prepared emulsion stabilized by the mixture of thermodynamically incompatible biopolymers (see Figure 3.4) (Dickinson and Semenova, 1992 Semenova el al., 1999a Tsapkina et al., 1992 Makri et al., 2005). It should be noted, however, that some hydrocolloids do cause a reduction in the protein emulsifying capacity by reducing the protein adsorption efficiency as a result of viscosity effects. 

There are a number of key points to be made about the variables used In the equations. First, the equation forms used for the high salt concentration (1.0 M NaCl) are simple quadratic and cubic forms using pH and the square and cube of pH as Independent variables. The high salt concentration negated the emulsion Inhibiting effects of the Isoelectric point. The percent of the variation In the function properties accounted for by these equations ranged from about 80 percent for emulsion viscosity to over 98 percent for emulsion capacity. 

Although they are a relatively small volume product—approximately 75,000 tons produced in 1949 (126)—interest in asphalt emulsion has continued at a high level. Abraham (6) has reviewed the patent literature relative to the types of emulsifying agents used, while commercial practice has been discussed by Day (16). The most common emulsifiers are sodium or potassium soaps of tall oil, abietic acid, or Vinsol resin, or colloidal clays such as bentonite for adhesive base emulsions. Lyttleton and Traxler (53) studied the flow properties of asphalt emulsions, and Traxler (122) has investigated the effect of size distribution of the dispersed particles on emulsion viscosity. A decrease in particle size uniformity was found to be accompanied by a decrease in consistency because particles of various size assume a more loosely packed condition than do those of the same size. 

For electrostatically or sterically interacting drops, emulsion viscosity will be higher when droplets are smaller. The viscosity will also be higher when the droplet sizes are relatively homogeneous, that is, when the drop size distribution is narrow rather than wide. The nature of the emulsifier can influence not just emulsion stability but also the size distribution, mean droplet size, and therefore the viscosity. To describe the effect of emulsifiers on emulsion viscosity Sherman [215] has suggested a modification of the Richardson Equation to the following form ... 

Several practical formulae have been developed for estimating the effect on emulsion viscosity of changes in key variables such as temperature, water content, and droplet size distribution, in which adjusting factors for each property are obtained from empirical correlations. An illustration is provided by Rimmer et al. [763]. Such formulae may also contain a term representing changes in emulsion droplet size due to droplet coalescence that occurs with time as the emulsion moves through the pipeline ( ageing ). 

Of the other factors which influence the viscosity of w/o emulsions, the effect of emulsifiers and solvents is well established (3). Although... 

Expressing the effective viscosity of the gas-liquid emulsion with Einstein s equation, and replacing the viscous shear stress T as the product of effective emulsion viscosity and the maximum liquid velocity gradient, and after combining Eq. (2) into Eq. (1), one arrives at the following relationship between a and a° ... 

This chapter presents a brief review of the rheological classification of fluids and instruments used for viscosity measurements. A discussion of the rheology of suspensions and how it relates to that of emulsions is given. Predictive correlations for emulsion viscosity are discussed in detail. The effect of added solids to an emulsion is fully treated, and its relation to a bimodel system is discussed. 

The concentric cylinder viscometers are supplied with different inner and outer cylinders such that various gap widths can be formed. For rheological measurements of emulsions and suspensions, care must be taken to ensure a gap width of at least 20 times the suspended particle size in order to avoid wall effects. Moreover, experiments should be conducted with different gap widths to ensure the absence of any wall slip that is usually encountered in emulsion viscosity measurements (J6). However, uniformity of shear rate can be achieved only when the ratio of the gap width to the inner cylinder radius is small. 

The chemical nature and the concentration of an emulsifying agent also play a role in determining the viscosity of emulsions (37). The average particle size, particle size distribution, and the viscosity of the continuous phase (to which an emulsifier is normally added) all depend upon the properties and concentration of emulsifying agent. Also, ionic emulsifiers introduce electroviscous effects, leading to an increase in the emulsion viscosity. 

Oil-in-water emulsions provide a cost-effective alternative to heated pipelines or diluents for transportation of heavy crude oil or bitumen. A typical ""transport emulsion is composed of 70% crude oily 30% aqueous phase, and 500-2000 ppm of a stabilizing surfactant formulation. The resulting emulsion has a viscosity in the 50-200-cP range at pipeline operating conditions. Nonionic surfactants have the advantage of relative insensitivity to the salt content of the aqueous phase. The ethoxylated alkylphenol family of surfactants has been used successfully for the formation of stable emulsions that resist inversion. Correlations have been developed for prediction of emulsion viscosity as a function of emulsion life and process conditions. The cost of stabilizing surfactants is estimated at 0.50 to 1.00 per barrel of crude oil for a transportation distance of200 to 400 miles. 

The emulsion water concentration has a double effect on pumping costs. For a fixed fiow rate of crude oil, as the water content increases, the total volume fiowing increases, which increases the cost of pumping. However, additional water in the emulsion also reduces its viscosity and thereby lowers pumping costs. These two factors tend to offset each other for emulsion crude-oil concentrations in the 50-70% range. At higher oil concentrations, the viscosity increases more rapidly, and this situation reduces the incentive for further water reductions. Correlations of emulsion viscosity as a function of water concentration, such as were described earlier, are required to perform this analysis. Calculations were performed for a hypothetical pipeline system with the following characteristics ... 

Figure 13. Effect of continuous phase composition on viscosity and emulsion viscosity. Experimental data Oemulsion, o organic phase fitted data —lnrj0 = exp -K (vol%)+K2] lnrj = lnri0 + K3.

Figure 15. Effect of phase ratio Vj/Vj on emulsion viscosity Tjem and splitting efficiency 7js Vj...total water content I>splitted .obtained continuous water phase after splitting.

Equations proposed by Sherman for predicting viscosities from apparent volume fractions and particle diameters were useful in analyzing the effects of formulation and preparation variables and aging on emulsion viscosities. 

The effect of oil viscosity on initial emulsion viscosity is not clear from these experiments. The St. Lina crude is about six times as viscous as the California crude. The apparent viscosity of the lower viscosity St. Lina Crude emulsion (2 x 10 moles NaOH/gram oil) is less than 50% greater than the lowest viscosity moderately stable California crude emulsion (4.0 x 10 NaOH). The average particle size of the St. Lina emulsion is 7 microns while that of the Shell crude emulsion is about 3 microns (see Figure 8). Since particle sizes, particle size distributions and types of oil are different, no conclusions can be drain about the influence of oil viscosity. There is, however one fact which should be emphasized, namely that viscosities 600 times lower than that of the crude were observed for 60% St. Lina crude emulsions. 

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