Viscosity electroviscous effect

Electroviscous effect occurs when a small addition of electrolyte a colloid produces a notable decrease in viscosity. Experiments with different salts have shown that the effective ion is opposite to that of the colloid particles and the influence is much greater with increasing oxidation state of the ion. That is, the decrease in viscosity is associated with decreased potential electrokinetic double layer. The small amoimt of added electrolyte can not appreciably affect on the solvation of the particles, and thus it is possible that one of the determinants of viscosity than the actual volume of the dispersed phase is the zeta potential. 

The electroviscous effect present with solid particles suspended in ionic liquids, to increase the viscosity over that of the bulk liquid. The primary effect caused by the shear field distorting the electrical double layer surrounding the solid particles in suspension. The secondary effect results from the overlap of the electrical double layers of neighboring particles. The tertiary effect arises from changes in size and shape of the particles caused by the shear field. The primary electroviscous effect has been the subject of much study and has been shown to depend on (a) the size of the Debye length of the electrical double layer compared to the size of the suspended particle (b) the potential at the slipping plane between the particle and the bulk fluid (c) the Peclet number, i.e., diffusive to hydrodynamic forces (d) the Hartmarm number, i.e. electrical to hydrodynamic forces and (e) variations in the Stern layer around the particle (Garcia-Salinas et al. 2000). 

The primary electroviscous effect occurs, for a dilute system, when the complex fluid is sheared and the electrical double layers around the particles are distorted by the shear field. The viscosity increases as a result of an extra dissipation of energy, which is taken into account as a correction factor pi" to the Einstein equation ... 

The increasing dilution of flexible polyelectrolytes at low ionic strength, the reduced viscosity may increase first, reach a maximum, and then decrease. Since a similar behavior can also be observed even for solutions of polyelectrolyte lattices at low salt concentration, the primary electroviscous effect was thought as a possible explanation for the maximum, as opposed to conformation change. 

For poly electrolyte solutions with added salt, prior experimental studies found that the intrinsic viscosity decreases with increasing salt concentration. This can be explained by the tertiary electroviscous effect. As more salts are added, the intrachain electrostatic repulsion is weakened by the stronger screening effect of small ions. As a result, the polyelectrolytes are more compact and flexible, leading to a smaller resistance to fluid flow and thus a lower viscosity. For a wormlike-chain model by incorporating the tertiary effect on the chain... 

The effect of pH on the intrinsic viscosity testing gives a minimum at the isoelectric point at pH 5.1 for gelatin B to pH 9.1 for gelatin A. from electroviscous effect analysis shows that 0.001 M ionic strength the hydrodynamic radius is at its maximum. 

Three electroviscous effects have been noted in the literature.27 The primary electroviscous effect refers to the enhanced energy dissipation due to the distortion of the diffuse layer from spherical symmetry during flow. The analysis for low diffuse layer potentials has been clearly reviewed by van de Ven28 and the result for the intrinsic viscosity with Ka—> oo is ... 

The secondary electroviscous effect is the enhancement of the viscosity due to particle-particle interactions, and this of course will control the excluded volume of the particles. The most complete analysis is that due to Russel30 and we may take this analysis for pair interactions as the starting point. Russel s result gives the viscosity as... 

Our objective in this chapter is modest, namely, to provide a general discussion of the electroviscous effects and to present a few equations that serve as guidelines for understanding the effects of colloidal forces on the viscosity of dispersions. The underlying theories are rather complicated and fall outside our scope. 

The electroviscous effects and the other effects discussed in Sections 4.7a-c lead to what is called non-Newtonian behavior in the flow of dispersions. In the next section, we begin with a brief review of the basic concepts concerning deviations from Newtonian flow behavior and then move on to consider how high particle concentrations and electroviscous effects affect the flow and viscosity. 

In Section 4.7c we outlined the types of effects one can expect in the response of charged dispersions to deformation. In this section, we present some results for the viscosity of charged colloids for which electroviscous effects could be important. As mentioned above, we shall not go into the theoretical details behind the equations since they require a fairly advanced knowledge of fluid dynamics and, in some cases, statistical mechanics. Moreover, some of... 

The first analysis of the primary electroviscous effect dates to 1916 when Smoluchowski presented the following equation for the intrinsic viscosity ... 

A corrected and more general analysis of the primary electroviscous effect for weak flows, i.e., for low Pe numbers (for small distortions of the diffuse double layer), and for small zeta potentials, i.e., f < 25 mV, was carried out by Booth in 1950. The result of the analysis leads to the following result for the intrinsic viscosity [rj] for charged particles in a 1 1 electrolyte ... 

The variations in the intrinsic viscosity predicted by the primary electroviscous effect are often small, and it is difficult to attribute variations in the experimentally observed [17] from the Einstein value of 2.5 to the above effect since such variations can be caused easily by small amounts of aggregation. Nevertheless, Booth s equation has been found to be adequate in most cases. Further discussions of this and related issues are available in advanced books (Hunter 1981). 

The intrinsic viscosity [17] in the above expression includes the primary electroviscous effect. The experimental data of Stone-Masui and Watillon (1968) for polymer latices seem to be consistent with the above equation (Hunter 1981). Corrections for a for large values of kRs are possible, and the above equation can be extended to larger Peclet numbers. However, because of the sensitivity of the coefficients to kRs and the complications introduced by multiparticle and cooperative effects, the theoretical formulations are difficult and the experimental measurements are uncertain. For our purpose here, the above outline is sufficient to illustrate how secondary electroviscous effects affect the viscosity of charged dispersions. 

How do charges on particles change the viscosity of a dispersion What are electroviscous effects How do they differ from the viscoelectric effect ... 

The electroviscous effect can influence the results of viscosity determinations, but this can be avoided by using a sufficiently high concentration of salt12 when linear specific viscosity concentration versus concentration curves are obtained. Gelation effects with Fe++, Fe+++ and Cu++ ions also occur127 and so should be avoided. 

The electroviscous effects are observed as variations of viscosity upon application of outer electric fields, and as build-up of potential gradients upon flow of such fluids. See also -> electroconvection, electrorheological... 

The effective viscosity of a suspension of particles of types other than rigid particles has also been theoretically investigated. Taylor [22] proposed a theory of the electroviscous effect in a suspension of uncharged liquid drops. This theory has been extended to the case of charged liquid drops by Ohshima [17]. Natraj and Chen [23] developed a theory for charged porous spheres, and Allison et al. [24] and Allison and Xin [25] discussed the case of polyelectrolyte-coated particles. 

In this chapter, we first present a theory of the primary electroviscous effect in a dilute suspension of soft particles, that is, particles covered with an ion-penetrable surface layer of charged or uncharged polymers. We derive expressions for the effective viscosity and the primary electroviscous coefficient of a dilute suspension of soft particles [26]. We then derive an expression for the effective viscosity of uncharged porous spheres (i.e., spherical soft particles with no particle core) [27]. 

Figure 15.3 indicates that for the silica microspheres, the potential and viscosity both follow the expected behavior predicted by the classical DLVO theory. On the other hand, the nanosize fnmed silica exhibits a discrepancy between the expectation of DLVO theory and the experimental results that is, as the of the nanosize fumed silica increases, viscosity sharply increases. Hence factors such as particle crowding, particle ordering, and electroviscous effects will also impact viscosity, in addition to aggregate or network formation. 

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. 

Addition of electrolytes to a suspension decreases the thickness of the double layer and reduces electroviscous effects, an effect reflected in the reduced viscosity of the suspension. 

Because of the polyelectroly tic nature, pectin solutions need to be made in excess of salt, usually in 0.05 0.1 M sodium chloride or phosphate, and use the same solvent for dilution (isoionic dilution) (Pals and Hermans, 1952). This is because, unlike neutrol polymers, the viscosity of dilute solution of polyelectrolytes displays unique dependence on concentration. As shown in Figure 9.5, the qsp of sodium pectate exhibits a maximum in pure water and low concentration of salt, a phenomenon caused by the so-called electroviscous effect. When the salt concentration is... 

Electroviscous Effect The increase in apparent viscosity of a dispersion of charged species caused by their mutual electrostatic repulsion. 

Electroviscous Effect Any influence of electric double layer(s) on the flow properties of a fluid. The primary electroviscous effect refers to an increase in apparent viscosity when a dispersion of charged colloidal species is sheared. The secondary electroviscous effect refers to the increase in viscosity of a dispersion of charged colloidal species that is caused by their mutual electrostatic repulsion (overlapping of electric double layers). An example of the tertiary electroviscous effect would be for polyelectrolytes in solution where changes in polyelectrolyte molecule conformations and their associated effect on solution apparent viscosity occur. 

A theoretical explanation for the increase of the viscosity t) could be found in the so-called electroviscous effect. It is well known that an electrolyte solution streaming between two charged walls shows an increase of the apparent shear viscosity. Assuming that the results obtained for plane parallel channels in a steady state by Levine et al. " may be used for the film situation, it was found that a maximum increase of about 20% can be expected for the viscosity of the solution inside the film compared to the bulk value. This electroviscous effect is expected to be important only in equilibrium films where an overlap of the electrical double layers occurs but nevertheless this phenomenon cannot explain the full discrepancy between theory and experiment. 

As outlined earlier (Chapter 3), electrically charged colloidal particles are surrounded by an electrical double layer. This leads to important rheological effects. The primary electroviscous effect is a consequence of the fact that the electrical double layer [Figure 8.10(a)] is deformed from its spherical shape by the shear field. Construction of the double layer ahead of the particle and its disintegration behind the particle take a finite time [Figure 8.10(b)], This causes an increase in the intrinsic viscosity, which for low zeta-potentials ( < 25 mV) is proportional to the square of this potential ... 

The sodium chloride was added to eliminate the electroviscous effect shown by sodium alginate solutions (6, 11, 12), Rose (11) found that O.IN sodium chloride swamped this effect, and higher concentrations, up to 0,5N sodium chloride, either had no additional effect or slightly increased the viscosity of 0.1 to 0.5% solutions of sodium alginate. 

Another important attribute of carrageenin sols is viscosity and here, again, the presence of salts must be considered. In the absence of salts, the electroviscous effect due to the predominance of the polymer as the negatively charged particle is observed (Figure 4) (7, 4). Addition of salts rapidly lowers the viscosity to... 

In the simplest experimental conditions, the viscosity of a suspension depends on the concentration, shape, and even potential (the "electroviscous." effects) of the particles (63-65). Figure 12 (63) is an example of experimental data reported on different systems the behavior observed is Newtonian except for very high-volume fractions, and in such concentrated suspensions the frequency of particle-particle collisions is so high that the stability of the whole system will be largely affected. It is hence difficult, if not impossible, to control the flow properties of pharmaceutical suspensions without the use of special additives, almost systematically included in practical formulations. 

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