Effect, electroviscous

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 ... 

Equation 46 suggests that, maintaining pi constant, q, must depend linearly on if only a first-order electroviscous effect exists, and an increase in the electrolyte concentration implies a decrease in the thickness, 1/k, of the electrical double layer. 

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... 

Fig. 3. Study of the electroviscous effect of NaCl on gelatin B. a-Hydrodynamic radius, b-r /r o. c- Zeta potential at different pH (O.OOIM NaCl).

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. 

Li Jiang, Dahong Yang, and Shing Bor Chen. Electroviscous Effects of Dilute Sodium Poly(styrenesulfonate) Solutions in Simple Shear Flow. Macromolecules 2001, 34, 3730-3735. 

Ohshima H. Primary Electroviscous Effect in a Dilute Suspension of Soft Particles. Langmuir 2008, 24, 6453-6461. 

Rubio-Hernandez F.J., Carrique F., Ruiz-Reina E. The primary electroviscous effect in colloidal suspensions. Advances in Colloid and Interface Science 107 (2004) 51-... 

Rubio-Hernandez F. J., Ruiz-Reina E., and Gomez-Merino A. I. Primary Electroviscous Effect with a Dynamic Stern Layer Low Ka Results. Journal of Colloid and Interface Science 226,180-184 (2000). 

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 ... 

Figure 3.18 The primary electroviscous effect estimated from Equation (3.59) for spherical particles dispersed in 10 2 M KCl...

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... 

Figure 3.23 The tertiary electroviscous effect observed for particles of polystyrene latex with a copolymer of polyacrylic acid at the outer surface. The experimental points were obtained at pH 3 and 10. The dry particle radius was 75 nm and Ka 25...

It is convenient to think of the diffuse part of the double layer as an ionic atmosphere surrounding the particle. Any movement of the particle affects the particle s ionic atmosphere, which can be thought of as being dragged along through bulk motion and diffusional motion of the ions. The resulting electrical contribution to the resistance to particle motion manifests itself as an additional viscous effect, known as the electroviscous effect. Further,... 

The electroviscous effects are usually classified in three categories depending on the origin of the underlying mechanism. 

The secondary electroviscous effect refers to the change in the rheological behavior of a charged dispersion arising from interparticle interactions, i.e., the interactions between the electrical double layers around the particles. 

The term tertiary electroviscous effect is applied to the changes in the conformation of poly electrolytes that are caused by //t/ramolecular double-layer interactions. It is customary to extend this definition to include all effects in which the geometry of the system is altered as a result of double-layer interactions. 

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 secondary electroviscous effect is often interpreted in terms of an increase in the effective collision diameter of the particles due to electrostatic repulsive forces (i.e., the particles begin to feel the presence of other particles even at larger interparticle separations because of electrical double layer). A consequence of this is that the excluded volume is greater than that for uncharged particles, and the electrostatic particle-particle interactions in a flowing dispersion give an additional source of energy dissipation. 

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. 

As mentioned in Section 4.7c, the tertiary electroviscous effect is at least partly due to the expansion and contraction of particles arising from the conformational changes of the polyelectrolytes (adsorbed or chemically bound to the surface of the particles) with changes in... 

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

Hunter, R. J., Zeta Potentials in Colloid Science Principles and Applications, Academic Press, London, 1981. (Advanced level. The focus of this book is on the role of electrical double layers and zeta potential on electrophoresis and electroviscous effects. This volume presents some details on electrical double layers around nonspherical particles not discussed in the present book.)... 

Discuss this electroviscous effect in terms of the concepts of this chapter and Chapter 4. 

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