Electroviscous effect fluids

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 ... 

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... 

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 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... 

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. 

The net effect of the charge forces close to the wall is a diminished flow in the main flow axis. The fluid appears to exhibit an enhanced viscosity if its flow rate is compared with the flow without the EDL effects. This increase in apparent viscosity of a fluid is the electroviscous effect [4-6]. 

Electroviscous effects on fluid flow for ionic fluids in microchannels have been evidenced over the last decade experimentally [7-9] and are still a subject of widespread theoretical research [4, 10]. Ren et al. [5] found... 

The primary electroviscous effect is the retardation of the particle velocity (or the fluid flow in pores) due the polarisation of the EDL, which is a consequence of the relative motion between diffuse layer and the particle surface (Fig. 3.4). The polarisation, in turn, causes a flux of ions and solvent that opposes and decelerates the relative motion. This kind of deceleration affects particle sedimentation (von Smoluchowski 1903 Ohshima et al. 1984 Keh and Ding 2000) capillary flow... 

The mechanism(s) of a particulate fluid electroviscous effect is still not fully resolved and quantified. It is not strictly relevant to this work and is therefore not dealt with in detail. At this stage it can only be said that it is a very multi-parameter and multidisciplinary event and, secondly, it should be understood that there is little change in the viscosity p of the fluid as it is normally defined in its continuum context save for a derived effective or non Newtonian viscosity sense. The term electroviscous, which has often been used to describe the present class of fluids, is misleading in this case. Rather, the held imposes a yield stress type of property on the fluid which is similar to, but not the same as, that which is a feature of the ideal Bingham plastic. This can readily be seen by referring to Figs. 6.63 to 6.66 inclusive. It is alternatively possible to claim that either the plastic viscosity changes with shear rate or the electrode surface yield stress does. 

It is important to emphasize the need to measure the surface conductance as well as the zeta potential. If the surface conductance is neglected in Eq. (18) the zeta potential can be severely underestimated. It should also be noted that the equations derived above neglect electroki-netic effects on the flow by assuming a Poiseuille type flow through the microchannel. In actuality the streaming potential creates a reverse electro-osmotic flow in the channel that decreases the overall flow rate. The decreased velocity creates the appearance of an increased fluid viscosity and is known as the electroviscous effect. Generally this effect is prevalent in microchannels less than 50 xm. 

The viscosities of liquids, colloidal suspensions and polyclcctrolytcs or polymeric systems with and without an external electric field can be well described with the free volume concept and the derived viscosity equations are remarkably consistent with experimental results. The main topic of this book is the ER effect of ER fluids, which typically operate under an external electric field. The electroviscous effect that generally doesn t need an external electric field is only briefly covered. Attention is paid to deriving a more universal viscosity equation that can account for colloidal suspensions, pure liquids, and polymeric systems. The free volume concept has proved to be extremely important for such a task. 

Electroviscous fluid is a fluid that can drastically change its viscoelasticity by applying an external electrical field, with this effect called ihe ER effect. Electrorheology materials can include fluids but also ER rubbers, ER gels, and other forms [26, 27]. 

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