Electroviscosity

Figure 4 Typical viscosity response of a polysaccharide polyanion and a neutral molecule to concentration, showing electroviscosity in a dilute dispersion of the polyanion (negative slope segment) and linearity resulting from interactions and cancellation of electroviscosity (positive slope). P represents the polyanion and P° represents its neutral counterpart.

An initial negative slope of iq, vs ct in a dilute water dispersion (electroviscosity) of a polysaccharide is indicative of polyanionic character. Electroviscosity disappears in excess electrolyte solution and is nonexistent in neutral polymer viscosity profiles. 

The size of a polysaccharide molecule is an indeterminate property molecular weight, for example, depends on the analytical method. Electroviscosity and solute-solvent and solute-solute interactions affect the volume-gram relationship (density) and conformational dimensions. The degree of polymerization is not affected and is therefore a constant, reliable indicator of the molar mass. 

When an electrolyte solution flows in a microchannel driven only by the pressure gradient, the charged wall surface will increase the resistance, which is defined as the electroviscosity effect. This effect has been proven experimentally [1]. Dynamic models based on the Navier-Stokes equations have been built to model this effect [1]. The following example investigates this effect using the LPBM. 

Velocity profiles for flow in a homogeneous channel are shown in Fig. 7 for a channel having a width of 1 x 10 m, = 10 M, and dF/dx = 1 X 10 Pa/m, and the surface zeta potentials, range from 0 to 200 mV. When Ixj/J is very small (<100 mV), the velocity profile is almost same as the non-EDL channel flow profile. When xj/J is larger than 100 mV, the electroviscosity effect becomes noticeable, and the effect increases with li/ l. Unlike the regular viscosity effect, the electroviscosity effect mainly affects the velocity distribution near the wall away from the parabolic shape. 

Streaming current and electroviscosity can only appear with an ionic fluid and a microchannel with a nonconductive inner surface such as fused silica. The surface charging mechanism with a liquid can have several origins [1]. One is the ionization and dissociation of a chemical group and another is ion adsorption, also called ion binding, on a surface which has a low charge... 

Streaming Current and Electroviscosity, Fig. 1 Hydration and ionization of a fused silica surface by H2O... 

To calculate electroviscosity, we first define the volunaic flow rate using... 

Fig. 3 Electroviscosity as a function of microtube radius for a pure deionized water flow with an electrical surface potential of 57 mV...

Equation 39 gives the electroviscosity in a microtube with an inner charged surface at the potential v[/o. The reduced forward volumic flow rate observed experimentally is interpreted as an increased apparent viscosity. Using the expressions of Ki, K2, Es, and "3 (respectively, Eqs. 15b, 15c, 29, and 30), it is possible to evidence that /xj/x is a function of neither the average fluid velocity (Uo) nor the pressure gradient (Pz) but only a function of the micrornbe diameter and the electrokinetic parameters. 

The case of two parallel plates has been studied by Mala et al. [6] and will not be presented here. The electroviscosity equation obtained differs slightly from Eq. 39 due to the geometry. 

Figure 3 presents the variation of the electroviscosity for a fluid flow of pure deionized water in a microtube with a radius ranging from 1 to 24 pm. The increase in electroviscosity in a microtube is not perceptible for diameters above 15 pm. Below 15 pm the increase becomes perceptible with a maximum of 25 % for a microtube with a diameter of 2 pm. The model previously described and the mathematical assumptions are correct since the radius of the microtube is greater than the electrokinetic distance, which is about 1 pm for pure water. Experiments with microtubes smaller than the electrokinetic distance have not as yet been reported in the literature. 

---