Viscosity of dilute Na-montmorillonite suspensions in electrostatically stable condition under low shear stress

We have studied the rheological behavior of dilute suspensions of electrostatically dispersed Na-montmorillonite to elucidate the importance of the electroviscous effect under the condition of zero stress limits. The viscosities of the suspensions were measured using an original spiral-type viscometer that consisted of two measuring cylinders connected with a one-meter-long capillary tube. Extremely dilute suspensions, whose volume fractions ranged from 2.0 × 10−4 to 2.0 × 10−3, were used for the experiments. The ionic strength of the suspensions was controlled to be less than 0.001 M. Thus, a fully developed electrical double layer was formed. It was confirmed that the measured viscosities of the suspensions increased with a decrease in their ionic strength, in accordance with a manifestation of the electroviscous effect. The viscosity increased markedly owing to the secondary electroviscous effect under the limit of the salt-free condition. To analyze this effect, we focused on the effective radii of the suspended montmorillonite particles. We estimated their effective radii by fitting the Dougherty–Krieger equation to the viscosity data. We found that the obtained effective radii depended significantly on the ionic strength in the low Peclet number regime, which was similar to the theoretical dependence of the effective radii determined from the interaction balance between the particles governed by hydrodynamic, electrostatic, and diffusive energies. Thus, the increase in the effective radius with a decrease in the ionic strength gives rise to the expansion of the electric repulsive force, resulting in an increase of the viscosity of this clay suspension. In addition, the magnitudes of the effective radii were greater than the Debye length. This indicates that even when the distance between the montmorillonite particles is greater than the Debye length, the electrical repulsive force affects the interaction between the particles.

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► We focused on the electroviscous effects of dilute Na-montmorillonite suspensions.
► The viscosity was measured using an original spiral type viscometer.
► An enhancement of viscosity was remarkable in the limit of salt free condition.
► We analyzed this effects based on the effective collision radius.