Negative Electrorheology of Self-Generated Networked Suspensions

Beyond the percolation concentration, suspended particles form a network in the medium, classifying the system as a networked suspension. The response of such suspensions to an applied electric field depends on the difference in dielectric constant between the particles and the medium. When non-polar particles (low dielectric constant) are dispersed in a polar medium (high dielectric constant), the application of an electric field can reduce the rheological parameters of the system; this effect is known as negative electrorheology. While the exact mechanism remains uncertain, proposed explanations include Quincke rotation and electromigration [1].

Crude oils at low temperatures behave as networked suspensions due to wax crystallization, and exhibit a reduction in viscosity and moduli upon electric field application [2,3]. This reduction is attributed to the breakage of the wax network into smaller aggregates. In these systems, the dispersed waxes have a low dielectric constant while the medium has a high dielectric constant. To investigate the underlying mechanism, model suspensions were prepared by suspending wax in solvents of different dielectric constants. Wax networks were formed by heating to dissolve the wax, followed by cooling to induce particle nucleation, growth, and the establishment of cemented interparticle connections.

About a 10-fold reduction in moduli is observed with the application of a 2 kV/mm field, and this reduction increases with increasing dielectric constant of the medium. The minimum electric field required for this reduction is found to vary inversely with the dielectric constant of the medium. The reduction in storage modulus increases exponentially with the duration of electric field application, and the corresponding rate constant varies linearly with the dielectric constant of the medium. The reduction in rheological parameters and the breakage of the wax network are attributed to Maxwell stresses generated under the applied electric field. Using the Taylor–Melcher leaky dielectric model, it is found to be compressive and sufficiently strong to break the network at high field strengths. These findings contribute to understanding the mechanism underlying the negative electrorheological effect observed in networked suspensions.