RHEOLOGY OF A FLUID OF ENERGY AND ENVIRONMENTAL INTEREST: RESPONSE TO OSCILLATORY SHEAR OF DENSE SUSPENSIONS

M. Minale
 
Department of Engineering, University of Campania “Luigi Vanvitelli”, Via Roma 29, 81031 Aversa (CE)
 
*Corresponding author: E-mail:  mario.minale@unicampania.it, Phone: +39 081 5010292.

Abstract

Suspensions are ubiquitous both in the environment and in energy applications. The thermal and rheological properties of the suspending fluid are significantly modified by the suspended particles whose microstructure primarily affects the suspension behavior. The microstructure can be altered by the flow, and we here investigate the response of a very simple Newtonian, non-Brownian, inertialess, dense suspension of rigid hollow glass spheres to oscillatory shear, both after the very first oscillatory cycles and after a long time sweep oscillatory experiment.

We first focus on the first two or three cycles of oscillations. Experimental and numerical results agree and allow to prove that at very small strain amplitudes the oscillatory shear only induces the rotation of few couples of touching particles and the complex viscosity results slightly smaller than the steady one, at intermediate amplitudes the oscillatory shear induces the breakage of particle clusters and the microstructure modifies so to minimize particle collisions, while for very large strains the oscillatory flow reshuffles the particles inducing a microstructure as clustered as the steady state one but with a different angular distribution function. We showed that most of the microstructure rearrangement occurs soon after the flow inversion of the first cycle.

In long time sweep oscillatory experiments the suspension response resulted dependent on the amplitude of the applied strain, and, unexpectedly, on the angular frequency. Two different regimes were individuated depending on the applied strain. For values smaller than 1 the complex viscosity depends on the frequency, for values larger than 1, it is rate independent. In the first regime, the dependence on the applied strain amplitude and the angular frequency can be lumped into a single parameter: The maximum shear rate. The presence of non-hydrodynamic force, so small to be neglected in simple shear, can explain the observed behavior. Using a minimal hydrodynamic model, we show that van der Waals attraction gives rise to this behavior showing also that the rate dependence is accompanied by diverging particle diffusivities and pronounced cluster formations after repeated oscillations. We also showed that in the presence of weak attractions a new transition to irreversibility occurs below an ω-dependent critical amplitude.

Keywords: Non-Brownian suspensions, rheology, oscillatory shear, microstructure.