Wall slip and bulk flow heterogeneity in a sludge under shear

TL;DR

This study explores the complex shear flow behavior of a nuclear industry sludge analog, revealing transitions from homogeneous flow to wall slip and oscillatory regimes, emphasizing the importance of local flow analysis.

Contribution

It provides new insights into the flow heterogeneity and wall slip phenomena in dilute sludge suspensions under shear, using combined rheometry and ultrasound imaging.

Findings

Flow transitions from homogeneous to plug-like with wall slip

Oscillatory slip velocity propagates along the vorticity direction

Large, quasi-periodic shear rate peaks near yield stress

Abstract

We investigate the shear flow of a sludge mimicking slurries produced by the nuclear industry and constituted of a dispersion of non-Brownian particles into an attractive colloidal dispersion at a total solid volume fraction of about 10%. Combining rheometry and ultrasound flow imaging, we show that, upon decreasing the shear rate, the flow transitions from a homogeneous shear profile in the bulk to a fully arrested plug-like state with total wall slip, through an oscillatory regime where strong fluctuations of the slip velocity propagate along the vorticity direction. When the shear stress is imposed close to the yield stress, the shear rate presents large, quasi-periodic peaks, associated with the propagation of local stick-and-slip events along the vorticity direction. Such complex dynamics, reminiscent of similar phenomena reported in much denser suspensions, highlight the importance of local flow characterization to fully understand sludge rheology.