Shear-thinning and shear-thickening emulsions in shear flows

TL;DR

This study uses numerical simulations to analyze how shear-thinning and shear-thickening behaviors of carrier fluids influence the rheology of emulsions, revealing that local shear rates critically affect effective viscosity.

Contribution

It demonstrates the impact of non-Newtonian carrier fluids on emulsion viscosity and highlights the importance of local shear rate distributions over mean shear rates.

Findings

Effective viscosity increases with shear-thickening fluids.

Effective viscosity decreases with shear-thinning fluids.

Local shear rate spectrum is crucial for understanding emulsion rheology.

Abstract

We study the rheology of a two-fluid emulsion in semi-concentrated conditions; the solute is Newtonian while the solvent an inelastic power law fluid. The problem at hand is tackled by means of direct numerical simulations using the volume of fluid method. The analysis is performed for different volume fractions and viscosity ratios under the assumption of negligible inertia and zero buoyancy force. Several carrier fluids are considered encompassing both the shear-thinning and thickening behaviors. We show that the effective viscosity of the system increases for shear-thickening fluids and decreases for the shear-thinning ones for all the viscosity ratios considered. The changes in the emulsion viscosity are mainly due to modifications of the coalescence in the system obtained by changing the carrier fluid property: indeed, local large and low shear rates are found in the regions between two interacting droplets for shear-thickening and thinning fluids, respectively, resulting in increased and reduced local viscosity which ultimately affects the drainage time of the system. This process is independent of the nominal viscosity ratio of the two fluids and we show that it can not be understood by considering only the mean shear rate and viscosity of the two fluids across the domain, but the full spectrum of shear rate must be taken into account.