Towards local rheology of emulsions under Couette flow using Dynamic Light Scattering

TL;DR

This study uses heterodyne Dynamic Light Scattering to measure local velocities in emulsions under shear, revealing wall slip effects and shear-thinning behavior, highlighting the importance of local measurements in rheology.

Contribution

It provides the first local velocity profiles of emulsions under shear, demonstrating the complexity of flow behavior and questioning the applicability of a single global flow curve.

Findings

Dilute emulsion is Newtonian.

Concentrated emulsion exhibits shear-thinning with exponent 0.4.

Local measurements reveal wall slip and complex flow behavior.

Abstract

We present local velocity measurements in emulsions under shear using heterodyne Dynamic Light Scattering. Two emulsions are studied: a dilute system of volume fraction $\phi=20$ % and a concentrated system with $\phi=75$ %. Velocity profiles in both systems clearly show the presence of wall slip. We investigate the evolution of slip velocities as a function of shear stress and discuss the validity of the corrections for wall slip classically used in rheology. Focussing on the bulk flow, we show that the dilute system is Newtonian and that the concentrated emulsion is shear-thinning. In the latter case, the curvature of the velocity profiles is compatible with a shear-thinning exponent of 0.4 consistent with global rheological data. However, even if individual profiles can be accounted for by a power-law fluid (with or without a yield stress), we could not find a fixed set of parameters that would fit the whole range of applied shear rates. Our data thus raise the question of the definition of a global flow curve for such a concentrated system. These results show that local measurements are a crucial complement to standard rheological tools. They are discussed in light of recent works on soft glassy materials.