Rheological behaviour of suspensions of bubbles in yield stress fluids

TL;DR

This study investigates how bubbles affect the flow and deformation of yield stress fluids, revealing that bubble stiffness and volume fraction influence rheological properties through capillary number effects.

Contribution

It introduces a tunable model system to systematically study the impact of bubble size and stiffness on yield stress fluid rheology, linking capillary numbers to macroscopic behavior.

Findings

Capillary numbers control the decrease of elastic and loss moduli.

Yield stress remains unaffected by bubble inclusion.

Gas volume fraction increases the consistency of the suspension.

Abstract

The rheological properties of suspensions of bubbles in yield stress fluids are investigated through experiments on model systems made of monodisperse bubbles dispersed in concentrated emulsions. Thanks to this highly tunable system, the bubble size and the rheological properties of the suspending yield stress fluid are varied over a wide range. We show that the macroscopic response under shear of the suspensions depends on the gas volume fraction and the bubble stiffness in the suspending fluid. This relative stiffness can be quantified through capillary numbers comparing the capillary pressure to stress scales associated with the rheological properties of the suspending fluid. We demonstrate that those capillary numbers govern the decrease of the elastic and loss moduli, the absence of variation of the yield stress and the increase of the consistency with the gas volume fraction, for the investigated range of capillary numbers. Micro-mechanical estimates are consistent with the experimental data and provide insight on the experimental results.