This work examines the steady shear viscosity of dilute polydisperse bubble suspensions generated in a mixture of mineral oil
and span 80. We proved theoretically that, in polydisperse bubble suspensions, the shear-thinning behavior spans a capillary
number (Ca) range between 0.01 and 100, instead of occurring at Ca~1, which is the case for monodisperse suspensions. However,
for the effect of polydispersity to become apparent, the bubble size distribution should be bimodal, with very small and very
large bubbles having similar volume fractions. In any other case, we can consider the polydisperse suspension as monodisperse,
with a volume-weighted average diameter (d43). To confirm the theoretical results, we carried out steady shear rheological tests.
Our measurements revealed an unexpected double power-law decay of the relative viscosity. To investigate this behavior further,
we visualized the produced bubble suspensions under shear. The visualization experiments revealed that bubbles started forming
clusters and threads at average capillary number around 0.01, where we observed the first decay of viscosity. CFD simulations
confirmed that under the presence of bubble clusters and threads the fluid streamlines distort less, thus resulting in a decrease of
the suspension viscosity. Consequently, we can attribute the first decay of the relative viscosity to the formation of bubble clusters
and threads, proving that the novel shear-thinning behavior we observed is due to a combination of bubble clustering and
deformation
