Puzzling bubble rise speed increase in dense granular suspensions

TL;DR

This study reveals that air bubbles rise faster in dense granular suspensions than in pure liquids of similar viscosity, due to reduced dissipation caused by shear-induced particle migration.

Contribution

It uncovers the counterintuitive phenomenon of increased bubble rise speed in dense suspensions and links it to non-uniform particle distribution and dissipation reduction.

Findings

Bubbles rise faster in suspensions than in particle-free liquids.

The rise speed increase correlates with particle volume fraction.

Experimental data aligns with Suspension Balance Model simulations.

Abstract

We present an anomalous experimental observation on the rising speed of air bubbles in a Hele-Shaw cell containing a suspension of spherical, neutrally-buoyant, non-Brownian particles. Strikingly, bubbles rise faster in suspensions as compared to particle-less liquids of the same effective viscosity. By carefully measuring this bubble speed increase at various particle volume fraction and via velocity field imaging, we demonstrate that this strange bubble dynamics is linked to a reduction in the bulk dissipation rate. A good match between our experimental data and computations based on Suspension Balance Model illustrates that the underlying mechanism for this dissipation-rate-deficit is related to a non-uniform particle distribution in the direction perpendicular to the channel walls due to shear-induced particle migration.