Abstract below

Sylvain Joubaud

Laboratoire de physique – Ens de Lyon

Bubble rise in granular suspensions

Many natural and industrial processes involve complex multiphase flows where gas, liquid, and solid particles interact. Understanding the physical mechanisms that govern these systems—particularly how microscale properties such as grain size, shape, and surface roughness influence macroscopic dynamics—remains a major challenge. In this talk, I will describe studies on the rise of gas bubbles in Newtonian fluids and dense granular suspensions through laboratory experiments.

In the first part, we analyze the rise of a single bubble in a Hele-Shaw cell. For Newtonian fluids, we characterize the evolution of bubble velocity, aspect ratio, and drag coefficient across the transition from viscous to inertial regimes. We then explore bubble dynamics in density-matched suspensions at low Reynolds numbers, quantifying how the vertical velocity depends on particle volume fraction. Surprisingly, bubbles rise faster in these suspensions than in particle-free liquids of the same effective viscosity.

In the second part, we study a 3D configuration: the draining of a bottle – an everyday yet rich example of two-phase fluid dynamics. While the alternation between rising bubbles and downward liquid jets is relatively well understood, we propose to further complicate the phenomenon by investigating the draining of suspensions. By varying both the suspension composition and the outlet diameter, we reveal counter-intuitive behaviors the flow rate remains constant during, even as the speed of the liquid free surface exhibits different regimes depending on particle accumulation.