[Soft Matter Café] Three-dimensional imaging of a deforming foam using fast X-ray tomography

Liquid foams are assemblies of bubbles within a continuous liquid phase. They are complex fluids, behaving as elastic solids below a yield stress and undergoing plastic flow above, with viscous dissipation. These rheological properties are difficult to relate to physical processes at the scale of bubbles, because foams are opaque. To circumvent this difficulty, we present the first experimental study coupling the controlled deformation of a foam in a rheometer with the simultaneous 3D imaging, using fast X-ray tomography. We quantify all quantities at the bubble scale: displacements, deformation, evolution of their contacts and, for the first time, a measure of the local elastic stress from the geometry of the gas/liquid interfaces. We validate this new measure by comparing it to the applied torque by the rheometer. We apply these descriptive tools to the study of the elementary plastic events in foams, the four-fold neighbour swapping of bubbles. We reveal that such events have a very clear precursor signature on the local stress. We also show that they redistribute the elastic stress in their surroundings with a quadrupolar symmetry, related to the fourfold neighbour swapping. These new experimental results provide unprecedented insight onto the rheology of foams, and more generally of amorphous materials, at the scale of the microstructure.

About the speaker: B. Dollet is a CNRS researcher and a part-time Associate Professor at Ecole polytechnique. He obtained a PhD in Physics in 2005 at the Université Joseph Fourier. After two years of post-doc in the group Physics of Fluids at the University of Twente (The Netherlands), he was appointed CNRS research in 2007 at the Institut de Physique de Rennes. He moved in 2017 in the LIPhy lab at the Université Grenoble Alpes. His scientific interests are various between soft matter physics, fluid mechanics and acoustics (acoustics and shock waves in foams, ultrasound contrast agent microbubbles, drops, bubbles and biomimetic studies in microfluidics, surfactant influence on thin film hydrodynamics, damping of sloshing waves…), but the study of foam rheology by image analysis methods has been a regular guideline in his career.