Rheology studies how materials react to mechanical stresses. However, conventional techniques only provide general information (such as viscoelastic modules), without access to what is happening at the microscopic level. This severely limits our understanding of certain complex materials such as amorphous materials known as ‘soft jammed’ materials. These materials, such as pastes, emulsions, foams, etc., have their elementary constituents (grains, droplets, bubbles, etc.) densely confined by their neighbours. However, under stress, these elements can become unblocked and reorganise themselves, giving rise to highly heterogeneous deformations and complex structural rearrangements.


To better capture these internal dynamics, researchers from an international collaboration involving three CNRS physics laboratories have developed a new experimental approach called tomo-rheoscopy, which combines a shearing device with time-resolved 3D X-ray tomography and applied it to liquid foams. This method makes it possible, for the first time, to individually track nearly 100,000 moving bubbles, while accessing mechanical stresses at all scales, from a single bubble to the entire foam sample. They were thus able to establish a direct link between microscopic rearrangements in the structure (neighbourhood changes) and the overall rheological response. These rearrangements are accompanied in particular by a redistribution of stresses according to a quadrupole pattern (see figure), predicted theoretically but measured experimentally here for the first time within a three-dimensional material.

The present study was carried out in the following CNRS laboratories:

• Laboratoire de Physique de l’ENS de Lyon (LPENSL, CNRS / ENS de Lyon)
• Institut de physique de Nice (INPHYNI, CNRS / Université Côte d'Azur)
• Laboratoire Interdisciplinaire de Physique (LiPhy, CNRS / Université Grenoble Alpes) 

This methodology can be transposed to other soft matter systems, opening up new perspectives for modelling and understanding the mechanical behaviour of many complex materials. Tomo-rheoscopy should thus become a reference tool for exploring the mechanics of amorphous materials. These results are published in the journal Nature Communications.