Sommaire:

When fluids are forced to flow, instabilities are commonly observed in simple and complex fluids due to inertia or to elasticity. Shear-thickening, the process by which the viscosity of concentrated particulate dispersions dramatically increases above some critical load, is another widespread phenomenon that can be interpreted in terms of an underlying instability. While recent theoretical work has narrowed down the potential instabilities at play, their experimental demonstration is sorely lacking and mechanisms accurately explaining shear-thickening are still a subject of heated debate.

Our experimental work uncovers for the first time the local origin of the unsteady large-scale dynamics characteristic of shear-thickening in a dense suspension of cornstarch grains. We solicit the suspension under a constant shear stress and image its flow using echography. Introducing and adapting statistical tools from turbulence physics, we pin down the origin of fluctuations: macroscopic regions organize as transient localized bands that travel perpendicular to the flow direction and proliferate as the shear stress is increased. These large-scale “coherent structures” are scattered in a random background reminiscent of turbulence. These key observations urge to introduce stress spatiotemporal correlations in models and to simulate dense three-dimensional systems together with the shearing boundaries

Pour plus d'informations, merci de contacter Truzzolillo D.