Stories of grains in fluids :
from impact in dense suspensions to micro-avalanches in plant cells

Yoël Forterre — January 23, 2018

Abstract :
Over the past twenty years, great progress has been made in our understanding of how granular matter flows, a topic of both basic and practical interest. A distinctive feature of granular flows is that they display a flow threshold and a maximal packing fraction, beyond which no permanent flow is possible (jamming transition). In this seminar, I will illustrate how physical phenomena at the particle level (fluid drag, colloidal forces, particle agitation) can profoundly affect the flow behavior of a mixture of grains and fluids close to the jamming transition. First, I will show that the short-time response of a dense suspension under impact is entirely controlled by the coupling between transient effects (Reynolds dilatancy) and the fluid pressure. I will then discuss how the addition of short-range repulsive forces between grains can conspire with friction to yield the dramatic shear-thickening behavior displayed by some suspensions like cornstarch. Finally, I will show how the sensor of gravity of plants relies on intracellular micro-avalanches flowing in an active liquid.

Biography :
Yoël Forterre is Research Director at CNRS and works in the Particulate Flow Group at the IUSTI laboratory (CNRS/Aix-Marseille Université). His main research topics concern the physics of granular flows, complex fluids and more recently plants biomechanics. He is co-authored of a book on the physics of granular media and is director of the GDR ‘Biophysique et biomécanique des plantes’ (CNRS/INRA). He received an ERC grant for the project PLANTMOVE in 2014 and was awarded in 2017 the “Ernest Dechelle” Physics price of the French Academy of Sciences.

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