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.

You can also watch this video on savoirs.ens.fr or YouTube