Laboratoire Matière et Systèmes Complexes

November 30th 11h30

There will be a live diffusion with Zoom. Contact Michael Berhanu to attend the video seminar.

Surface instability of shear-thickening suspensions down an inclined plane

Baptiste Darbois Texier, Chargé de recherche CNRS

Laboratoire Fluides, Automatique et Systèmes Thermiques, Université Paris Saclay, Orsay

Shear-thickening suspensions exhibit a dramatic increase of viscosity above a critical shear stress. This phenomenon is very well known for a mixture of cornstarch and water, a fluid that is widely used for scientific dissemination. Despite its notoriety, it is only recently that shear-thickening has been understood as resulting from a frictional transition which occurs above a critical stress set by a repulsive interaction between particles. In this presentation, we will focus on the stability of a thin film of cornstarch suspension flowing down an inclined plane. I will detail the experiments that we realized to determine the onset of destabilization of this free-surface flow. These measurements first reveal that, at low packing fractions, the destabilization corresponds to a Kapitza instability that is well-established for Newtonian fluids. The main surprise here is that at high packing fractions, a new instability emerges at Reynolds numbers much smaller than the Kapitza threshold. We will explore how the recent framework developed to model shear-thickening, and particularly the existence of S-shape flow curves at high packing fractions, allows to rationalize this new instability. Finally, we will propose a simple mechanism for this destabilization and discuss its extension beyond the particular case of cornstarch suspension.