Baptiste Darbois Texier (FAST) - Surface instability of shear-thickening suspensions down an inclined plane 

When you mix cornstarch with water, you get a very strange fluid that flows like milk when gently stirred but turns rock-solid when impacted at high speed. This phenomenon, called shear-thickening, has made cornstarch suspensions very popular 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. This progress has provided rheological laws that now permit to move forwards and tackle the behavior of shear-thickening suspension in hydrodynamical configurations. In this presentation, we will focus on the stability of a thin film of cornstarch suspension flowing down an inclined plane and on the regular and prominent waves that develop at the free-surface (see image below). I will present our measurements of the onset of destabilization and the characteristics of the surface waves. 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. I will explain 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 generic mechanism for this destabilization that occurs at low inertia and we will discuss its extension beyond the particular case of cornstarch suspension.