Fluid-particle interactions at the surface of a granular bed: erosion and accretion

The interactions of particles and liquids contribute to the complexity of many environmental, industrial and biological processes, including the erosion of a sediment bed, the preparation and transport of slurry, and the decontamination of surfaces. However, the local fluid-particle interactions that lead to the suspension or deposition of grains at the surface of a granular bed cannot be captured by classical rheological approaches. In this talk, I will discuss two examples of recent studies in which we address these complex couplings through a quantitative experimental description of both the fluid flow and particle transport. I will first describe the erosion of a granular bed by a fluid flow in a biologically inspired system. Indeed, some bottom-dwelling fish, such as flounders, are seen flapping their fins with oscillating motions until the sand particles are lifted up, before falling on their backs. In our model experiment, a foil is placed above a sand bed to mimic the fin motion. We characterize the vortices generated by the oscillations of the plate and the conditions to erode the granular bed. In the second part of my talk, I will consider the flow of a dry granular material on a wet granular bed. This situation is important in industrial blending and geophysical processes, yet it remains poorly understood because of the interplay between the particles, the fluid and interfacial effects. Our experiments show that the wet granular bed grows over time by trapping of the flowing dry particles. The accretion rate exhibits a complex time dependence that we rationalize by accounting for the local mechanisms of grain capture.