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.