Solid boundaries: a new way to to a pick a length scale at low Reynolds number

At low Reynolds number, hydrodynamic interactions between suspended particles are long ranged and strongly depend on the presence of nearby boundaries. In particular, collective motion in a suspension can be strongly modified by a nearby wall. Combining experiments, large scale numerical simulations and continuum models, we investigate two surprisingly overlooked examples that illustrate the effect of a wall on collective motion: (1) the fingering instability in suspensions of particles rotating above a floor (microrollers) (2) the lengthscale selection in suspensions of particles sedimenting parallel to a wall. We will show how these two examples unveil a universal mechanism of lengthscale selection by boundaries at low Reynolds number.