Emergent Structures in Colloidal Suspensions Sedimenting Down an Incline

The presence of a nearby no-slip boundary strongly affects the structures emerging in sedimenting colloidal suspensions. We combine experiments, theory and large scale numerical simulations to study the dynamics of colloidal suspensions sedimenting down an incline. By varying the inclination angle we observe different regimes of sedimentation. At low angles, when sedimentation is mostly directed toward the floor, the suspensions forms a monolayer with a dense traveling front. This traveling front can be described using a simple one dimensional nonlocal PDE. The front then transitions into finger-like structures whose width depends on the particle size and height from the floor. As the inclination angle increases, more and more particles are lifted away from the floor due to hydrodynamic interactions. The particle height distribution becomes bimodal: a second layer of more rapid particles forms, the fingers then move faster and exhibit a larger wavelength. After a characteristic time, the fingers sediment back on the incline, only to be lifted again by the flows generated by the particles at the rear. This cycle leads to unusual and rich long-time dynamics.