Sedimentation to form rough, quasi-one-dimensional interfaces

The quasi-two-dimensional sedimentation of silica particles in viscous fluids results in quasi-one-dimensional rough surfaces. These surfaces are rough on all length scales between the particle size and the cell size, but different roughness exponents are observed in two well defined length-scale regimes. The range of hydrodynamic forces should play an important role in determining which, if either, length-scale regime shows universal properties. The strong similarity between the height-height correlations of the surface and the density-density correlations inside the flow at longer length scales suggests that the roughness at longer length scales is very closely tied to the hydrodynamic interactions in the fluid. Measurements have been performed at three different cell aspect ratios and at three different fluid viscosities and in no case is there an observable change in the crossover length scale of the system.