Striped Patterns in Radially Driven Suspensions with Open Boundaries

Abstract

We study the motion of radially driven fluid-immersed particles in a novel Hele-Shaw cell with open boundaries. The initially uniform suspension forms a striped pattern within a specific range of horizontal oscillation frequencies and for sufficiently large amplitudes. We observe that the initial coarsening dynamics of the stripes gradually slows down and the pattern reaches a steady state after a few minutes. The distance between the stripes in the steady state exhibits an exponentially saturating increase with increased oscillation amplitude or frequency. The width of the stripes decreases as a power-law with the frequency while its amplitude dependence follows a logistic function. We propose a mechanism-based on the interplay between shear stress, hydrodynamic interactions, and frictional forces-to link the structural characteristics of the stripes to the properties of the oscillatory external drive. We discuss the similarities of the striped patterns in our experiments to the ripples observed along curved sandy coasts.