  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.
