To realize a highly parallel optical detection in bead-based bioanalytical assays, we investigate the hydrodynamic aggregation of bead suspensions in a hexagonally periodical monolayer by a pressure-driven flow through a microfluidic structure. This device consists of one inlet channel connected to a shallow chamber with a depth that only slightly exceeds the diameter of the beads. To enforce the aggregation of the beads, the flow leaves the chamber via outlet channels possessing a depth smaller than a bead diameter. This way the outlets act as barriers to the beads and force them to accumulate in the chamber.