The transport behavior of a system of gravitationally driven colloidalparticles is investigated. The particle interactions are determined by thesuperparamagnetic behavior of the particles. They can thus be arranged in acrystalline order by application of an external magnetic field. Therefore themotion of the particles through a narrow channel occurs in well-defined lanes.The arrangement of the particles is perturbed by diffusion and the motioninduced by gravity. Due to these combined influences a density gradient formsalong the direction of motion of the particles. A reconfiguration of thecrystal is observed leading to a reduction of the number of lanes. In thecourse of the lane reduction transition a local melting of thequasi-crystalline phase to a disordered phase and a subsequent crystallizationalong the motion of the particles is observed. This transition is characterizedexperimentally and using Brownian dynamics (BD) simulations.
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