The transport behavior of a system of gravitationally driven superparamagnetic colloidal particles is investigated. The motion of the particles through a narrow channel is governed by magnetic dipole interactions, and a layered structure forms parallel to the walls. The arrangement of the particles is perturbed by diffusion and the motion induced by gravity leading to a density gradient along the channel. Our main result is the reduction of the number of layers. Experiments and Brownian dynamics simulations show that this occurs due to the density gradient along the channel. DOI: 10.1103/PhysRevLett.97.208302 PACS numbers: 82.70.Dd, 61.72.ÿy, 64.60.Cn, 75.40.Mg Lattice defects can be introduced in perfectly ordered crystals by deformations due to external forces. Despite their importance, the dynamical behavior of single dislocations is still rather poorly understood, because they are difficult to generate and observe on the atomic scale. Therefore, the observation of this behavior in a model system will lead to insights that can be transferred to a broad range of systems. Studies of isolated lattice defects have recently become possible in static systems of colloidal crystals [1]. Experiments on the behavior of such defects in a nonequilibrium system can lead to a better understanding of the transport behavior of a wide range of systems.In biological systems, the transport of interacting particles through narrow constrictions is of high importance for many processes, for example, for the size selectivity of transport in ion channels [2]. The complexity of such systems allows one to only make hypotheses on the underlying physics governing such phenomena. Model systems that can be easily accessed experimentally can reveal many of the underlying processes [3]. This requires studies of particle transport through channels with variously shaped walls. In order to perform these studies, transport through channels with straight walls has to be well understood in experiments shown in this Letter.In this work, we report on studies of the transport behavior of colloids in a quasi-two-dimensional (2D) setup. The colloids are superparamagnetic; therefore, the interaction energy can be continuously tuned by the application of an external magnetic field [4]. The particles are driven by gravity through a narrow constriction (channel). Such driven diffusive systems serve as model systems for theoretical studies of nonequilibrium behavior [5]. In addition, such a system resembles the classical case of a quantum point contact in mesoscopic electronics [6,7] or in metallic single atom contacts [8,9]. These contacts exhibit transport in electronic channels due to quantization effects. These quantum channels can be seen as analogous to the layers in the macroscopic transport, since both occur due to the interaction of the particles with the confining potential. A classical version of a similar scenario can be built on a liquid helium surface, which is loaded with charges. For such a system, the formation of layers has been repor...

The behavior of particles driven through a narrow constriction is investigated in experiment and simulation. The system of particles adapts to the confining potentials and the interaction energies by a self-consistent arrangement of the particles. It results in the formation of layers throughout the channel and of a density gradient along the channel. The particles accommodate to the density gradient by reducing the number of layers one by one when it is energetically favorable. The position of the layer reduction zone fluctuates with time while the particles continuously pass this zone. The flow behavior of the particles is studied in detail. The velocities of the particles and their diffusion behavior reflect the influence of the self-organized order of the system.

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