Glass transition in a two-dimensional system of magnetic colloids

We describe experiments on binary mixtures of superparamagnetic colloidal particles confined by gravity to a flat horizontal water-air interface. The colloids repel each other because of their magnetic dipole moments induced by a vertical external magnetic field B. By tuning B, the effective temperature of the system can be adjusted over several orders of magnitude. Particle coordinates are monitored by video-microscopy over more than five decades in time. Measured radial pair-distribution functions g(r) and mean-square displacements illustrate that this system is an ideal model of a two-dimensional (2D) glass former. We find that the effects of small amounts of aggregated particles only weakly affect the averaged structure and dynamics. Locally, a small number of elementary structural elements are observed each characterized by a special triangular shape. These triangles arrange in dense mostly space-filling arrays and account for the essential features of g(r). The long-time a-relaxation is related to drifts of arrays as well as erosion due to single particle and collective hopping events.