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 α‐relaxation is related to d...