Structure and tracer-diffusion in quasi–two-dimensional and strongly asymmetric magnetic colloidal mixtures

– We study theoretically and experimentally static and diffusional properties of a strongly asymmetric binary mixture of super-paramagnetic colloids confined to an air-water interface. The colloids interact via repulsive dipolar forces, induced by a magnetic field applied perpendicular to the planar interface. Brownian dynamics (BD) computer simulations are employed to analyse the microstructure and the tracer-diffusion of both components. The profound interaction asymmetry leads to unusual features in the partial pair distribution functions, and to significantly enhanced tracer-diffusion. We find good agreement between our experimental data and the computer simulation results. Introduction. – A lot of effort has been devoted over the past years to study the structural and dynamical properties of quasi–two-dimensional (Q2D) model dispersions of monodisperse super-paramagnetic colloidal spheres confined to a liquid-gas interface and interacting by induced repulsive dipolar forces [1–4]. These model systems are of particular interest both from the experimental and theoretical point of view, since the interaction potential is well characterised and the two-dimensional trajectories can be followed over an extended time range using video imaging. The interaction strength can be precisely tuned by the applied magnetic field. This allows to study in great detail interesting phenomena like the freezing and melting transition via an intermediate hexatic phase of quasi–long-range orientational order [5]. While static properties are independent of the solvent flow, it is known that the dynamics of Q2D monodisperse systems is strongly affected by the solvent-mediated hydrodynamic interactions (HI). This gives rise to remarkable effects like the divergence of certain shorttime diffusional transport coefficients [3, 4, 6], and the enhancement or de-enhancement of (∗) E-mail: [email protected]