Rotation dynamics and internal structure of self-assembled binary paramagnetic colloidal clusters.

Abstract

We study experimentally and theoretically the dynamics of two-dimensional self-assembled binary clusters of paramagnetic colloids of two different sizes and magnetic susceptibilities under a time-varying magnetic field. Due to the continuous energy input by the rotating field, these clusters are at a state of dissipative nonequilibrium. Dissipative viscoelastic shear waves traveling around their interface enable the rotation of isotropic binary clusters. The angular velocity of a binary cluster is much slower than that of the magnetic field; it increases with the concentration of big particles, and it saturates at a concentration threshold. We generalize an earlier theoretical model to successfully account for the observed effect of cluster composition on cluster rotation. We also investigate the evolution of the internal distribution of the two particle types, reminiscent of segregation in a drop of two immiscible liquids, and the effect of this internal structure on rotation dynamics. The binary clusters exhibit short-range order, which rapidly vanishes at a larger scale, consistent with the clusters' viscoelastic liquid behavior.