Condensed phases of paramagnetic colloids

We experimentally and theoretically study the condensed phases of spatially confined superparamagnetic spherical colloids. Experimentally we observe the micron sized colloids confined between glass walls by video microscopy, while further insight into the observed structures is provided by Monte Carlo simulations and by ground state energy minimization with genetic algorithms. The colloidal interactions are induced by external magnetic fields with one component perpendicular to the plane of confinement and the other component rotating in the plane. Depending on the relative magnitudes of these two components the system changes from purely repulsive to purely attractive. Additional control over the profile of the interaction is obtained by varying the thickness of the confining cell.

In the case of core softened repulsive interactions we observe clustering of colloids without the attraction. A large part of the theoretically predicted phases in systems of core-softened particles, including expanded and close-packed hexagonal, square, chain-like, stripe/labyrinthine, and honeycomb phase have been experimentally and numerically confirmed. When the in-plane component is large enough so that the interaction is purely attractive, the kinetics becomes an important factor which determines the foam-like structures observed in the experiment. In the intermediate regime with long-range repulsion and short-range attraction curious froth phases appear that can be explained only when many-body demagnetization effects are accounted for. Very similar phases have recently been observed in type-I superconductors.

Co-authors:
P. Ziherl, N. Osterman, D. Babic, I. Poberaj, J. Fornleitner, G. Kahl, and D. Frenkel