Condensation and ordering of colloidal spheres dispersed in a ferrofluid

Polystyrene microspheres of the same size dispersed in ferrofluid produce voids which have been denoted magnetic holes. The magnetic moment of one sphere (hole) is equal to the magnetic moment of the ferrofluid displaced by the sphere. The system may essentially be made two-dimensional by confining the dispersion between two glass plates with separation slightly larger than the sphere diameter. It is thus possible to have a model system of spheres with dipolar interactions which may be varied by external magnetic fields. Experimental studies to be discussed include aggregation of magnetic holes in external fields and the dynamics of magnetic holes in rotating external fields. The temporal evolution of the very complex trajectories are classified according to knot theory.