Structure and dynamics of charged magnetic colloids

By coating cobalt ferrite nanoparticles with a silica shell, the polydispersity of the resulting core–shell particles can be reduced. Hereby, opposite to the case for conventional ferrofluids, self-organization to liquid-like and even crystalline structures in aqueous media is enabled. The resulting structures mainly originate from the predominant electrostatic repulsion of colloidal macroions bearing charged groups at the surface of the silica shell. Due to the small magnetic moment of the cobalt ferrite cores, however, these structures can be influenced by external magnetic fields or field gradients. While field gradients act as a magnetic trap for these particles, homogeneous fields induce an aligning of the magnetic momenta. Hereby a decrease of symmetry from spherical to cylindrical symmetry of the structures appearing can be observed. Due to collective phenomena, even interactions significantly smaller than the thermal energy can induce clearly observable structural distortions. Even in the absence of an external field, suspensions of such magnetic particles show an unexpected slow diffusion caused by hydrodynamic interactions.