Reorientation kinetics of superparamagnetic nanostructured rods

The attractive interactions between oppositely charged species (colloids, macromolecules etc) dispersed in water are strong, and the direct mixing of solutions containing such species generally yields a precipitation, or a phase separation. We have recently developed means to control the electrostatically driven attractions between nanoparticles and polymers in water, and at the same time to preserve the stability of the dispersions. We give here an account of the formation of supracolloidal aggregates obtained by co-assembly of 7 nm particles with copolymers. Nanostructured rods of length comprised between 5 and 50 mum and diameter 500 nm were investigated. By application of a magnetic field, the rods were found to reorient along with the magnetic field lines. The kinetics of reorientation was investigated using step changes of the magnetic field of amplitude pi/2. From the various results obtained, among which is an exponential decay of the tangent of the angle made between the rod and the field, we concluded that the rods are superparamagnetic.