  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 to a precipitation, or to 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 microns 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 90 degrees. From the various results
obtained, among which an exponential decay of the tangent of the angle made
between the rod and the field, we concluded that the rods are
superparamagnetic.
