Self-assembly in model magnetic inks

We have studied the stability of ordered structures in simulated suspensions of magnetic particles similar to those used in the manufacture of magnetic tape. We previously reported (1997) that a monodisperse colloidal suspension of acicular uniformly magnetized particles forms a smectic ordered phase consisting of planar layers within which the particles are oriented perpendicular to the plane. One reason why these structures have not been seen experimentally may be that real magnetic inks have a distribution of particle sizes. We have generalized the simulation to allow this, using a log-normal length distribution. We find that the smectic structure is stable against a small amount of polydispersity-a system of particles with a 10% length variation forms a smectic structure similar to the monodisperse system, whereas 25% variation is sufficient to suppress the ordering. Work is under way to quantitatively establish the phase boundaries for the smectic ordering.