Field-induced smectic ordering in model magnetic inks

We have performed Brownian dynamics simulations of model colloids of acicular (cigar-shaped) magnetic particles with aspect ratio 6.5, similar to Toda F ceramic-coated metal particles. In the absence of a magnetic field, these have a gel-like structure. We have investigated the effect of a magnetic field on this structure. At very low volume fractions the particles (after orienting along the field) form chains in the field direction. At more realistic volume fractions (3%-9%), we find a unique filamentary structure, in which the filaments themselves are smectically ordered. That is, the particles are stacked side-by-side with their poles in a common plane. The net effect of the orienting field is to promote aggregation into filaments. (C) 1997 American Institute of Physics.