Thermal coarsening of uniaxial and biaxial field-structured composites

When a suspension of colloidal particles is subjected to a strong electric or magnetic field, the induced dipolar interactions will cause the particle s to form organized structures, provided a sufficient permittivity or perme ability mismatch exists, respectively, between the particles and the suspen ding liquid. A uniaxial field will produce uniaxial structures, and a biaxi al field, such as a rotating field, will produce biaxial structures, and ei ther of these structures can be pinned by polymerizing the continuous phase to produce field-structured composites. We have previously reported on the coarsening of field-structured composites in the absence of thermal effect s, i.e., Brownian motion. Athermal simulations are primarily valid in descr ibing the deep quenches that occur when the induced dipolar interactions be tween particles greatly exceed k(B)T. However, deep quenches can lead to ki netic structures that are far from equilibrium. By introducing Brownian mot ion we have shown that structures with significantly greater anisotropy and crystallinity can form. These structures have enhanced material properties , such as the conductivity, permittivity, and optical attenuation. Careful anneals at certain fixed fields, or at continuously increasing fields, shou ld produce more anisotropic structures than the deep quenches we have used to synthesize real materials. (C) 1999 American Institute of Physics. [S002 1-9606(99)70309-0].