We have observed both two dimensional [1] and three dimensional [2]
ordered structures in electromagnetorheological (EMR) fluids by using
uniform-sized microspheres coated with a inner layer of nickel and
outer insulating materials. For the 2D structures, the magnetic
microspheres floating on a liquid meniscus were seen to form planar
crystal structures with lattice constant which is variable as a
function of applied normal magnetic field. Other symmetries, such as
oblique, centered rectangular, square, and even local formations of
quasicrystals with five-fold symmetry, were also obtained under a
tilted external magnetic field, where the structural transition from
2D to 1D can be also observed. Here the balance between the repulsive
magnetic interaction and the ``attractive'' interaction, due to the
weight of the particles projected along the surface tangent, is the
basic underlying physical mechanism. Three dimensional ordered
structures with body-centered tetragonal (BCT) or face centered cubic
(FCC) symmetries were obtained by dispersing the coated microspheres
in liquid and applying crossed electric and magnetic fields.
* Work done in collaboration with L. Zhang, P. Sheng, N. Wang, H. Ma, Z. Lin, W. Y. Tam, and C. T. Chan