Two and three dimensional ordered structures in electro-magneto-rheological fluids*

Weijia Wen

Department of Physics
Clear Water Bay
Hong Kong

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