Abstract
An electrorheological and magnetorheological fluid has small spherical particles in a space where a uniform electric field is perpendicular to a uniform magnetic field. When the ratio between the electric field and magnetic field varies, structural transitions occur. Lf the electric (or magnetic) field is dominant, the ideal structure of the system is a body-centered tetragonal lattice with its fourfold rotational axis in the electric (or magnetic) field direction. When the electric field and magnetic field are compatible, under the dipolar approximation , the system may have a hexagonal close-packed (hcp) structure. However, because the energy difference between the hcp structure and face-centered cubic (fcc) lattice is small and compatible with the thermal energy at room temperature, the system may develop into a hcp-fce mixed structure when the electric field and magnetic field are compatible.