Two-and three-dimensional ordered structures formed by electro-magnetorheological colloids

Abstract We have observed two- and three-dimensional ordered structures in electro-magnetorheological fluids by using uniform-sized microspheres coated with an inner layer of nickel and an outer layer of insulating material. The coated microspheres, when suspended in oil, exhibit electro-magnetorheological responses. The 2D structures were formed by floating the microspheres on a liquid meniscus. A planar hexagonal structure was observed, with variable lattice constant as a function of applied normal magnetic field. Other planar symmetries, such as oblique, centered rectangular, and square lattices, were also obtained by tilting the external magnetic field away from the normal. The balance between the magnetic interaction and the “attractive” interaction, due to the weight of the particles projected along the surface tangent, is the basic underlying physical mechanism responsible for the planar structures. Three-dimensional ordered structures, with body-centered tetragonal or face-centered cubic symmetries, were obtained by dispersing the coated microspheres in liquid and applying crossed electric and magnetic fields. A martensitic transition between these two crystal symmetries was observed as a function of the ratio between the magnetic field and the electric field.