Microstructured particles for electromagneto-rheological (EMR) applications

Common electrorheological (ER) fluids are suspensions of simple dielectric particles in non-conducting fluid where the fluid properties are controllable through the application of an external electric field. Guided by first-principles calculations based on the minimization of free energy density, we have fabricated novel doubly-coated micro-particles with a conducting inner layer and an insulating outer layer for ER applications. In good agreements with the calculations, orders of magnitude enhancement in the yield stress under an electric field was obtained using the doubly-coated particles when compared to that of bare or singly-coated (without the conducting layer) particles. We have also fabricated multiply-coated particles of glass spheres that exhibit simultaneous electro- and magneto- responses. Under crossed electric and magnetic fields, a transition from the body-centered-tetragonal (BCT) structure to the face-centered-cubic (FCC) structure was observed when the ratio between the magnetic and the electric fields exceeded a minimum value. Both the FCC crystal formation and the BCT-FCC transition scenario are correctly predicted by the first-principles calculations