Single and ensemble response of colloidal ellipsoids to a nearby ac electrode

Abstract

Anisotropic colloids can be used as building blocks for fabricating assemblies with complex microstructure and function. External electric fields are one key tool used to assemble such colloidal structures. Herein, we utilized a parallel plate electrode geometry to drive motion of micrometer scale ellipsoidal colloids. The response of pseudo 2D layers of ellipsoids with systematically altered aspect ratios was measured as a function of electric field strength and frequency. We found there to be a weak dependence of ellipsoid ensemble structure on aspect ratio. Namely, regions of aggregation and separation were only slightly dependent on particle aspect ratio. We found that ellipsoids with a smaller aspect ratio tended to align with the electric field, while high aspect ratio ellipsoids did not in response to an electric field of the magnitude and time scale used herein. In addition, we found the kinetics of aggregation of colloids to be very similar regardless of aspect ratio. Complementary experiments utilizing these data demonstrated long-lasting deposition of ellipsoids in energetically unfavorable orientations, which is being used as a model system for the measurement of light scattered from an evanescent wave by an ellipsoid.