(340e) Broken Symmetry in the Electrokinetic Flow Surrounding Asymmetric Colloidal Dimers

We report electric field induced propulsion of asymmetric colloidal dimers including both dielectric and metal-dielectric dimers. Under a perpendicularly applied AC electric field, the asymmetric dimers exhibit lateral motion on conductive substrates. Interestingly, the direction of the propulsion can be reversed by tuning both frequency and salt concentration. In addition, dielectric dimers can further assemble into chiral clusters, which behave like micro-propellers and rotate in opposite directions based on their handiness. However, all autonomous motions will disappear when the dimers stand up (i.e., align with the external field) at high frequencies. Neither spheres nor symmetric dimers show such propulsion under similar experimental conditions. All of the above observations can be explained by the symmetry-breaking in the electrohydrodynamic flow surrounding the dimers. By solving the standard electrokinetic model, we develop both a theoretical model and numerical simulations to investigate the detailed distribution of both electric and flow fields. Moreover, the delicate balance between the electrohydrodynamic and induced-charge electroosmotic flow, which have very different frequency dependence, could well explain the rich behavior that we have observed for the metal-dielectric dimers.