Lateral Propulsion of Asymmetric Particles Under Perpendicular AC Electric Fields

We have discovered lateral propulsion of dielectric dimers in directions that are perpendicular to the applied AC electric field. We provide comprehensive evidence to show that asymmetric properties in geometry, composition, and surface charge can all influence the EHD flow surrounding two sides of the particles differently in both flow direction and magnitude. Such a difference results in an unbalanced EHD flow surrounding asymmetric particles and induces particle motion. Built upon a simple dumbbell model, a scaling law, and detailed electrokinetic calculation, we can predict both the propulsion direction and speed of dimers quantitatively. These predictions are validated by our experiments for different types of asymmetric dimers. The propulsion mechanism revealed here is not limited to colloidal dimers and should be universal for other types of asymmetric particles. Such knowledge is important for both building intelligent colloidal robots and studying the out-of-equilibrium behavior of active matter.