(9c) Propulsion of Asymmetric Dielectric Particles Under High Frequency AC Electric Fields

Under an AC electric field that is applied perpendicularly between two electrodes, it has been shown that asymmetric particles can propel along the substrate due to a few different mechanisms. For example, metallodielectric Janus particles propel with the dielectric hemisphere leading due to the induced charge electrophoresis. On the other hand, dielectric dumbbells made from silica and polystyrene spheres can also move with the silica lobe leading due to unbalanced electrohydrodynamic flow. These particles typically move at frequencies below 10 kHz. We recently, however, discover that the same silica-polystyrene dumbbells can also propel in the high frequency regime (~100 kHz - MHz), which is well beyond the charge relaxation time of electric double layer near the electrode. Interestingly, the propulsion direction of the asymmetric dumbbells at high frequency is opposite to that in the low frequency regime, i.e., the polystyrene lobe is leading. Our experiments with tracer particles further show that there is a strong and contractile hydrodynamic flow surrounding both silica and polystyrene lobes. In fact, the flow is so strong that the whole dumbbell, while propelling, is lifted at least ten microns beyond the substrate. The flow surrounding the polystyrene lobe is stronger than the silica lobe, which generates a net flow towards the silica lobe, consistent with the net propulsion direction towards the polystyrene side. We further show that this unbalanced electrohydrodynamic flow at high frequencies is a universal phenomenon not only applied for inorganic particles but also for biological cells such as yeast. By exploiting this flow, we build microscopic motors that can selectively bind, transport, and deliver specific types of cells among a mixture.