Visualization and analysis of 3-dimensional AC electroosmotic flow in an elongated droplet*

When an alternating voltage is applied between two parallel electrodes, the charge induced on the electrode surface causes a slip velocity in the direction perpendicular to the electrodes, regardless of the polarity of voltage. This flow has been extensively studied as an alternating current electroosmotic flow(AC EOF), and its typical form is paired vortices on the electrodes. Due to its handful control in microchannel, AC EOF has been employed as the core mechanism of trapping and aligning particles in the electrolyte, or generating a manipulated flow as a micropump using asymmetric electrodes. However, two-dimensional approximation is often invalid for AC EOF within droplets where 3-dimensional constriction is dominant. In this study, we visualized three-dimensional motions of AC EOF in an electrolyte existing in the form of elongated droplets on symmetric planar electrodes and conducted a 3D numerical simulation with liquid-gas interface. In this system, we demonstrated the generation of flow in the direction parallel to the electrodes, which cannot be predicted by two-dimensional assumption and analyzed the cause of the flow.