Motion and mixing of a viscous droplet located on a super-hydrophobic surface using resonant vibration under the vertical and horizontal Ac field

To develop an electrostatic stirrer or mixer for a small amount of liquid, resonant vibrating motion of a water droplet located on a super-hydrophobic surface was investigated experimentally. A water droplet under an ac electric field with resonant frequency changes drastically its shape, repeating extension and shrinkage state alternatively. To enhance liquid flow in a droplet, both the horizontal and vertical electric fields were applied to the droplet. From the images of the droplet taken by high-speed video camera, the degree of droplet deformation was evaluated by deformation rate for a width and a height. Viscosity of a droplet was adjusted by concentration of polyethylene glycol. With a resonant frequency, the deformation ratio was increased significantly even for a viscous droplet. Moreover, the droplet height at a shrinkage state during a vibration varied periodically with a longer period. The mixing of an ionized water droplet and a viscous droplet were demonstrated.