In this work, we found experimentally that there exist fairly strong fluid flows in AC electrowetting, which can be utilized as a means to mix the fluids in EWOD-based micro-devices. We visualized the internal flow. There may exist two distinct flow-generation mechanisms; one is the droplet oscillation, and the other is the electrohydrodynamic flow. The flow pattern is significantly dependent on the applied AC frequency. At low frequencies (represented here by 1 kHz), the center of the vortices is located somewhat randomly and the flow directs upward near the symmetric axis. At high frequencies (represented by 128 kHz), however, a pair of vortices having quite a regular structure is clearly visible and the flow directs downward near the symmetric axis. The flow patterns are strongly dependent on the position of the point electrode. The droplet surface undergoes a periodic oscillation (visualized by a high-speed camera) with a frequency exactly twice the frequency of applied electrical signal. The oscillating interface can generate a steady streaming. However the numerical results show that there exists no electric field at low AC frequencies. On the contrary, there exists quite a strong electric field inside the droplet at high frequencies. It means the electrohydrodynamic flow cannot be generated at the low frequency region, and the droplet oscillation might cause the flow generation at low frequencies. We also demonstrated the flow can be beneficially utilized as a mixing method.

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