Keywords: Electrowetting, mixing, electrothermal flow Abstract: Mixing is a key issue in microfluidics, including droplet-based “digital” microfluidics. In electrowetting, internal flow patterns inside drops can be generated without any lateral translation if the drops are excited with AC voltage. Two regimes can be distinguished: at AC frequencies of the order of the eigenfrequency (typically O(1kHz) or less), the drops periodically oscillate between states of high and low contact angle. Despite the periodicity, there is a symmetry breaking in the drop shape between the spreading and the receding phase, which causes a time-averaged net flow inside the drop that promotes mixing. This process can be described using a model based on capillary wave-driven Stokes drift. For somewhat higher frequencies, this mechanism becomes progressively inefficient, because the liquid cannot follow the applied voltage anymore. At substantially higher AC frequencies (typically >>10kHz, depending on the salt concentration), however, a new driving mechanism for internal flows sets in, as reported by Nichols and Gardniers (Anal. Chem. 79, 8699 (2007) and by Ko et al. (Langmuir 24, 1094, 2008). Under these conditions, the liquid no longer acts as a perfect conductor. The electric field penetrates into the drop and generates local Ohmic currents. These currents produce Joule heating and ultimately giving rise to electro-thermal flows inside the drops. Solving numerically for the distribution of the electrostatic field, the flow field, and the temperature distribution, we show that the flow velocity scales with the fourth power over a wide range of both the applied voltage and the AC frequency – in good agreement with the experiments by Ko et al.