Towards enhanced bubble detachment within a thin liquid film by electrowetting with voltage modulation

This paper extends our previous bubble actuation study using a simple constant voltage by including the oscillating effect created by voltage modulation. Rather than normal contact angle change due to the constant voltage, voltage modulation exhibits preferable characteristics of periodical contact angle variation which is proved to be helpful for bubble detachment within a thin liquid film. Different waveform and frequency modulations were evaluated to acquire an optimal signal input for the purpose of inducing the maximum oscillation effects with which bubble detachment in a thin liquid film can be enhanced. The thick liquid film results show that the square waveform coupled with a frequency of 1 Hz allows for maximum contact angle change scope and induces the largest vertical bubble velocity. With the optimal signal, the tests of bubble detachment within a thin liquid film were conducted and characterized. Three different bubble detachment modes were observed and classified as follows: direct detachment, delayed detachment, and non-detachment. The actuation mechanism of the electrowetting effect on the bubble behavior within a thin liquid film was analyzed. The dimensionless parameter, Weber number, was used to characterize the bubble deformation. A high-speed frame analysis shows that a Weber number greater than 0.5 × 10^-3 is necessary to break the energy barrier of the ultra-thin film and achieve the direct detachment mode. It is expected that a proper electrowetting actuation mechanism causing a relatively large Weber number can effectively enhance the bubble detachment within a thin liquid film which will provide promising applications to improve two-phase heat transfer.