AC electrowetting promoted droplet shedding on hydrophobic surfaces
11 May 2020-Applied Physics Letters (AIP Publishing LLC AIP Publishing)-Vol. 116, Iss: 19, pp 193701
TL;DR: In this paper, the influence of AC electrowetting fields on short-duration droplet shedding on hydrophobic surfaces was studied, with three parameters being varied (voltage, AC frequency, and device geometry).
Abstract: Condensation is significantly enhanced by condensing vapor as droplets (instead of a film), which rapidly shed-off. Electrowetting (EW)-induced coalescence and shedding of droplets have been recently shown to accelerate condensation. This work studies the influence of AC electrowetting fields on short-duration droplet shedding on hydrophobic surfaces. Experiments involve tracking the shedding of an ensemble of water droplets under the influence of EW fields, with three parameters being varied (voltage, AC frequency, and device geometry). Significant physical insights into EW-induced droplet shedding are obtained. First, EW enables almost complete removal of water (dry area fraction ∼98%) in very short time durations (∼ 1 s). Second, while the dry area fraction does depend on the applied voltage, significant water shedding can be achieved without needing to apply voltages significantly higher than the threshold voltage. Third, the frequency of the AC waveform does not influence the dry area fraction (for voltages above the threshold voltage); however the time constant associated with droplet shedding strongly depends on the AC frequency. Fourth, the orientation of the device influences water removal due to electrostatic pinning of droplets. Importantly, the measured water removal fluxes immediately after the application of an EW field are two orders of magnitude higher than those measured over a long-duration condensation experiment; this highlights the benefits of intermittent EW fields as opposed to continuous EW fields. Overall, these results suggest that EW on hydrophobic surfaces offers benefits comparable to those offered by superhydrophobic surfaces.
Citations
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References
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TL;DR: In this paper, the authors compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high.
Abstract: Electrowetting has become one of the most widely used tools for manipulating tiny amounts of liquids on surfaces. Applications range from 'lab-on-a-chip' devices to adjustable lenses and new kinds of electronic displays. In the present article, we review the recent progress in this rapidly growing field including both fundamental and applied aspects. We compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high. We discuss in detail the origin of the electrostatic forces that induce both contact angle reduction and the motion of entire droplets. We examine the limitations of the electrowetting equation and present a variety of recent extensions to the theory that account for distortions of the liquid surface due to local electric fields, for the finite penetration depth of electric fields into the liquid, as well as for finite conductivity effects in the presence of AC voltage. The most prominent failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, is discussed in a separate section. Recent work in this direction indicates that a variety of distinct physical effects?rather than a unique one?are responsible for the saturation phenomenon, depending on experimental details. In the presence of suitable electrode patterns or topographic structures on the substrate surface, variations of the contact angle can give rise not only to continuous changes of the droplet shape, but also to discontinuous morphological transitions between distinct liquid morphologies. The dynamics of electrowetting are discussed briefly. Finally, we give an overview of recent work aimed at commercial applications, in particular in the fields of adjustable lenses, display technology, fibre optics, and biotechnology-related microfluidic devices.
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