An Experimental Investigation of Electrowetting Modulated Nucleate Boiling
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Electrowetting (EW) has drawn significant research interests in droplet-based microfluidics, and most applications focus on electronic displays, lab-on-a-chip devices and electro-optical switches, etc. In this work, we report a novel application of EW in enhancing nucleate boiling heat transfer. The working approach capitalizes on the complimentary roles of hydrophobicity and hydrophilicity in the fundamental processes of nucleate boiling, and takes advantage of the ability of EW to dynamically alter the surface wettability at different thermal loads. For instance, at low-to-moderate heat fluxes, the boiling surface remains hydrophobic so that onset of nucleate boiling (ONB) commences spontaneously and excellent boiling heat transfer can be obtained. When the bubble growth and merger intensify at high fluxes, EW will be activated to change the surface to hydrophilic, thereby delaying the critical heat flux (CHF). In this work, we demonstrate the creation of such tunable adaptive boiling surfaces, and examine the effects of both direct current (DC) EW and alternating current (AC) EW on the overall boiling heat transfer characteristics. A synchronized high-speed optical imaging and infrared thermography approach is taken to obtain simultaneous measurements of the bubble dynamics and temperature distribution on the boiling surface. Boiling curves are constructed and the boiling heat transfer coefficients are computed for an EW-modulated surface, and the comparison with those for hydrophilic and hydrophobic surfaces shows clearly the efficacy of using EW to drastically improve the boiling heat transfer performance. Some insights are also offered on the boiling heat transfer mechanisms under the influence of EW.