Electrowetting (EW) has drawn significant interests due to the potential applications in electronic displays, lab-on-a-chip microfluidic devices and electro-optical switches, etc. However, current understanding of EW is hindered by the inadequacy of available numerical and theoretical methods in properly modeling the transient behaviors of EW-actuated droplets. In the present work, a combined numerical and experimental approach was employed to study the EW response of a droplet subject to both direct current (DC) and alternating current (AC) actuating signals. Computational fluid dynamics models were developed by using the Volume of Fluid (VOF)-Continuous Surface Force (CSF) method. A dynamic contact angle model based on the molecular kinetic theory was implemented as the boundary condition at the moving contact line, which considers the effects of the contact line friction and the pinning force. The droplet shape evolution under DC condition and the interfacial resonance oscillation under AC condition were investigated. It was found that the numerical models were able to accurately predict the key parameters of electrowetting-induced droplet dynamics.
Single-phase convective heat transfer of nanofluids has been studied extensively, and dif ferent degrees of enhancement were observed over the base fluids, whereas there is still debate on the improvement in overall thermal performance when both heat transfer and hydrodynamic characteristics are considered. Meanwhile, very few studies have been devoted to investigating two-phase heat transfer of nanofluids, and it remains inconclu sive whether the same pessimistic outlook should he expected. In this work, an experimen tal study of forced convective flow boiling and two-phase flow was conducted for Al20 3-water nanofluids through a minichannel. General flow boiling heat transfer char acteristics were measured, and the effects of nanofluids on the onset of nucleate boiling (ONB) were studied. Two-pliase flow instabilities were also explored with an emphasis on the transition boundaries of onset of flow instabilities (OFl). It was found that the presence of nanoparticles delays ONB and suppresses OFl, and the extent is correlated to the nanoparticle volume concentration. These effects were attributed to the changes in available nucleation sites and sin face wettability as well as thinning of thermal boundary layers in nanofluid flow. Additionally, it was observed that the pressure-drop type flow instability prevails in two-phase flow of nanofluids, but with reduced amplitude in pressure, temperature, and mass flux oscillations.ALO^-water nanofluids with two nanoparticle volume concen trations (0.01 vol. % and 0.1 vol. %) were used in this work. They were prepared following the same method described in Ref.[40],
Journal of Heat Transfer
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