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Controlling transport phenomena in liquid and gaseous media through electrostatic forces has brought new important scientific and industrial applications. Although numerous EHD applications have been explored and extensively studied so far, the fast-growing technologies, mainly in the semiconductor industry, introduce new challenges and demands. These challenges require enhancement of heat transfer and mass transport in small scales (sometimes in molecular scales) to remove highly concentrated heat fluxes from reduced size devices. Electric field induced flows, or electrohydrodynamics (EHD), have shown promise in both macro and micro-scale devices. Several existing problems in EHD heat transfer enhancements were investigated in this thesis. Enhancement of natural convection heat transfer through corona discharge from an isothermal horizontal cylindrical tube at low Rayleigh numbers was studied experimentally and numerically. Due to the lack of knowledge about local heat transfer enhancements, Mach-Zehnder Interferometer (MZI) was used for thermal boundary layer visualization. For the first time, local Nusselt numbers were extracted from the interferograms at different applied voltages by mapping the hydrodynamic and thermal field results from numerical analysis into the thermal boundary layer visualizations and local heat transfer results. A novel EHD conduction micropump with electrode separations less than 300 µm was fabricated and investigated experimentally. By scaling down the pump, the operating voltage was reduced one order of magnitude with respect to macro-scale pumps. The pumping mechanism in small-scales was explored through a numerical analysis. The measured static pressure generations at different applied voltages were predicted numerically. A new electrostatically-assisted technique for spreading of a dielectric liquid film over a metallic substrate was proposed. The mechanism of the spreading was explained through several systematic experiments and a simplified theoretical model. The theoretical model was based on an analogy between the Stefan’s problem and current problem. The spreading law was predicted by the theoretical approach and compared with the experimental results. Since the charge transport mechanism across the film depends on the thickness of the film, by continuing the corona discharge exposure, the liquid film becomes thinner and thinner and both hydrodynamic and charge transport mechanisms show a cross-over and causes different regimes of spreading. Four different regimes of spreading were identified. For the first time, an electrostatically accelerated molecular film (precursor film) was reported. The concept of spreading and interfacial pressure produced by a corona discharge was applied to control an impacting dielectric droplet on non-wetting substrate. For the first time, the retraction phase of the impact process was actively suppressed at moderate corona discharge voltages. At higher corona discharge strengths, not only was the retraction inhibited but also the spreading phase continued as if the surface was a wetting surface.

Photoresponsive titania surfaces are of great interest due to their unique wettability change upon ultraviolet light illumination. However, their applications are often limited either by the inability to respond to visible light or the need for special treatment to recover the original wettability. Sensitizing TiO2 surfaces with visible light-absorbing materials has been utilized in photovoltaic applications. Here we demonstrate that a dye-sensitized TiO2 surface can selectively change the wettability towards contacting liquids upon visible light illumination due to a photo-induced voltage across the liquid and the underlying surface. The photo-induced wettability change of our surfaces enables external manipulation of liquid droplet motion upon illumination. We show demulsification of surfactant-stabilized brine-in-oil emulsions via coalescence of brine droplets on our dye-sensitized TiO2 surface upon visible light illumination. We anticipate that our surfaces will have a wide range of applications including microfluidic devices with customizable wettability, solar-driven oil–water clean-up and demulsification technologies., Controlling surface wettability using visible light is highly attractive for a range of liquid separation technologies. Here, Varanasi, McKinley and colleagues fabricate dye-sensitized photocatalytic TiO2 surfaces on which liquid droplet motion can be externally manipulated by visible light illumination.