Rational Surface Nanoengineering for Phase Change and Separation Applications: From Desublimation Control to Sunlight-driven Antifogging

Abstract

As the title suggests, my thesis investigates the use of rationally nanoengineered surfaces to control phase changes and phase separation. More precisely I look at the areas of desublimation frosting, sublimation defrosting, fogging, and oil-water separation. Each of these applications lie within the bounds of technologies that will improve our ability to more efficiently use energy and water, the two most important commodities on earth. Understanding the fundamentals of desublimation frosting is important for many applications, most notably for transportation and environmental building control. Improving the efficiently of defrosting within the sublimation regime adds additional efficiency improvements for such applications. Controlling the formation of condensation can not only be applied in energy generation, but also governs the engineering rules for designing antifogging surfaces, applicable in areas such as eye-wear, windows, and sensing. Oil-water separation has obvious applications in the area of wastewater remediation, such as industrial wastewater treatment or oil-spill cleanup. Each chapter of this thesis is dedicated to research into a novel aspect on one of these respective topics. In the first chapter, we study the fundamentals of frost formation---both from water condensation (followed by freezing) and in particular from desublimation (direct growth of ice crystals from vapor)---and its implications for designing intrinsically icephobic surfaces. Guided by nucleation physics, we investigate the effect of material composition and surface texturing (atomically smooth to nanorough) on the nucleation and growth mechanism of frost for a range of conditions within the sublimation domain. Surprisingly, we observe that on silicon at very cold temperatures---below the homogeneous ice solidification nucleation limit---desublimation does not become the favorable pathway to frosting. Furthermore, we show that surface nanoroughness makes frost formation more probable, facilitated by capillary condensation, consistent with Kelvin's equation. The findings show that such nanoscale surface morphology imposed by design to impart desired functionalities---such as superhydrophobicity---or from defects can be highly detrimental for frost icephobicity at low temperatures and water vapor partial pressures. The second chapter consists of a feasibility study, containing on-going work, in which we introduce a novel phenomenon that can be used to enhance the efficiency of deicing. Here, we observed that ice crystals, upon sublimation, may spontaneously detach from the substrate before they completely sublimate. We present the spontaneous detachment phenomenon and initially examine the conditions at which this happens. This initial analysis provides us with insight allowing us to postulate the mechanism for detachment. We recognize that this phenomenon has obvious applications to enhance ice removal in areas such as transportation and energy generation and its work is therefore planned to continue beyond the scope of this thesis. In the third chapter we introduce a novel approach for energy sustainable antifogging on transparent surfaces. Going beyond state-of-the-art techniques, such as superhydrophilic and superhydrophobic coatings, we rationally engineer solar absorbing metasurfaces that maintain transparency to offer an alternative passive antifogging solution. Upon illumination, induced localized heating significantly delays the onset of surface fogging and decreases defogging time. For the same environmental conditions, we demonstrate that our metasurfaces are able to reduce defogging time by up to 4-fold and under supersaturated conditions inhibit the nucleation of condensate, outperforming conventional state-of-the-art approaches in terms of visibility retention. The research here illustrates a durable and environmentally sustainable approach to antifogging and defogging for transparent surfaces, opening up the opportunity for large-scale manufacturing that can be applied to a range of materials, including polymers and other flexible substrates. In the final chapter, we investigate the implications of surfactants on oil-water membrane separation. Due to the stabilization of the dispersed phase in surfactant-laden emulsions, membrane filtration has become a promising technology to improve separation and flux efficiencies. Here we investigate the fundamental wetting and transport behavior of such surfactant-stabilized droplets and the flow conditions necessary to perform sieving and separation of these stabilized emulsions. We show that, for water-soluable surfactants, such droplets are completely repelled by a range of materials (intrinsically underwater superoleophobic) due to the detergency effect; therefore, there is no need for surface micro-/nanotexturing or chemical treatment to repel the oil and prevent fouling of the the filter. With this new understanding, we demonstrate the use of a commercially available filter---without any additional surface engineering or functionalization---to separate oil droplets ($d < 100$ \textmu m) from a surfactant-stabilized emulsion with a flux of $\sim 11,000$ L m\textsuperscript{-2}