In this work, we were motivated to develop novel devices for water harvesting inspired by natural trees, and to understand their collection efficiency and working principles. We accomplished that with scale-model and large-scale fog harps, floating leaves, and synthetic trees. Fluids mechanics, physics, and thermodynamics were applied to solve the problems and rationalize the results. Redwood-inspired fog harps were designed with stainless steel vertical wires, using 3D-printing and laser-cutting techniques. Fog harps always harvested more water than any of the meshes, tested both under heavy fog and light fog conditions. The aerodynamic efficiency, deposition efficiency, and sliding efficiency were calculated to compare the fog harvesting performance. These findings provide insight into the new design of fog harvesters with high-efficiency fog harvesting performance, and future development of large fog harps, applied into regions even with light fog conditions, as an economically viable means. synthetic trees were fabricated with a nanoporous ceramic disk and silicone tubes. This tree system was tested in an environmental chamber (6 cm short trees) or a plant growth chamber (3m tall trees), both with controlled ambient humidities. The system pressure was calculated with Darcy's equation, Poiseuille equation and Laplace equation. The stable transpiration can happen to any scalable tree, which pumps water up an array of large tubes. Our synthetic trees, like natural trees, have the ability to lift water across a wide range of water temperatures and ambient humidities. They can be used as the large-scale evaporation-driven hydraulic pump, for example, pumped storage hydropower, filtration, underground water extraction. / Doctor of Philosophy / The purpose of this work is to investigate and characterize novel techniques for water harvesting that are inspired by natural trees. We are interested in two modes of water harvesting in particular: fog harps and synthetic trees. Fog harps were comprised of only vertical wires, inspired by the parallel structures of redwoods, which can capture and shed off fog droplets efficiently. Fog harps harvested more water than the traditional mesh nets, both under heavy fog and light fog conditions. Redwood-inspired fog harps have the high-efficient fog harvesting performance. They can be set up at coastal deserts to collect water from fog, where there is scarce rainfall but plenty of fog, like Chile, Peru and South Africa. Synthetic trees were designed with nanoporous disk (leaf) and tubes (xylem conduits), inspired by the transpiration process in natural trees. This transpiration-powered pump can lift water against the gravity at large scales, driven by the water evaporating from the nanopores. They can be used as the large-scale evaporation-driven hydraulic pump, for example, pumped storage hydropower, filtration, underground water extraction.

This dissertation delves into the theory, design and fabrication, and practical uses of synthetic trees that replicate the transpiration mechanisms of natural trees. The first chapter provides an in-depth explanation of how natural trees utilize hydraulic mechanisms to draw water from the soil, through their roots, and up to their leaves, sustaining hydration through transpiration. This process is reliant on the difference in relative humidity between the leaf and the ambient to promote evaporation, and synthetic trees replicate this cycle by integrating reservoirs and conduits with wetted nanopores, mimicking the negative Laplace pressure seen in natural trees. Chapter 2 presents a detailed theoretical framework for transpiration in synthetic trees. These trees feature a vertical array of tubes connected to a nanoporous synthetic leaf. Our model considers the impact of convective gas flow on the leaf, minimizing the diffusive boundary layer and directly influencing the leaf's negative Laplace pressure. We next analyze how the rate of evaporation and tree morphology affect the required Laplace pressure for mass conservation, in an ambient environment with an appreciable diffusive boundary layer. Our model considers the changing dynamics of the menisci, including their capability to adjust their contact angle and withdraw into nanopores to self-stabilize. We then determine conditions where transpiration is limited by evaporation or constrained by the leaf's maximum Laplace pressure, across various tree geometries and ambient conditions. In Chapter 3, the focus shifts to a practical application, as the insights from the previous chapters guide the creation of a synthetic tree for water harvesting. Solar steam generation employing a porous evaporator, with a 3D design extending beyond the free surface to mitigate heat losses, is used to demonstrate how transpiration, rather than capillarity, can raise water up glass tubes, and improve liquid transport heights over conventional methods. Chapter 4 expands on the synthetic tree concept, proposing a mobile desalination water container driven by transpiration. The container features a ring-shaped fin designed to absorb solar heat, increasing water evaporation from a nanoporous synthetic leaf. This approach combines reverse osmosis and thermal evaporation, offering a promising solution for obtaining fresh water from seawater. In Chapter 5, the study explores transpiration-powered oil-water filtration using synthetic trees. Our approach showcases the potential for natural separation of oil and water in various applications, without the need for a pump and in opposition to gravity. Chapter 6 modifies the synthetic tree design to selectively absorb and retain oil from oil-water emulsions. When water evaporates from the synthetic leaf, enabled by the generated negative suction within, oil is then drawn and contained within the system through oleophilic and hydrophobic membranes. This approach offers a sustainable method for oil spill clean-up, oil extraction and purification. Chapter 7 experimentally investigates how to eliminate the capillary driving force in synthetic trees. By over-filling the synthetic leaf's top surface to remove existing concave menisci, the study hypothesizes gravity as a replacement mechanism for negative pressure, with the water in hydrostatic columns held in tension by the overlying water supported within the porous leaf. In summary, these engineered hydraulic systems offer novel approaches to water harvesting, desalination, oil-water filtration, and the cleanup of oil spills, and the study of synthetic trees opens up a realm of possibilities for sustainable water management and environmental remediation, showcasing the potential of biomimicry in solving pressing global challenges. / Doctor of Philosophy / This dissertation explores the concept of synthetic trees designed to mimic the transpiration cycle of natural trees for various applications. The first chapter provides a detailed explanation on how this is achieved. The second chapter introduces the theoretical model, highlighting the interplay between suction pressure, spontaneous flow, and tree geometry in surface tension powered water flow. In Chapter 3, the findings inform the design of a synthetic tree for water harvesting through solar steam generation. Overcoming constraints of floating evaporators, this tree demonstrates enhanced water condensation compared to traditional reservoirs, and the use of transpiration in the tubes allow for greater height flexibility. Chapter 4 presents a theoretical design for a portable desalinating water bottle powered by transpiration. Inspired by mangrove trees, the bottle utilizes solar heat absorption, a nanoporous synthetic leaf, and reverse osmosis to spontaneously enable desalination. The hybrid approach enhances thermal evaporation and pre-filters salt, potentially producing a daily extraction of one liter of fresh water from seawater. Chapter 5 explores oil-water filtration using surface tension power in synthetic trees. Operating without pumps and against gravity, this spontaneous phase separation demonstrates potential applications in oil spill cleanup, wastewater purification, and oil extraction. In Chapter 6, the synthetic tree is further modified to selectively take up and contain only oil from an oil-water emulsion. Driven by the surface tension mechanism, oil enters the tree through oil loving and water membranes, yielding high-purity oil samples, and offering innovative solutions for various environmental and industrial challenges. Chapter 7 investigates how to stop capillary forces in synthetic trees. When water evaporates from the leaves, it creates suction, pulling water from the soil through the xylem to keep the tree hydrated. We filled the top of the synthetic leaf to remove the curved surfaces that cause capillary tension. Surprisingly, water in the vertical tubes still held against gravity. This led us to consider a new idea: gravity might be replacing surface tension, with columns of water in the tree held in tension by the water above them in the leaf. Overall, this research on synthetic trees suggests exciting new ways to address environmental issues and manage water resources sustainably, underlying the power of nature-inspired solutions.