CAREER: Synthetic Mangrove Trees for Passive Desalination and Water Harvesting

用于被动海水淡化和水收集的合成红树林世界上至少有三分之一的人口无法获得足够的淡水，预计到2025年这一比例将增加到三分之二。将海水通过过滤器泵入的反渗透装置对于大规模海水淡化很有用，但每立方米纯净水需要大约2千瓦时的大量电力消耗。受红树林的启发，该项目寻求开发一种利用蒸腾作用获取淡水的替代方法，不需要任何主动能源输入。人造红树林树叶将使用3D打印制造，并连接到一系列模仿树木木质部导管的微通道上。当水从合成叶子的纳米孔中蒸发时，由于每个纳米孔内的水半月板的凹曲率，仍然在叶子内的水将表现出负的（吸力）压力。这种吸入压力将在合成木质部产生压力差，以允许从水库或潮湿土壤中连续泵送水。最终目标是获得足够强大的吸入压力，使海水通过除盐过滤器，而不需要机械泵，类似于红树林在海水中生长的方式。为了吸引更多的观众，一棵已经完工的人造红树林将被用来设计弗吉尼亚自然历史博物馆的一个新展览。该项目的目标是开发一种合成红树，能够通过产生超过3兆帕的蒸腾诱导的水力负荷来被动地淡化海水。我们已经知道，由于半月板的凹曲率，水从纳米多孔介质中蒸发会引起高度负的水压。然而，目前的合成树表现出非常低的水力导度，并且没有蒸腾叶片上的气孔来帮助稳定水分，这将水力负荷限制在1 MPa以下，并且需要不切实际的环境湿度超过85%以避免干燥或沸腾不稳定。在这里，通过3D打印合成树叶界面上的一系列气孔室和气孔，并将树叶连接到密集的微通道阵列（木质部），水流的稳定性和吞吐量将大大提高。假设是，即使在高度亚饱和的环境中，气孔下腔室也可以局部增加湿度以避免空化，而微通道阵列电导的增加应该可以防止叶片干燥。通过将木质部连接到放置在质量平衡上的水库，加热叶片的底部，并将叶片的顶部暴露在受控的亚饱和环境中，可以测量质量通量和延伸的水力负荷。利用聚焦木质部微通道的自顶向下显微镜捕捉空化/干化事件的发生和动力学。用开尔文方程、拉普拉斯方程和经典成核理论分析了水的亚稳态。水在树木中的流动将使用泊泽维尔定律（木质部）、达西定律（纳米孔）和菲克定律（气孔）来建模。这些理论见解将与实验测量相关联，以优化最终合成树的设计配置。受控纳米加工、实验表征和理论分析的协同混合将独特地揭示木质部、纳米孔、气孔下腔室和气孔的结构如何协同控制水分在树木中的蒸腾速率。

Synthetic Mangrove Trees for Passive Desalination and Water HarvestingAt least one-third of the world population does not have enough access to fresh water, and this is predicted to increase to two-thirds by 2025. Reverse osmosis plants, which pump ocean water through filters, are useful for large-scale desalination but require a large power consumption of about 2 kilowatt-hours for every cubic meter of purified water. Inspired by mangrove trees, this project seeks to develop an alternative means of harvesting fresh water that is powered by transpiration and does not require any active energy input. Synthetic mangrove leaves will be fabricated using 3D printing and connected to an array of micro-channels that mimic the xylem conduits of trees. As water evaporates from the nano-pores of the synthetic leaves, the water still inside of the leaves will exhibit a negative (suction) pressure due to the concave curvature of the water meniscus within each nano-pore. This suction pressure will generate a pressure differential across the synthetic xylem to allow for continuous pumping of water from a reservoir or moist soil. The ultimate goal is to achieve a suction pressure strong enough to pull ocean water through a salt-excluding filter without requiring a mechanical pump, analogous to how mangrove trees are able to grow in ocean water. To reach out to a broad audience, a completed artificial mangrove tree will be used to design a new exhibit at the Virginia Museum of Natural History. The objective of this project is to develop a synthetic mangrove tree capable of passively desalinating ocean water by generating transpiration-induced hydraulic loads exceeding 3 MPa. It is already known that water transpiring from a nanoporous medium can induce a highly negative water pressure due to the concave curvature of the menisci. However, current synthetic trees exhibit a very low hydraulic conductance and do not feature stomata on the transpiring leaves to help stabilize the water, which has constrained the hydraulic load to under 1 MPa and required impractical ambient humidities of over 85% to avoid dryout or boiling instabilities. Here, the stability and throughput of water flowing through synthetic trees will be dramatically improved by 3D printing an array of substomatal chambers and stomatal apertures at the interface of the synthetic leaves and by connecting the leaves to a dense array of micro-channels (xylem). The hypothesis is that the substomatal chambers serve to locally increase the humidity to avoid cavitation even in highly subsaturated ambient environments, while the increased conductance of the micro-channel array should prevent leaf dryout. The mass flux and by extension the hydraulic load will be measured by connecting the xylem to a water reservoir placed on a mass balance, heating the underside of the leaf, and exposing the top of the leaf to a controlled subsaturated ambient. The onset and dynamics of cavitation/dryout events will be captured using a top-down microscope focused on the xylem micro-channels. The metastability of the water will be analyzed using the Kelvin equation, Laplace equation, and classical nucleation theory. The flow of water through the tree will be modeled using Poiseuille's law (xylem), Darcy's law (nano-pores), and Fick's law (stomata). These theoretical insights will be correlated with the experimental measurements to optimize the design configuration of the final synthetic tree. The synergistic blend of controlled nanofabrication, experimental characterization, and theoretical analysis should uniquely reveal how the configuration of the xylem, nano-pores, substomatal chambers, and stomata serve to cooperatively govern the transpiration rate of water through trees.