At 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.

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