Water purification and transportation are becoming increasingly important due to the extremely uneven distribution of water source on the earth and environmental issues for water pollutions. About one year ago, Nature Communications reported the plant leaves inspired sunlight powered water purification based solar steam generation technique and membrane-based technology to take full utilization of sunlight (Nat. Commun. 2019, 10, 1512, https://www.nature.com/articles/s41467-019-09535-w). The combinations of the steam generation and desalination by osmosis phenomenon promoted the water collection rate up to 4.2 kg m‒2 h‒1 under one sun irradiation, which represents the highest value at that moment. In our case, most of the water (3.1 2 kg m‒2 h‒1) is collected by a liquid to liquid collection way, rather than the conventional liquid-steam-liquid way, which requires much higher energy consumption and further procedures to recollect the steam.
Based on similar mechanism, we took advantage of the reversible coil-globule phase transformation of the poly(N-siopropylacrylamide), realized a sunlight-powered water pump (Angew. Chem. Int. Ed. 2018, 130, 15661, https://onlinelibrary.wiley.com/doi/full/10.1002/ange.201808835) and Intelligent solar water evaporation (Angew. Chem. Int. Ed. 2018, 57, 16343, https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201810345). Inspired by the natural botanical phenomena, e.g. the stomatal opening/closing of plant leaves, we used thermally responsive and microstructured graphene/poly(N‐isopropylacrylamide) (mG/PNIPAm) membrane to regulate the water transpiration rate. The as-prepared smart solar water evaporation system shows fast water evaporation through reversible pore opening under weak light and high water evaporation rate under intense light irradiation by controlling the open and closure of the microstructures. Similarly, we modified the inner wall of a poly(dimethylsiloxane) (PDMS)/reduced graphene oxide (rGO) tube with the poly(N-siopropylacrylamide) hydrogel (Figure 1), a device consists of two layers for stimuli responsive water transportation was obtained: a photothermal outer layer (PDMS/rGO) and a reconfigurable inner layer (thermal responsive hydrogel). The inner hydrogel layer exhibits reversible hydrophilic/hydrophobic transformation and reconfiguration as the outside is self-heated on exposure to sunlight irradiation. This tubular shape integrates capillary force and wettability gradients, allowing complicated water manipulation and long-distance water transportation.
Figure 1. Schematic structure of the sunlight‐powered water pump (PDMS/rGO‐PNIPAm pump) that is established via incorporating a sunlight responsive skeleton (PDMS/rGO) and a reconfigurable inner hydrogel (PNIPAm).
Following this plant leaves inspired structure, we further replicate the natural trunk system by integrating this leave-like structure with a meters-long artificial trunk (Figure 2). The artificial tree consists of directional microchannels in meters long for continuous water capillary rise, and the top leaf structure to evaporate water under sunlight irradiation (Global Challenges. 2020, 4, 2000042, https://onlinelibrary.wiley.com/doi/full/10.1002/gch2.202000043). Water is automatically drawn by capillary rise through the microchannels and transpired into air by the above mentioned leaf structure. The loss of water by evaporation provides the built-in negative pressure, pulling up water continuously. Furthermore, this work demonstrated the evaporative cooling and sufficient interfacial exchange technique that finished a complete water transportation-evaporation-recollection cycle. The design principles underlying the artificial trunk-leaf structure could also be used to transform solar energy into potential energy.
Figure 2. Schematic demonstration of continuous antigravity water transport in an artificial tree based on the directional microchannels in the artificial trunk with a photothermal steam generator on the top leaf. The water capillary rise is performed in acetylated chitosan channels, and a double network hydrogel consisting of poly(vinyl alcohol) (PVA), melamine‐formaldehyde (MF), and reduced graphene oxide (rGO) generates continuous water flows.
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