Smart "salt" dance, dynamic "steaming" and "present": a research on adaptive dynamic temperature-controlled photothermal interface water evaporator
Figure 1. The development process of interfacial solar evaporation technology – from mechanism to equipment to clean water. (Figure source: Nat Water 1, 587–601 (2023).
In recent years, interfacial solar evaporation technology, with its innovative clean water harvesting methods, has opened up new avenues for applications such as desalination (Figure 1). However, the core challenge of technology diffusion is salt accumulation, which exacerbates equipment corrosion, deteriorates efficiency, affects system stability, and leads to higher O&M costs. Especially under conditions of high salinity and organic matter contamination, achieving efficient and self-sustaining solar evaporation remains challenging.
Professor Michael Tam's team at the University of Waterloo, Canada, and Professor Wang Zuankai's team at the Hong Kong Polytechnic University jointly developed a double-layer solar evaporator (SDWE), equipped with a temperature-sensing dynamic hydrophilic and hydrophobic conversion system, which can autonomously switch between high-efficiency thin water layer evaporation and salt cleaning modes. Unlike conventional stereotyped solar evaporators, our SDWE has switchable water transfer channels that can be adaptively adjusted to changes in temperature during evaporation, resulting in a continuous increase in evaporation efficiency in brine with high salt concentrations. Using heat-responsive pollen particles as a switchable water gate, the authors cleverly designed a double-layer solar evaporator (SDWE) with a dynamic fluid flow mechanism. By capturing the temperature fluctuations in the evaporation process and switching the water supply channel, the rapid and continuous evaporation of thin layer water under light and heat and the self-cleaning of large water flows under salt pollution can be realized. The long-term and efficient evaporative water collection of 3.58 kg m-2 h-1 was realized, which provided a new design idea for the development of evaporators suitable for seawater desalination and high-salt wastewater treatment. This dynamic water transport mechanism is superior to traditional day-night cycles, has inherent thermal adaptability, and enables continuous, efficient evaporation, opening up new possibilities for the next generation of solar-driven evaporation technology. The results were published in Nature Communications under the title "Thermo-adaptive interfacial solar evaporation enhanced by dynamic water gating" [1].
Graphic analysis
Using nickel foam as a substrate, the researchers fabricated the evaporator with a two-layer structure and integrated two key components: an interfacial polydopamine nanosphere assembly layer (PDA) and a bottom heat-responsive pollen layer (PNm-g-SEC). Specifically, the PDA layer acts as a photothermal interface, while the heat-responsive pollen particles in the lower layer act as a switchable water flow channel. When the temperature exceeds the low critical solution temperature (LCST) of PNm-g-SEC, the layer transitions to a hydrophobic state, and water flows through the core adsorption through the PDA channel, ensuring a continuous supply of thin aquifers and faster evaporation efficiency. At lower temperatures, the PNm-g-SEC layer becomes hydrophilic, attracting a large amount of water to flow back and removing the accumulated salts.
Figure 2 illustrates the structural design and basic working principle of SDWE.
In order to present and validate the concept of "dynamic water flow control", the authors conducted a comprehensive study of the flow of water in an evaporator, using a confocal laser microscope (as shown in Figure 3) to observe the dynamic water flow under different morphological conditions. At the same time, three-dimensional imaging micro-CT was used to quantitatively analyze the thickness of the water film (as shown in Fig. 4).
Figure 3 illustrates the engineering design of a dynamic water flow in an evaporator: a. Schematic diagram of water transport within a switchable channel in SDWE. b. Thin aquifer supply in a capillary force-driven PDA assembly microchannel. c. Confocal microscope image showing the water flow/filling process over time in p-SDWE: at 20 °C, water fills the internal macropores of nickel foam. At 36 °C, water transports a thin layer of water along the PDA assembly channel on a nickel foam backbone.
Figure 4 illustrates the water flow behavior driven by different temperatures, the thickness of the water layer, and its effect on evaporation efficiency: a. At low temperatures (20°C), the entire foam structure is filled with water. At high temperatures, the superhydrophobic PN10-g-SEC layer blocks a large amount of water and pumps the water to the p-PDA layer through the thin water transfer channel of the p-PDA. c. Change in the amount of water produced by p-SDWE in response to solar radiation. d. Changes in the amount of water produced by p-SE in response to solar radiation. Thin aquifer in p-SDWE measured by micro-CT.
Notably, unlike traditional evaporators, which rely on non-interactive salt removal processes, such as day-night cleaning cycles. This thermally adapted dynamic water control system innovatively introduces a self-cleaning mechanism that takes advantage of temperature fluctuations caused by scaling or salt accumulation during the evaporation process for effective autonomous salt self-cleaning. When the accumulated salts affect the photothermal efficiency of the system, the thermally responsive layer of our SDWE structure changes from a superhydrophobic state to a hydrophilic state, effectively removing contaminants through capillary action. When the pollutants are removed, the temperature rises further, the layer structure changes from hydrophilic to superhydrophobic, and the evaporation mode of the thin aquifer is restored, so as to achieve long-term efficient recycling of solar evaporators. Obviously, such a system is significantly superior to traditional evaporators that rely solely on salt removal or round-the-clock cleaning (as shown in Figure 5).
Figure 5 shows a schematic diagram of salt dissolution and reflux of p-SDWE and compares the long-term cycling effect of an existing solar water evaporator: a-f. Schematic diagram of a simulated salt crystallization experimental set-up, temperature distribution tracked by an infrared camera and a top-view digital image of the evaporation surface. g. Change in collection rate of p-SDWE compared to p-ShE and p-SE in one cycle. Long-term cycling performance of seawater was simulated using 10 wt%, where p-ShE was used as a reference sample, representing an evaporator decorated with a superhydrophobic SEC layer.
Figure 6 illustrates the evaporation performance of solar energy in an outdoor environment.
Summary and prospects: The concept and design of "dynamic water flow control" reported in this paper give the evaporator the ability to switch between high-performance evaporation and effective cleaning modes autonomously, making the equipment flexible, proactive, and responsive. It demonstrates the great potential to promote the evolution of the next generation of solar-driven evaporation technology, and illuminates new ideas for the abundance and development of clean energy water harvesting systems [2].
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Links to relevant literature
[1] Wang, Y., et al. Thermo-adaptive interfacial solar evaporation enhanced by dynamic water gating. Nat Commun 15, 6157 (2024).
https://doi.org/10.1038/s41467-024-50279-z
[2] Wang, Y., et al. Biomimetic surface engineering for sustainable water harvesting systems. Nat Water 1, 587–601 (2023).
https://doi.org/10.1038/s44221-023-00109-1 Source: Frontiers of Polymer Science