More recently, interfacial solar-driven evaporation has emerged as a sustainable approach to generate clean water using solar energy. However, the ultralow diffusivity of salt in water leads to significant salt accumulation, causing fouling, reducing the evaporation rate, and degrading device reliability. Recent advancements have addressed both high efficiency and salt rejection. Despite their efficiency and reduced salt fouling, current systems are encumbered by their rigid structures and passive operation cycles, limiting their long-term viability. Achieving autonomous solar-driven water evaporation, particularly in environments with high salt concentrations and organic pollutants, remains a formidable challenge.
 In this paper, we describe the development of a bilayer-structured solar evaporator (SDWE) featuring a dynamic water-thermal contro that autonomously shifts between efficient thin water evaporation and salt washing. Unlike previous reported solar evaporator with rigid structures, our SDWE features a switchable water transport channel that adapts to temperature changes during evaporation, enabling improved evaporation efficiency, even in high-concentration saline brine. We fabricated SDWEs using nickel foam as the substrate and incorporated two key components: the interfacial polydopamine nanosphere-assembled layer (PDA) and the bottom thermo-responsive sporopollenin-engineering layer (PNm-g-SEC). More precisely, the upper PDA layer serves as the photothermal interface, while the lower layer consisting of thermo-responsive sporopollenin, acts as a switchable gating layer. When the temperature rises above the lower critical solution temperature (LCST) of PNm-g-SEC, this layer transitions to a superhydrophobic state that directs water through specific PDA-assembled channels, ensuring a consistent supply of thin water layers and optimizing the evaporation process. At lower temperatures, the PNm-g-SEC layer becomes hydrophilic, drawing bulk water to backflow accumulated salt (Figure 1).
Figure 1. Illustration of the working principles of SDWEs: Water transport occurs along the larger pores within the nickel foam at low temperature (right), while thin water transport takes place along the intine PDA layer at high temperature (left), driven by the thermodynamic modulation induced by the water gating layer of PNm-g-SEC.
To illustrate this concept, we conducted a comprehensive study on the high-efficiency evaporation phenomena generated by this dynamic water control. For example, we examined the dynamic water flow under varying morphological conditions using confocal laser microscopy (Figure 2). While the engineered water flow could be detected by various characterization techniques, such as micro-CT characterization of the thickness of the water film, please refer to the manuscript for detailed results.
Figure 2 a. Schematic illustration of water transport within switchable channel of SDWEs, b. Thin water supply within PDA-assembled microchannel driven by capillary force, Confocal microscopic images of switchable water transport along p-SDWE using a time-series model at the z-axis: c. bulk water fill the inner large pore of NiF foam, where the injecting water phase at 20°C, d. thin water transport along the PDA-assembled channels on the skeleton of NiF foam, where the injecting water phase at 36°C.
Notably, unlike traditional evaporators that depend only on salt rejection or day-night cleaning cycles, this thermal-adaptive dynamic water control system incorporates a self-cleaning mechanism. It capitalizes on temperature fluctuations caused by fouling or salt accumulation during solar-driven evaporation, which triggers an effective, autonomous salt self-washing process. When the accumulated salts reduce the system’s photothermal efficiency, our SDWE structure’s thermal-responsive layer shift from superhydrophobic to hydrophilic states, enhancing capillary action for effective pollutant removal via the convective transport of bulk water. Once the pollutants are eliminated, the temperature will increase further, triggering a transformation from hydrophilic back to superhydrophobic state. This reinstates the thin water evaporation mode, thereby enabling a long-term, efficient, cyclic utilization of the solar evaporator. Obviously, this system markedly outperforms conventional evaporators, which rely solely on salt rejection or day-night washing (Figure 3).
Figure 3. Illustration of Salt dissolving and backflow of p-SDWE under 5 sun illumination over 6 hours: a. Experimental setup, Temperature distribution traced by IR Camera and top view digital images of the evaporation surface: b1 and b2. Salt crystallization, c1 and c2. Salt dissolution after simulator light deactivation, d1 and d2. Salt dissolution after 20 min, e1 and e2. Salt dissolution after 30 min, f1 and f2. Salt dissolution after 40 min. g. Long-term cycling performance using 10 wt% simulated seawater, where the reference sample of p-ShE denoting as the evaporator decorated with superhydrophobic SEC layer, h. The operational regime varying the collection rate during one cycle of p-SDWE compare with p-ShE and p-SE.
This ground-breaking innovation endows the evaporator with the capability to autonomously switch between high-performance evaporation and effective cleaning modes. This adaptability equips the device with flexible, active, and responsive behavior, demonstrating the potential to propel the evolution of the next-generation solar-driven evaporation technologies.Â