A novel application area of high-voltage electrostatic flocking: solar-driven water evaporators.

Solar interfacial evaporators are devices that convert abundant solar energy into localized thermal energy to produce clean water. The key to optimizing such devices lies in enhancing heat capture capability through photothermal conversion and minimizing thermal energy loss to improve evaporation efficiency. Electrostatic flocking technology is commonly applied in two aspects of solar evaporators: as a light-absorbing layer and as a thermal insulation layer. [10-12, 45, 103] This application fully leverages the excellent light-absorbing properties of the array and the superhydrophobicity and water-retention characteristics of this hydrophobic fiber flocking layer.

Guo et al. developed a solar-driven evaporator using electrostatic flocking technology, as shown in Figure e. The device primarily consists of a three-layer structure: from top to bottom, a black hydrophilic nylon flocking layer, a hydrophilic fabric, and a white hydrophobic nylon flocking layer. [11] When sunlight irradiates the upper flocking fibers, the fiber array captures light due to the light-trapping effect, achieving an excellent light absorption rate of over 94%. The lower electrostatic flocking layer forms an air layer between the fabric and the water surface, effectively reducing heat transfer to the water. Overall, the vertically arranged hydrophobic flocking layer reduces energy loss by 17.1%.

In recent years, innovations in the row-combination structure and regional dimensions of simple electrostatic flocking evaporators have continued. Tu et al. developed an insertable flocking evaporation system comprising multiple electrostatic flocking evaporation panels (Figure f). [12] By vertically inserting surface fluidized panels into the substrate, the evaporation surface area is effectively expanded, and air convection on the surface of the fluidized panels is enhanced. Under 1 kW/m³ illumination, the evaporation rate can reach 2.09 kg/m²/h. Tian et al. [10] constructed a three-dimensional micro-array structure composed of vertically aligned, layered, and hydrophilic carbon fibers. They successfully fabricated a large-area electrostatic fluidized evaporator (Figure g) and are exploring electrostatic fluidized evaporators with different 2D/3D morphologies (Figure h) [10], aiming to enhance light absorption efficiency and evaporation area. Additionally, Tian's team found that hydrophilic fiber arrays can generate capillary effects at different scales, forming a three-dimensional evaporation surface covered with micro-scale menisci and nano-scale thin water layers. This increases the water/air interface available for evaporation, significantly improving the evaporation rate.

[10] Meanwhile, Tian et al. [45] reported an eight-stage solar-driven multi-stage device composed of a copper plate (CP), non-woven fabric (NWF), and insulating ethylene-vinyl acetate foam (EVA foam) for seawater desalination (Figure i). [45] A three-dimensional VACF structure was constructed on the TCA/CP surface using electrostatic flocking technology to form a photothermal layer. [45] Under the synergistic effect of the eight stages, this multi-stage evaporator achieved a water collection efficiency of 2.25 kg/m²/h under single sunlight exposure. [45] This innovative approach provides new insights into solar-driven water splitting, demonstrating the great potential for highly efficient and sustainable water production using renewable energy.

In addition to enhancing light absorption efficiency, electrostatic flocking also improves anti-salt scaling capability. As shown in Figure j, the thin water layer formed by capillary action in the electrostatic flocking structure facilitates rapid ion diffusion, effectively preventing salt accumulation. [10, 11] Carefully optimized pore sizes ensure the robustness of capillary behavior and the adequacy of fluid transport capacity, further enhancing performance. [11, 12] This study achieved a stable evaporation rate in brine over 10 days.

e) Schematic diagram of AE based on vertical array structure, demonstrating light-trapping and air-entrapping functions; reproduced with permission. [11] Copyright 2020, Wiley-VCH Verlag GmbH & Co. KGaA. Solar-driven water evaporator.
f) Six multi-surface panels vertically embedded in a planar铺垫板 (substrate) of BNF; reproduced with permission. [12] Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
g) Flowchart of the fabrication process for the large-area LO-VACFs/CWF evaporator; reproduced with permission. [10] Copyright 2024, Wiley-VCH Verlag GmbH & Co. KGaA.
h) Schematic diagrams of LO-VACFs/CWF evaporators with different morphologies: 2D floating type, 2D suspended type, 3D conical type, and 3D spherical type; reproduced with permission. [10] Copyright 2024, Wiley-VCH Verlag GmbH & Co. KGaA.
i) Schematic diagram of the structure of the solar-driven multi-stage device; reproduced with permission. [45] Copyright 2024, American Chemical Society.
j) Schematic diagram of salt diffusion and potential salt accumulation; reproduced with permission. [11] Copyright 2020, Wiley-VCH GmbH.

References
[10] Y. Tian, R. Song, Y. Li, R. Zhu, X. Yang, D. Wu, X. Wang, J. Song, J. Yu, T. Gao, F. Li, Advanced Functional Materials 2024, *34*.
[11] Y. Guo, M. Javid, X. Li, S. Zhai, Z. Cai, X. Xu, Advanced Sustainable Systems 2020, *5*, 2000202.
[12] C. Tu, W. Cai, X. Chen, X. Ouyang, H. Zhang, Z. Zhang, Small 2019.
[45] Y. Tian, Y. Jiang, R. Zhu, X. Yang, D. Wu, X. Wang, J. Yu, Y. Li, T. Gao, F. Li, Environmental Science & Technology 2024, *58*.
[103] V. S. Mironov, M. Park, Combinatorial Science and Technology 2000, *60*.