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Compared to evaporators with a single structure, hydrogel evaporators with aligned/radial hybrid channels have been proven to synergistically utilize the rapid water transport of aligned channels and the strong thermal confinement capability of radial channels, achieving exceptional evaporation performance (Small 2025, 21, 2411780). However, in the aforementioned composite structures, the size of their aligned structure is not adjustable, and the potential mechanism of how the sizes of micro- and macro-scale channels simultaneously affect water transport and heat transfer, thereby controlling the balance between water supply and heat loss, is not fully understood.
Recently, a research team led by Professor Cheng Si from the College of Textile and Clothing Engineering at Soochow University, in collaboration with Assistant Researcher Zhang Shenglong from the Energy Institute of Qilu University of Technology (Shandong Academy of Sciences), published their findings in the internationally renowned journal Advanced Functional Materials, titled "Effects of Microchannel/Macrochannel Configurations on the Evaporation Performance of Internal Vertical/External Radial Hydrogel Solar Evaporators: Experiments and Simulations Insights". The co-first authors of the paper are Wen Yong, a 2022 graduate student, and Tan Jiewei, a 2023 graduate student, both from Soochow University. The corresponding authors are Professor Cheng Si and Assistant Researcher Zhang Shenglong. This study successfully fabricated a composite hydrogel evaporator with excellent salt tolerance and ultra-high evaporation performance by precisely regulating the synergistic effect between the internal aligned vertical channels and the external radial structure. By adjusting the size of the internal aligned vertical channels, the research investigated how the size of the aligned channels affects internal convection, thermal confinement, and overall evaporation performance. Systematic experiments and numerical simulations demonstrated that the internal vertical microchannels significantly enhance convective water transport, while the external radial structure effectively confines heat diffusion. Under one sun illumination, the size-optimized aligned/radial hybrid structure evaporator achieved an optimal evaporation rate of 5.01 kg m⁻² h⁻¹ and an evaporation efficiency of 165.13%. Further expanding the microchannels of the aligned structure to macroscopic vertical channels amplified the convection effect, increasing the evaporation rate further to 8.60 kg m⁻² h⁻¹. The results indicate that finely balancing the micro- and macro-scale channel structures can simultaneously optimize water transport and thermal confinement, providing new guidelines for the design of high-performance hydrogel evaporators.
Figure 1 a) Preparation process of PMS-As. b) Digital photo of the top view of PMS-A. c) SEM image of the top view of PMS-A. d) Digital photo of the longitudinal section of PMS-A. e) SEM image of the top view of PMS-A. f, g) SEM images of the side view of PMS-A.
Structural Design Innovation: Size-Tunable Aligned + Radial Hybrid Channels
The research team constructed an aPAN/MXene/SA (PMS) hydrogel evaporator with an internal vertically aligned channel structure and an external radial horizontal channel structure using the freeze-casting method (Figure 1). This structure not only retains the efficient water transport capacity of the vertical channels but also combines the excellent thermal confinement performance of the radial structure, achieving synergistic water-thermal regulation.
Figure 2 a) Light absorption spectrum of PMS. b) Surface temperature changes of PMS-V, PMS-R, PMS-C-1, PMS-C-2, PMS-C-3 evaporators and pure water under one sun illumination. c) Mass changes of pure water, PMS-V, PMS-R, and the PMS-C series (C-1 to C-3) under 1 sun illumination; control experiment shows mass change of water under dark conditions. d) Evaporation rate and efficiency of PMS-V, PMS-R, PMS-C-1, PMS-C-2, and PMS-C-3 under 1 sun illumination. e) Schematic diagram of different states of water in PMS-C-3 including free water, intermediate water, and bound water. f) Comparison of evaporation enthalpy between PMS-C-3 and pure water. g) Evaporation performance of PMS-C-3 under different concentrations of NaCl solution under 1 sun illumination. h) Time-dependent evaporation rate of the PMS-C-3 evaporator in 3.5 wt.% NaCl solution under 1 sun illumination. i) Evaporation rate of the PMS-C-3 evaporator over 7 cycles in 3.5 wt.% NaCl solution.
Performance Breakthrough: High Evaporation Rate and Efficiency
Under 1 sun illumination, PMS-C-3 (vertical zone diameter 1.0 cm) achieved an evaporation rate of 5.01 kg m⁻² h⁻¹ and an evaporation efficiency of 165.13%.
Figure 3 Numerical simulation of PMS hydrogel evaporators. (a-e) Schematic diagrams of the hydrogels: (a) PMS-R; (b) PMS-C-1; (c) PMS-C-2; (d) PMS-C-3; (e) PMS-V. (f-j) Simulated water pressure distribution at hydraulic equilibrium: (f) PMS-R; (g) PMS-C-1; (h) PMS-C-2; (i) PMS-C-3; (j) PMS-V. (k-o) Simulated fluid flow velocity and temperature distribution at thermal equilibrium: (k) PMS-R; (l) PMS-C-1; (m) PMS-C-2; (n) PMS-C-3; (o) PMS-V.
Mechanism Analysis: Dual Verification by Simulation and Experiment
Through simulations of the water pressure, fluid flow velocity, and temperature distribution in five hydrogel evaporators (PMS-R, PMS-C-1, PMS-C-2, PMS-C-3, and PMS-V) (Figure 3), the key roles of aligned and radial channels in achieving stable, high-performance solar desalination in the PMS composite hydrogel were elucidated. Figures 3f-j show that as the vertical zone area expands, the hydrogel gradually forms a higher relative water pressure overall, indicating that the aligned structure significantly enhances water transport capability (PMS-V > PMS-C-3 > PMS-C-2 > PMS-C-1 > PMS-R). Furthermore, the vertical design also enhances thermal localization at the evaporation interface, further accelerating water transport. As shown in Figures 3k-o, the fluid flow velocity in the PMS-C series exhibits a unique pattern: fluid moves vertically upward along the internal central region, while a portion of the flow diverges horizontally before rising. Notably, the flow velocity increases with the expansion of the aligned area (PMS-V > PMS-C-3 > PMS-C-2 > PMS-C-1 > PMS-R). Simulation results indicate that although PMS-R has the slowest fluid flow, its heat loss is the smallest (Figures 3k-o), whereas the thermal confinement capability of PMS-C worsens as the vertical zone increases. The simulation results are consistent with the experimental results in Figures 2c-d, demonstrating that PMS-C combines the advantages of both aligned and radial structures, and that the evaporation performance is closely related to the dynamic balance between the evaporator's water transport capacity and thermal confinement capability.
Figure 4 a) Digital photo of PMS-T-3. b) Surface temperature changes of PMS-T-1, PMS-T-2, and PMS-T-3 evaporators and pure water under one sun illumination. c) Mass changes of PMS-T-1, PMS-T-2, and PMS-T-3 over 1 hour under 1 sun illumination. d) Evaporation rate and efficiency of PMS-T-1, PMS-T-2, and PMS-T-3 under 1 sun illumination. e) Time-dependent evaporation rate of the PMS-T-3 evaporator for 3.5 wt.% NaCl solution under 1 sun illumination. f) Evaporation rate tested over 7 cycles in 3.5 wt.% NaCl solution. g) Evaporation performance of PMS-T-3 under different NaCl concentrations under 1 sun illumination. h) Overall performance of PMS-T-3 in removing heavy metals. i) Comparison of evaporation performance under 1 sun illumination for PMS-C-3 and PMS-T-3 hydrogels with previously reported evaporators.
Performance Breakthrough: High Evaporation Rate and Efficiency
To further enhance the convection effect, the microchannels were expanded into macrochannels to construct a tubular evaporator (PMS-T). As the diameter of the hollow region increased, the evaporation rate significantly improved. The evaporation rate of PMS-T-3 increased to 8.60 kg m⁻² h⁻¹, with an efficiency as high as 286.33%, significantly outperforming most reported evaporators.
Excellent Salt Resistance and Stability
PMS-T-3 maintained a high evaporation rate of 7.98 kg m⁻² h⁻¹ even in 20 wt.% high-salinity brine. Its performance remained stable during 7 days of continuous cycling tests with no significant decay. The quality of the desalinated water met WHO standards (Figure 4).
Figure 5. Simulation of thermal convection at the top of evaporators with different structures. (a) PMS with random channels, (b) PMS-R, and (c) PMS-T-3. (d-f) Schematics of numerical simulations for PMS-T hydrogel evaporators with different hollow sizes. (d) PMS-T-1, (e) PMS-T-2, and (f) PMS-T-3. (g-i) Simulation of fluid velocity and temperature distribution of hydrogels at thermal equilibrium. (g) PMS-T-1, (h) PMS-T-2, and (i) PMS-T-3.
In-depth Mechanism: Dual Verification by Simulation and Experiment
COMSOL multiphysics simulations revealed the synergistic mechanism between thermal convection, fluid flow velocity, and the temperature field (Figure 5). When the central "evaporation dead zone" in PMS-R was removed to form a macroscopic vertical channel, thermal convection was significantly enhanced. The simulation results show that the radial-structured hydrogel effectively reduces the impact of the evaporation dead zone on thermal convection, and removing the evaporation dead zone enhances thermal convection, thereby improving evaporation efficiency. The enhanced flow is attributed to two mechanisms: (1) The cavity channel increases the actual evaporation area, improving evaporation performance; (2) Stronger thermal convection accelerates water evaporation on the inner wall of the cavity. These synergistic effects ultimately enhance the overall evaporation rate, which is consistent with the experimental observations.
Figure 6 a) Self-cleaning capability of the PMS-C-3 evaporator. b, c) Salt resistance of PMS-C-3 and PMS-T-3 in 20 wt.% NaCl solution under 1 sun illumination. d) Schematic diagram of water transport and salt resistance mechanism during evaporation in the composite solar evaporator. e) Schematic diagram of water transport, salt resistance, and thermal convection mechanism during evaporation in the tubular solar evaporator. f) Comparison of salt resistance under 1 sun illumination for PMS-C-3 and PMS-T-3 with previously reported evaporators.
To evaluate the salt resistance of PMS-C-3, the research team uniformly deposited 0.4 g of NaCl crystals on its surface and recorded their dissolution process (Figure 6a). In a 20 wt.% NaCl solution, PMS-C-3 exhibited excellent salt resistance, with surface crystallization observed only after 15 hours (Figure 6b). PMS-T showed slightly lower salt resistance, with crystallization occurring after 10 hours (Figure 6c). These results confirm the critical role of the vertical zone in enhancing the overall salt resistance of the hydrogel. The outstanding salt resistance of the PMS evaporator series originates from two synergistic structural features:
(1) The vertically aligned channels in the oriented zone enable brine to be transported to the underlying water via the shortest path (Figures 6d-e);
(2) The external radial horizontal layered structure acts as a low-salinity ion transfer interface, significantly shortening the diffusion distance between the top high-salinity zone and the bottom low-salinity zone. The lamellar structure with micropores in the external radial structure promotes effective salt ion exchange between adjacent compartments, effectively mitigating salt accumulation pressure (Figure 6e). Furthermore, the salt precipitation time of the PMS hydrogel series in 20 wt.% NaCl solution was compared with previously reported evaporators, demonstrating a clear advantage for this hydrogel evaporator (Figure 6f).
In summary, this study successfully fabricated a solar interface evaporator with tunable internal vertical channels and an external radial structure, systematically investigating the regulatory effect of the intermediate micro-vertical channel size on its performance. Under one sun illumination, the evaporator (PMS-C-3) achieved an outstanding evaporation rate of 5.01 kg m⁻² h⁻¹ and an evaporation efficiency of 165.13%, maintaining high performance even after prolonged operation in high-concentration salt solutions. Moreover, by further expanding the microchannels into macro vertical channels, a tubular evaporator (PMS-T-3) was obtained, achieving an evaporation rate and efficiency of 8.60 kg m⁻² h⁻¹ and 286.33%, respectively. Experimental and simulation results indicate that increasing the size of the vertical channels in the composite structure significantly enhances the water transport capacity of the evaporator and minimizes heat loss. This remarkable enhancement stems from the active participation of the macro vertical channels in the evaporation process and the thermal convection that accelerates vapor escape within the tubular structure. The study provides crucial insights into understanding the balance mechanism between water transport and thermal management in evaporators and emphasizes the critical role of channel size regulation in optimizing the overall performance of evaporators.
Paper link: https://doi.org/10.1002/adfm.202519509
