Abstract:

Objective Solar-driven interfacial evaporation (SDIE) has emerged as a promising strategy for sustainable seawater desalination, yet its practical application is often constrained by inefficient water transport, significant heat loss, and complex fabrication processes. This study aims to develop a biomimetic tree-like evaporator by a simple braiding technique that integrates hydrophilic cotton yarns with carbon fibers of high photothermal conversion efficiency. The objective is to investigate systematically the influence of yarn exposure height (5-20 cm) on evaporation performance and to elucidate the synergistic mechanism between capillary water supply and heat management for structural optimization.

Method Carbon fiber/cotton composite yarns were fabricated using a semi-automatic braiding machine and bundled into three-dimensional evaporators with heights of 5, 10, 15, and 20 cm. The surface morphology and wettability were characterized by digital microscopy and contact angle measurement, respectively. The photothermal absorption property was evaluated by UV-Vis-NIR spectroscopy across the full solar spectrum (300-2 400 nm). Evaporation performance was assessed under simulated sunlight (1 kW/m²) by real-time mass monitoring. Infrared thermal imaging and wicking height tests were conducted to analyze temperature distribution and water transport capability across different heights.

Results UV-Vis-NIR spectroscopy revealed that the carbon/cotton composite yarn exhibited excellent broadband light absorption, reaching 97.0% in the ultraviolet region, maintaining 96.5%-95.0% in the visible spectrum, and retaining 94.5%-91.0% in the infrared region, with an average absorption exceeding 90% across the entire 300-2400 nm range. Infrared thermal imaging demonstrated a distinct temperature gradient along the evaporator height, with top temperatures decreasing as height increased due to enlarged sidewall heat dissipation and prolonged heat conduction paths. Wicking height tests confirmed that water transport time increased with yarn length, indicating greater water supply difficulty for taller evaporators. The evaporator with a 15 cm height achieved the highest evaporation rate of 1.95 kg/(m²·h) under 1 kW/m² illumination, surpassing the 5, 10, and 20 cm counterparts by 48.85%, 17.47%, and 34.48%, respectively. The 15 cm-height evaporator also exhibited excellent stability over 10 h of cyclic operation, maintaining an evaporation rate of approximately 1.99 kg/(m²·h). In salt resistance tests, it retained a high evaporation rate of 1.45 kg/(m²·h) in 15% NaCl solution. When applied to natural seawater from the South China Sea, the system achieved over 99% rejection of Na⁺, Mg²⁺, Ca²⁺, and K⁺, with post-treatment ion concentrations meeting the drinking water standards set by the World Health Organization and the U.S. Environmental Protection Agency.

Conclusion This study successfully demonstrates that a three-dimensional evaporator fabricated from carbon fiber/cotton composite yarns, with optimized height, enables high-performance solar desalination through synergistic capillary water supply and thermal managements, providing a scalable and cost-effective textile-based strategy for sustainable freshwater production, with strong potential for application in coastal regions facing water scarcity.

Key words: carbon fiber, woven structure, photothermal evaporation, structural regulation, seawater desalination