On Solar-Motivated Evaporation of Saline from an Open Meniscus in a Capillary Tube

Seawater desalination and distributed water treatment demand efficient and controllable interfacial evaporation technologies. Micro-scale channels offer a robust platform for achieving stable and high-efficiency evaporation. In this work, a multiphysics model was developed for solar-driven brine evaporation within capillaries, coupling heat transfer, gas-liquid convection-diffusion and interfacial solute transport. The microscale flow and mass transfer behaviors near the meniscus were numerically reconstructed, exposing the interactions among temperature gradients, solute concentration and phase-change convection, along with their spatial patterns. A detailed examination of interfacial phase change, vapor diffusion and local flow clarified the physical mechanisms behind evaporation and energy transfer pathways. The heat distribution within system is governed by the competition among physical processes of solid conduction, gas convection and interfacial phase change. In this process, heat flux distribution exhibits high sensitivity to geometric conditions. The heating width and liquid column height govern the radial heat-mass distribution and axial transport scale, serving as key geometric factors regulating overall evaporation performance. A wider heating region strengthens interfacial heat input, raising the normalized peak flux from about 0.25 to 0.88, but also intensifies convective and conductive losses, reducing evaporation efficiency to about 20%. While a taller liquid column impedes downward heat conduction, further raising the normalized peak flux to about 0.9, and enhancing evaporation efficiency by roughly sixfold under non-overcooled conditions, it also increases salt accumulation and crystallization risks. In addition, based on a balance between evaporation rate and efficiency, the optimal operating range in this study was identified. These findings provide quantitative guidance for structural design of interfacial evaporators and offer theoretical support for long-term operation in distributed seawater desalination and thermal management systems.