Unraveling the impact of uneven wetting induced by temperature distribution on wetting restoration by water flushing

Membrane distillation (MD) provides an effective solution for hypersaline wastewater treatment. However, its widespread industrial application is hindered by surfactant-induced wetting that leads to a decline in membrane performance. Removing surfactants through water flushing has emerged as a practicable strategy for wetting restoration. Nevertheless, the uneven distribution of temperature along the membrane axis can affect the uniformity of wetting, which may in turn influence the overall recovery of membrane wetting. To address this issue, this work studied the impact of non-uniform wetting on wetting restoration realized by water flushing. The ultrasonic time-domain reflectometry was employed to enable real-time observation of the wetting interface migration at various membrane locations during flushing. The results showed an enhanced wetting restoration along the membrane’s axial direction, ascribed to reduced wetting depths from inlet to outlet positions. Decreasing the flow rate and the water temperature accelerates the cleaning efficacy of water flushing because it dramatically enlarges the cleaning depth. These findings were further supported by optical coherence tomography, computational fluid dynamic simulations and force balance theoretical calculations. Our results provide a comprehensive evaluation of local wetting restoration along membrane and shed light on effective maintenance strategies for hydrophobicity recovery of MD systems.