Numerical simulation of droplet transport behavior in plasma-assisted fabricated asymmetric wettability Janus fiber membranes

The asymmetric wetting Janus fiber membrane exhibits many unique properties when interacting with liquids due to its significant difference in wetting properties on both sides. Therefore, it has broad application prospects in fields such as microfluidics and biomedicine. The directional transport of droplets is one of the key functions of Janus fiber membranes, and its transport mechanism and regulation rules are crucial for practical applications. However, there is currently insufficient research on how wettability gradient and pore structure regulate the directional transport behavior of droplets. This study established a two-phase flow phase-field model, and the model's reliability was validated through droplet transport experiments conducted on plasma-assisted fabricated Janus fiber membranes. Building on this foundation, the directional transport behavior of droplets within the membrane was systematically investigated. Results had shown that the spontaneous transport of droplets from hydrophobic to hydrophilic sides was driven by a synergistic effect of surface free energy gradient, Laplace pressure difference, and capillary force. It was found that hydrophobic layer thickness, hydrophilic layer thickness, wettability gradient and pore structure were key factors in regulating transport efficiency. Compared with traditional structures, Janus fiber membranes with wettability gradients could significantly improve the directional transport speed of droplets, and the wettability of the hydrophilic side showed a significant positive correlation with transport velocity. Although increasing pores could accelerate droplet transport, it concurrently reduced the steady-state spreading area on the hydrophilic side. This study provides an important theoretical basis for optimizing the Janus fiber membrane structure and achieving efficient and precise manipulation of droplets.