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非对称润湿性Janus纤维膜凭借其两侧显著的润湿性差异, 在与液体相互作用时展现出诸多独特性能, 因此在微流控和生物医学等领域具有广阔的应用前景. 液滴定向输运作为Janus纤维膜的关键功能之一, 其输运机制与调控规律对于实际应用至关重要. 然而, 目前对于润湿性梯度及孔隙结构如何调控液滴定向输运行为的研究尚不充分. 本文建立了两相流-相场模型, 结合等离子体辅助构筑的Janus纤维膜液滴输运实验, 验证了模型的可靠性; 在此基础上, 系统研究了液滴在膜内的定向输运行为. 研究表明, 液滴从疏水侧向亲水侧的自发输运由表面自由能梯度、Laplace压差及毛细力协同驱动; 疏水层厚度、亲水层厚度、润湿性梯度和孔隙结构是调控输运效率的关键因素. 相较于传统非对称润湿性结构, 具有润湿性梯度的Janus纤维膜可显著提升液滴定向输运速度, 且亲水侧润湿性与输运速度呈显著正相关; 增大孔隙虽能加速液滴输运, 却会导致其在亲水侧的稳态铺展面积减小. 本研究为优化Janus纤维膜结构、实现液滴的高效与精准操控提供了重要理论依据.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. In this study, a two-phase flow phase-field model is established, and the reliability of the model is 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 is systematically investigated. The results show that the spontaneous transport of droplets from hydrophobic side to hydrophilic side is driven by a synergistic effect of surface free energy gradient, Laplace pressure difference, and capillary force. It is found that hydrophobic layer thickness, hydrophilic layer thickness, wettability gradient, and pore structure are key factors in regulating transport efficiency. Compared with traditional structures, Janus fiber membranes with wettability gradients can significantly improve the directional transport speed of droplets, and the wettability of the hydrophilic side shows a significant positive correlation with transport velocity. Although increasing pores can accelerate droplet transport, it simultaneously reduces 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 fabrication of droplets.
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Keywords:
- Janus membrane /
- plasma modification /
- two-phase flow phase field model /
- asymmetric wettability /
- droplet transport
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图 1 等离子体辅助构筑非对称润湿性Janus纤维膜流程示意图
Fig. 1. Schematic diagram of plasma assisted construction of asymmetric wetting Janus fiber membrane process.
图 2 液滴在Janus纤维膜中定向输运的(a)仿真模型与(b)网格划分
Fig. 2. (a) Simulation model and (b) grid division for directional transport of droplet in Janus fiber membrane.
图 3 等离子体辅助构筑的Janus纤维膜(a)疏水侧和(b)亲水侧润湿状态和水接触角
Fig. 3. Wetting state and water contact angle on (a) hydrophobic side and (b) hydrophilic side of plasma assisted construction of Janus fiber membrane.
图 4 液滴从疏水侧到亲水侧自输运过程的(a)仿真演化过程及(b)实验监测结果
Fig. 4. (a) Simulation evolution process and (b) experimental monitoring results of droplet self-transport from hydrophobic side to hydrophilic side.
图 5 液滴滴于亲水侧的(a)仿真演化过程及(b)实验监测结果
Fig. 5. (a) Simulation evolution process and (b) experimental monitoring result of droplet spreading on the hydrophilic side.
图 6 液滴克服重力输运的(a)实验监测结果及(b)仿真演化过程
Fig. 6. (a) Simulation evolution process and (b) experimental monitoring result of droplet overcoming gravity transport from hydrophobic side to hydrophilic side.
图 7 液滴定向输运过程中受力分析 (a) 液滴从Janus纤维膜疏水层到亲水层的受力情况; (b) 液滴滴于Janus纤维膜亲水层受力情况
Fig. 7. Force analysis during droplet directional transport: (a) Force distribution of droplet movement from the hydrophobic layer to the hydrophilic layer of Janus fiber membrane; (b) force distribution of droplet movement on the hydrophilic layer of Janus fiber membrane.
图 8 液滴在不同HoLT、HiLT和DD的Janus纤维膜中单向输运的数值仿真结果
Fig. 8. Numerical simulation results of unidirectional transport of droplets in Janus fiber membranes with different HoLT, HiLT, and DD.
图 9 液滴在不同润湿梯度的Janus膜中运动特性仿真结果 (a) 液滴顶端位移变化; (b) 液滴底端位移变化; (c) 液滴铺展直径变化
Fig. 9. Simulation results of droplet kinetic characteristics on Janus fiber membranes with different wetting gradients: (a) Variation in upper contact point displacement of droplet; (b) variation in lower contact point displacement of droplet; (c) variation in spreading diameter of droplet.
图 10 液滴在不同孔间隙的Janus膜中运动特性仿真结果 (a) 液滴顶端位移变化; (b) 液滴底端位移变化; (c) 液滴铺展直径变化
Fig. 10. Simulation results of droplet kinetic characteristics on Janus fiber membranes with different pore gaps: (a) Variation in upper contact point displacement of droplet; (b) variation in lower contact point displacement of droplet; (c) variation in spreading diameter of droplet.
表 1 不同润湿性梯度下Janus纤维膜的水接触角及孔间隙参数
Table 1. Water contact angle and pore gap parameters of Janus fiber membrane under different wettability gradients.
类型 y(mm)方向
0—0.35 mm处
WCA/
(°)y(mm)方向
0.35—0.4 mm处
WCA/
(°)孔间隙
/mmA-on-B 50 150 0.03 A-to-B1 114.3y+50 1200y–330 A-to-B2 142.9y+40 1200y–330 
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表 2 不同孔间隙下Janus膜的接触角参数
Table 2. Water contact angle parameters of Janus fiber membrane under different pore gaps.
孔间隙/mm y(mm)方向
0—0.35 mm处WCA/(°)y(mm)方向
0.35—0.40 mm处WCA/(°)0.03 114.3y+50 1200y–330 0.04 114.3y+50 1200y–330 0.05 114.3y+50 1200y–330
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