基于自相似多级褶皱的超疏水表面构筑及其水下减阻应用研究

Friction is the main source of drag for underwater vehicles. Reducing friction drag could significantly enhance the performance of under vehicles and increase fuel efficiency. Superhydrophobic surface has been proven that it could greatly reduce drag in both laminar and turbulent flow. The efficient superhydrophobic surface for drag reduction application requires an ordered structure with strong structural anisotropy to reduce spanwise slip and special design such as nano-micro hierarchical structure to stabilize the trapped air bubble. So far there is no method that could fabricate such a surface in large areas with low cost. Therefore a scalable fabrication method capable of fabricating the required microstructures for drag reduction is highly desired. Due to its ease in materials handling, fabrication, and control, surface wrinkling has been proven to be a facile approach to generate a variety of controllable surface topographies. The object of this program is to fabricate a superhydrophobic hierarchical wrinkling surface for drag reduction, which is composed of nanoscale wavy wrinkles on microscale wavy wrinkles with same orientation. The capability of drag reduction of superhydrophobic hierarchical wrinkling surface will be quantitatively verified and analyzed. The success of this project could greatly promote the practical application of superhydrophobic surface for drag reduction and find applications in oil pipe transportation and naval ships.

降低水面水下舰艇表面摩擦阻力可以显著提高航行速度节约燃料，具有重要经济和军事价值。超疏水表面是一种有效的减阻途径，但是高效减阻需要其表面形貌符合流体力学规律，增加了制造难度和成本，因此大面积低成本制备超疏水减阻表面是本领域研究目标之一。本项目拟利用自相似多级褶皱结构制备超疏水表面，并将其用于水下减阻。本项目提出如下假说：波纹状褶皱可以抑制展向滑移，提升超疏水表面湍流减阻能力；微纳多级结构的存在可以提升超疏水表面气层在水下的稳定性。本项目具体研究内容包括：通过实验和模拟深入研究多层薄膜基底体系后屈曲过程，给出自相似多级褶皱的形成条件；探索多级褶皱的制备方法，得到基于自相似多级褶皱的超疏水表面；通过压差测量法和粒子图像测速研究超疏水多级褶皱表面的减阻性能，优化表面形貌设计。本项目旨在为低成本制备高效减阻超疏水表面提供新思路，推进超疏水表面减阻技术的实用化进程。

本课题基于薄膜结构面内压缩屈曲力学原理，结合有限元模拟和理论建模分析，深入研究了多层薄膜/基底体系在后屈曲范围内的非线性图案演化过程，得到了小褶皱叠加大褶皱的自相似多级表面褶皱结构的形成条件，发现其本质为正弦曲面基底上“高刚度”薄膜的全局屈曲现象，并据此得到了薄膜全局起皱临界应变和特征屈曲波长随薄膜无量纲厚度和曲面基底凸起宽高比两个重要参数变化的定量关系。基于力学理论指导，利用等离子体梯度交联PDMS前驱体，实验制备了大小褶皱相互垂直以及大小褶皱相互平行的两种自相似多级褶皱结构。液滴接触角测试发现，所得表面褶皱结构具有157°的静态接触角和4.7°的滚动角，显示出良好的超疏水性。微凹槽表面可能降低舰艇表面摩擦阻力，提高航行速度节约燃料。进一步通过计算流体力学模拟研究了多级褶皱表面的减阻性能和机理，获得了不同流速下减阻率与褶皱形貌的关系，发现沿流动方向的大褶皱可以减小流动粘性阻力，而垂直于流动方向的小褶皱可以减小由大褶皱引起的压力阻力。因此当大褶皱与小褶皱的协同作用时，对于一定雷诺数范围的流体，净流动阻力减小了最大约8.6%。研究成果深化了对于多层薄膜基底结构褶皱现象的理解，扩展了薄膜基底结构后屈曲图案库，并且对于表面减阻微凹槽结构的设计应用具有一定参考价值。