超疏水表面与水之间具有极低的粘附和摩擦，有望为诸多与环境、能源、生物医疗、军工国防等领域所亟需解决的自清洁、高效冷凝换热、流动减阻等科学问题提供解决方案，因而成为当前一个重要的研究方向。然而，超疏水表面的稳定性问题一直是制约其真正走向大规模实际应用的关键。超疏水表面的稳定性可以分为三类：化学稳定性、结构稳定性和润湿状态稳定性。其中要求水与材料接触区处于Cassie状态的润湿状态稳定性需求最为广泛，因此成为超疏水表面的阿喀琉斯之踵。由于可控的结构参数，具有规则微纳米结构的超疏水表面成为研究润湿状态稳定性的优良载体。在该表面上的准静态实验和理论研究表明，存在满足Cassie状态为唯一稳定状态的表面，称之为单稳态表面。相比于准静态润湿行为，单稳态表面的动态润湿行为更加接近实际应用的需求。从表面微纳米结构角度出发，无论是由外而内的液滴冲击，还是由内而外的液滴冷凝，动态润湿现象都普遍存在于日常生活和工业生产中。因此理解单稳态表面的动态润湿特性对于促进其走向实际应用具有重要意义。本文针对上述问题开展研究，主要取得了如下创新性成果：1、发现液滴冲击单稳态表面过程中的多次自相似的Cassie-Wenzel-Cassie 润湿状态转换；通过实验和理论分析澄清了液滴在Wenzel接触区微结构间的铺展和回缩动力学行为特征。当液滴冲击被颗粒污染的表面时，部分液滴进入微尺度结构间隙中带走大量内部污染颗粒，从而发现并制备出具有深度自清洁的超疏水表面。该研究为制备和设计具有高效自清洁功能的表面提供了方向。2、通过调控单稳态表面的温度，实现了对冲击液滴在该表面的粘附区域的控制。通过对液滴在微米结构间隙中的运动接触特征时间和纳米结构间隙中的冷凝成核特征时间之间的竞争分析，获得粘附与脱粘附的分界点，建立了基底温度与液滴粘附区域大小的关系。该研究为实现低粘附表面的液滴吸附提供了思路。3、基于液滴的透镜效应，发现了单稳态表面液滴的三维复合冷凝生长过程。次液滴在微结构间周期性的快速冷凝与吸收，使其上方主液滴的体积生长速率提升高达170%。澄清了微米尺度的结构尺寸对受限空间中液滴冷凝的影响，并基于统计平均模型给出了不同微结构尺度上次液滴对主液滴生长的贡献和生长标度指数。该研究为提高单稳态表面冷凝效率和抑制局部超疏水性能失效提供了指导。
The superhydrophobic surface has extremely low adhesion and friction with water, which is promising to solve the key problems in the fields of environment, energy, and biomedicine, such as the realization of self-cleaning, condensation and thermal dissipation with high efficiency and drag reduction. Thus, the study on the spectacular properties of superhydrophobicity become an important research direction. However, the stability of superhydrophobic surfaces has always been a key challenge restricting its large-scale applications. Specially, the stability of superhydrophobicity could be divided into three categories: chemical stability, structural stability and wetting state stability. Among them, the stability of the wetting state, which is required to be stable in the Cassie state, has the most extensive demand and becomes the Achilles heel of the super-repellent surface. Due to the controllable structural parameters, the micro-nano regular structure surface is an excellent carrier for studying the stability of the wetting state. The experimental and theoretical results under quasi-static state show that there exists a kind of surface on which the Cassie wetting state is the only one stable state. We name this surface the monostable surface. In fact, dynamic wetting behaviors are ubiquitous in a variety of practical applications to meet the need, rather than its quasi-static counterpart. From the perspective of the micro/nano-structured surface, whether it is the droplets that hitting the outer surface, or the condensation that occurring inside of structures, dynamic wetting happens common in daily production and life. Therefore, understanding the dynamic wetting characteristics of monostable superhydrophobic surfaces is of great significance to promote practical applications. This article has conducted research on the aforementioned questions. The main achievements in this dissertation are as follows:1. We found multiple self-similar Cassie-Wenzel-Cassie state transitions in the process of droplets impacting on a monostable superhydrophobic surface; through experimental and theoretical analysis, the kinetic behavior of the spreading and retraction of the droplets on the Wenzel contact region was obtained. In the process of droplets impacting the surface contaminated by particles, a part of liquid enters the micro-scale structure gaps to take away a large number of internal contaminated particles, showing that the surface has a deep self-cleaning function. This research provides an optimized direction for the preparation and design of high-efficiency self-cleaning surfaces.2. By adjusting the temperature of the monostable superhydrophobic surface, we realized controllable solid-liquid adhesion region of the impacting droplet. Through the competition between the characteristic time of droplet movement in the microstructure gap and the characteristic time of condensation and nucleation in the nanostructure gap, the critical condition between adhesion and de-adhesion is obtained. Furthermore, the relationship between the substrate temperature and the size of the droplet adhesion area is established. This research provides ideas for the adsorption of droplets on low adhesion surfaces.3. Based on the lens effect of the droplet, we discovered a kind of composite condensation growth process of droplets on monostable superhydrophobic surfaces. The sub-droplets periodically condensed and absorbed between the microstructures at high speed, which enhances the volume growth rate of the main droplet above the sub-droplets by up to 170%. We have figured out the influence of the micro-structure size on the condensation of droplets in the confined space. Based on the statistical average model, the contribution of the sub-droplets of different scales to the growth of the main droplet and the growth scaling exponent are given. This research provides guidance for improving the efficiency of monostable surface condensation and inhibiting the failure of superhydrophobic performance.
