液滴碰撞固体表面是自然界和工程领域中常见的一种现象，碰撞过程涉及到的液滴动力学和质热传递等问题对具体应用有着重要影响。如对于喷墨打印或喷雾冷却技术等，往往需要增强液滴在表面的沉积或与表面的传热效率。但在材料防水保温和抗结冰等场合，却需要限制液滴与表面之间的接触以及热量交换。已有研究发现当表面具有较强的疏水性时，液滴碰撞之后经历铺展和收缩阶段，能够从表面完全反弹。疏水性表面液滴弹跳，较短的固液接触时间和较小的固液接触面积，在控制液滴接触和传热方面具有重要的应用前景。由于质热传递主要发生在接触期间，可通过缩短液滴碰撞接触时间或调控界面传热等方式实现对液滴接触和传热的抑制。但目前表面结构和润湿性、液滴尺寸和碰撞速度等参数对接触时间和传热过程的影响规律尚待揭示。因此，本文开展了疏水性表面液滴碰撞接触时间和碰撞过程传热机理的基础研究，主要包括以下几个方面： 基于液滴非对称弹跳缩短接触时间的思路，研究了液滴碰撞不同尺度圆柱超疏水表面非对称弹跳的接触时间变化规律和内在机理。采用电火花线切割结合化学刻蚀的方法制造了亚毫米到毫米宏观尺度的圆柱超疏水表面，结合高速摄像法分析不同速度的液滴碰撞表面后的弹跳形式和对应的接触时间。实验发现，液滴在只有微纳米结构的超疏水平面上常规对称弹跳，固液接触时间基本保持恒定。但在不同直径的圆柱超疏水表面上表现出多种弹跳形式，接触时间得到了减少，且当液滴碰撞相近大小的圆柱超疏水表面时，接触时间最短。建立了基于液滴收缩速度的理论模型，证明圆柱直径小于液滴时，接触时间通常随着圆柱直径和液滴韦伯数的增加而减少；而当圆柱直径大于液滴时，在较小的圆柱直径和较大的液滴韦伯数条件下，其与表面的接触时间较短。理论分析揭示了液滴在不同尺度圆柱超疏水表面弹跳接触时间缩短的机理，即通过减少柱面上方的液膜厚度，提高液滴的轴向收缩速度，进而缩短液滴的收缩阶段时间。 受到水稻叶各向异性润湿性的启发，设计制造了亚毫米宏观尺度的沟槽超疏水表面，研究液滴在表面的弹跳形式和接触时间的变化规律。开展液滴碰撞实验，通过调整液滴韦伯数，发现了液滴在沟槽超疏水表面的一种“瓣状弹跳”快速脱离现象。与超疏水平面上的常规对称弹跳相比，该弹跳形式将液滴与表面的接触时间缩短了约70％。理论建模证明了该弹跳形式归因于液滴在向下穿入沟槽阵列过程中储存了界面能，在随后的向上毛细排空过程中界面能转化为液滴的动能，从而实现液滴的整体快速抬升。在沟槽深度较低的超疏水表面上，碰撞液滴接触到沟槽底部，由于拉普拉斯力的作用，液滴沿着沟槽方向形成具有竖直向上速度分量的明显翅状液体分支，进一步促进了液滴的向上运动，从而更加缩短接触时间。较高速度的液滴碰撞沟槽超疏水表面，破碎成更多更均匀的小液滴，缩短了总体接触时间，从而扩展了接触时间减少的液滴韦伯数范围。 通过调控微柱阵列表面的固／气组份和疏水性，开展了液滴常规对称弹跳过程传热的理论和实验研究。结合光刻技术和涂层法制造了不同空气份数的硅基疏水性表面，开展液滴碰撞传热实验。利用高速红外成像法分析液滴碰撞表面过程的温度数据，计算液滴对热表面无量纲冷却效率的实验值。采用相似性解法进行理论建模，计算接触界面的瞬时热流密度和热表面传递到液滴的热量，得到液滴对表面冷却效率的理论值，并与实验值进行比较。模型考虑了液滴内部流场，与简化的瞬态热传导模型相比，液滴内部对流增强了接触界面传热。结果表明，空气份数越大的表面疏水性越强，液滴碰撞后的接触时间越短，同时界面热流密度越小；模型预测和实验测量的液滴对表面总冷却效率随液滴韦伯数的增加而增大，但随液滴直径和表面空气份数的增加而减小。 为了揭示疏水性表面液滴弹跳的局部瞬时传热规律，实验研究了液滴碰撞薄壁疏水表面在接触时间段内的界面传热过程。在薄壁铝片表面加工涂层制得光滑疏水表面，实现碰撞液滴从表面的完全脱离，同时采用亲水表面作为对比。分析液滴碰撞表面的动力学过程和壁面温度的分布与变化，根据柱坐标系下薄壁的传热原理，将有限差分法用于界面热流密度和传热量的计算，并且研究了液滴碰撞速度和表面亲疏水性等的影响。实验表明，三相接触线附近热流密度最大，各位置的热流密度都随时间逐渐减小；疏水表面上的液滴收缩阶段，热流密度最大值的位置逐渐向中心区域回退，亲水表面上的高热流密度始终位于液滴最大铺展处；液滴韦伯数和表面润湿性不会明显改变局部热流密度，但会改变液滴的铺展和收缩速度及固液接触面积，进而影响液滴与表面间的传热量。 通过研究，揭示了表面结构和润湿性、液滴尺寸和速度等对液滴碰撞接触时间和传热特性的影响规律，为设计疏水性表面控制液滴接触和传热提供了理论和实验依据。 关键词：超疏水表面，微纳制造，液滴碰撞，接触时间，界面传热

 Droplet impact on solid surfaces is a ubiquitous phenomenon in nature and industry field. The droplet dynamics as well as heat and mass transfer characteristics involved during droplet impact have significant effect on specific applications. For inkjet printing or spray cooling technology, it is desirable to enhance droplet deposition or heat transfer effectiveness between impacting droplets and underlying surfaces. However, when design water-repellent and insulating or anti-icing surfaces, it is often necessary to limit the contact and heat exchange between droplets and surfaces. It was reported that an impacting droplet can bounce off a surface which has good hydrophobicity, after spreading and retraction stages. For droplet bouncing on hydrophobic surfaces, short contact time and small contact area between droplets and surfaces endow it important application prospects in controlling droplet contact and heat transfer. Since heat and mass transfer mainly occurs during the contact period, the suppression of heat transfer can be achieved by shortening the contact time or regulating interfacial heat transfer rate. However, the effects of surface structure, droplet size and impact velocity on the contact time and corresponding heat transfer process need to be revealed. Therefore, this study performs basic research on the contact time between droplets and hydrophobic surfaces as well as heat transfer mechanisms, including the following aspects. Based on the idea of reducing droplet contact time through asymmetric bouncing, the contact time variations and inherent mechanisms for droplet impact on different scale cylindrical superhydrophobic surfaces were studied. A series of cylindrical superhydrophobic surfaces from submillimeter to millimeter scales were fabricated by Wire Electrical Discharge Machining (WEDM) and chemical etching. Droplet impact dynamics and corresponding contact time on the surfaces were analyzed through high-speed imaging. It was found that the contact time of conventional bouncing droplets on a superhydrophobic flat surface with micronano structures was almost constant during the experiments. The contact time between impacting droplets and the cylindrical surfaces with different diameters was reduced compared to the conventional bouncing, spanning several bouncing regimes. The contact time was generally shortest for droplets impacting a cylindrical surface with a comparable diameter. A theoretical model based on the retraction speed of droplets was established. It is verified that the contact time generally decreases with the cylinder diameter and the Weber number when the cylinders are smaller than the droplets. Besides, the contact time is shorter if droplets impact cylinders with smaller diameters or at larger Weber numbers, when the cylinders are larger. The contact time reduction mechanism for droplet impact on different scale cylindrical superhydrophobic surfaces is revealed theoretically. That is reducing the liquid film thickness above the cylinders to accelerate droplet retraction and reduce the retraction time. Inspired by the anisotropic wettability of rice-leaves, sub-millimeter macrostructure grooved superhydrophobic surfaces were designed and fabricated to study droplet bouncing types and contact time variations. By changing the Weber number of droplets, a rapid detachment phenomenon named “petal bouncing” appeared on the grooved superhydrophobic surfaces in droplet impact experiments. Compared to the conventional bouncing on a superhydrophobic flat surface, the contact time was reduced by ～70%. Theoretical model proves that it is attributed to the interfacial energy stored during the downward penetration of impacting droplets. In the subsequent capillary emptying process, the interfacial energy partially transforms into kinetic energy, lifting the droplets rapidly. On the superhydrophobic surface with a lower groove depth, the impacting droplets touch the bottom of the grooves. Due to the Laplace force, the droplets turn into distinct wing-like liquid branches with vertical velocity components along the grooves. It further promotes the upward motion of the droplets and shortens the contact time. Impacting droplets with higher velocities break into smaller and more uniform child droplets on the grooved superhydrophobic surfaces. This shortens the overall contact time, extending the range of the Weber number for contact time reduction. Heat transfer process for conventional symmetric droplet bouncing was studied theoretically and experimentally, by regulating the solid/gas components and hydrophobicity of micro-pillared surfaces. Hydrophobic surfaces with different cavity fractions were fabricated by using photolithography and coating techniques. Droplet impact heat transfer experiments were performed on the heated surfaces. Temperature distributions and variations of impacting droplets were analyzed by high-speed infrared imaging, to calculate the temperature increase of the droplets and further estimate the experimental values of the dimensionless cooling effectiveness. Similarity solution was used to theoretically model the instantaneous heat flux at the interface. The total heat absorbed by bouncing droplets was calculated to obtain the theoretical values of cooling effectiveness, and compared with the experimental ones. The convection of fluid inside impacting droplets is considered in the model, which enhances the interfacial heat transfer. Results show that a surface with a higher cavity fraction is more hydrophobic, the droplet contact time is shorter and the interfacial heat flux is also weaker on the surface. Theoretical and experimental cooling effectiveness of droplets increases with the Weber number, but decreases with droplet diameter and surface cavity fraction. To reveal the transient heat transfer to bouncing droplets on hydrophobic surfaces, the interfacial heat transfer process during droplet contact period on hydrophobic foil surfaces was experimentally studied. Thin aluminum foils were coated to make the surfaces hydrophobic, from which impacting droplets can completely detach. Smooth hydrophilic surfaces were also used for comparison. Droplet impact dynamics and the variations of substrate temperature were recorded and analyzed. The heat transfer principle of a thin foil was analyzed in the cylindrical coordinate system. Finite difference method was applied to calculate the interfacial heat flux and total heat transfer. The effects of droplet impact velocity and surface wettability were also studied. Experimental results show that the interfacial heat flux is the highest near the threephase contact line, and it decreases with time. During the retraction stage of droplets on the hydrophobic surface, the positions of the maximum heat flux gradually return toward the central region, while the highest heat flux always appears at the maximum spreading diameter on the hydrophilic surface. The Weber number and surface wettability do not significantly change the local heat flux at the interface, but they change the spreading and retracting speed of the impacting droplets as well as the solid/liquid contact area, thereby affecting the heat transfer between droplets and the underlying surfaces. Through the above research, the effects of surface structure and wettability as well as droplet diameter and velocity on the contact time and heat transfer characteristics for droplet impact are revealed. This provides a theoretical and experimental basis for the design of hydrophobic surfaces to control droplet contact and heat transfer. Key Words: Superhydrophobic surface; Micro-nano fabrication; Droplet impact; Contact time; Interfacial heat transfer