Drop impact on nonwetting nanoporous surfaces: formation of a novel air film and its influencing factors

液滴冲击固体表面是自然界和工艺过程中普遍存在的现象。人们普遍认为，薄薄的空气膜被困在撞击液滴的下面。这种气膜的动力学特性对跌落冲击的结果至关重要，并影响到工艺过程中的换热效率和阻力。对于光滑的衬底，详细研究了撞击液滴下薄空气膜的动力学。然而，在粗糙或纳米多孔表面上，仍然缺乏对气膜稳定性和动力学作为液滴和衬底特性函数的基本了解。在非润湿纳米多孔氧化铝表面的初步实验中，我们在环境条件下观察到了一种新型的撞击液滴下的气膜。我们认为，闭合气孔中的空气耦合到撞击液滴和衬底之间的滞留空气。这种串扰有助于形成新颖的空气膜。冲击过程中气膜的形成和动力学受表面结构、液体性质和环境条件的影响。本课题重点研究了这种新型气膜在纳米孔表面的形成机理和动力学，阐明了影响因素的影响，并解决了这种新型气膜对液滴碰撞动力学的贡献。我们打算使用不同的纳米多孔表面(具有开放或闭合的孔)，并系统地改变孔直径和孔长度。我们使用扫描电子显微镜和原子力显微镜来表征表面结构。使用高速相机的侧视和俯视成像以及底视高速共焦成像揭示了撞击过程的动力学。液滴撞击实验是在控制良好的条件下进行的，系统地改变环境空气压力、表面温度、表面倾角、液体表面张力和粘度。这些参数变化使我们能够揭示气膜的起源和动力学，给出关于最大液滴半径、水面接触区半径、气膜半径和厚度、气膜的动力学和寿命以及液滴反弹和飞溅的临界撞击速度的定量数据。加热的衬底导致液滴的蒸发增强，并在液滴和衬底之间添加蒸汽源。较低的环境压力减少了液滴和衬底之间的气体量。此外，具有开放毛孔的表面由于空气流过毛孔而减少了气膜。综上所述，我们的目标是定量地了解气膜在液滴撞击纳米孔表面时的稳定性及其对液滴扩散、反弹和飞溅动力学的贡献。我们的结果有助于合理设计功能表面，以控制气膜的动力学，实现特殊的非润湿性能，并控制传热和流体阻力。

Drop impact on solid surfaces is a ubiquitous phenomenon in nature and technological processes. It is generally accepted that a thin air film is entrapped underneath impinging drops. The dynamics of such air film is crucial for the outcome of drop impact and affects the heat transfer efficiency and drag in technological processes. For smooth substrates, the dynamics of the thin air films underneath impinging drops have been studied in detail. However, on rough or nanoporous surfaces, a fundamental understanding of the stability and dynamics of the air film is still missing as a function of drop and substrate properties. In preliminary experiments on nonwetting nanoporous alumina surfaces, we observed a novel kind of air film underneath impinging drops under ambient conditions. We suppose that the air in the closed pores couples to the entrapped air between impinging drops and the substrate. This crosstalk contributes to the formation of the novel air film. The surface structure, the liquid properties and the surrounding conditions affect the formation and dynamics of the air film during the impact. This project focuses on the formation mechanism and dynamics of this novel air film on nanoporous surfaces, elucidating the effects of the influencing factors involved and resolving the contribution of the novel air film on the drop impact dynamics. We intend to use diverse nanoporous surfaces (with either open or closed pores) and vary systematically the pore diameter and pore length. We apply scanning electronic microscopy and atomic force microscopy to characterize the surface structure. Side-view and top-view imaging using high-speed cameras and bottom-view high-speed confocal imaging reveal the dynamics of the impact process. Drop impact experiments are performed under well-controlled conditions, varying systematically the ambient air pressure, surface temperature, surface inclination, liquid surface tension and viscosity. These parameter variations enable us to unravel the origin and dynamics of the air film, giving quantitative data on the maximum drop radius, the radius of the water-surface contact area, and the radius and thickness of the air film, the dynamics and lifetime of the air film, as well as the critical impact velocities for drop bouncing and splashing. Heated substrates lead to an enhanced evaporation of the drop and add a vapor source between the drop and the substrate. Lower ambient pressures reduce the amount of gas between the drop and the substrate. Also, surfaces with open pores reduces the air film due to air flow through the pores. To summarize, we aim for a quantitative understanding of the stability of the air film in drop impact on nanoporous surfaces and its contribution on the drop spreading, bouncing and splashing dynamics. Our results contribute to the rational design of functional surfaces to control the dynamics of the air film, to achieve special nonwetting properties and to control heat transfer and fluid drag.