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

固体表面的跌落冲击是自然界和工艺过程中普遍存在的现象。一般认为，在撞击液滴下面夹有一层薄的空气膜。这种空气膜的动态特性对于跌落冲击的结果至关重要，并且影响工艺过程中的传热效率和阻力。对于光滑的基板，薄的空气膜下的冲击滴的动力学进行了详细的研究。然而，在粗糙或纳米多孔表面上，空气膜的稳定性和动力学的基本理解仍然缺失作为液滴和基底性质的函数。在非润湿纳米多孔氧化铝表面的初步实验中，我们观察到一种新型的空气膜下的环境条件下的冲击滴。我们假设封闭孔中的空气与撞击液滴和基底之间的截留空气耦合。这种串扰有助于形成新的空气膜。表面结构、液体性质和周围条件影响冲击过程中气膜的形成和动态。本项目的重点是这种新型的空气膜在纳米多孔表面上的形成机制和动力学，阐明所涉及的影响因素的影响，并解决这种新型的空气膜对跌落冲击动力学的贡献。我们打算使用不同的纳米多孔表面（具有开放或封闭的孔），并系统地改变孔径和孔长度。我们采用扫描电子显微镜和原子力显微镜来表征表面结构。使用高速照相机的侧视图和俯视图成像以及仰视图高速共焦成像揭示了撞击过程的动态。跌落冲击实验在良好控制的条件下进行，系统地改变环境空气压力、表面温度、表面倾斜度、液体表面张力和粘度。这些参数的变化使我们能够解开的起源和动态的空气膜，给出定量的数据上的最大滴半径，水面接触面积的半径，和空气膜的半径和厚度，动态和寿命的空气膜，以及临界冲击速度为滴反弹和飞溅。加热的基底导致液滴的蒸发增强，并在液滴和基底之间添加蒸汽源。较低的环境压力减少了液滴和基底之间的气体量。此外，具有开放孔的表面由于空气流过孔而减少了空气膜。总而言之，我们的目标是定量地了解纳米多孔表面上的液滴冲击中的空气膜的稳定性及其对液滴扩散、弹跳和飞溅动力学的贡献。我们的研究结果有助于合理设计的功能表面，以控制动态的空气膜，以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.