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

液滴对非润湿纳米多孔表面的影响：新型空气膜的形成及其影响因素

• 批准号: 456180046
• 负责人: Dr. Günter K. Auernhammer
• 金额: --
• 依托单位: Institut Physikalische Chemie und Physik der Polymere
• 依托单位国家: 德国
• 项目类别: Research Grants
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/456180046?language=en
• 关键词: Drop; impact; nonwetting; nanoporous; surfaces

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.

液滴对固体表面的冲击是自然界和工艺过程中普遍存在的现象。人们普遍认为，在撞击液滴的下面有一层薄薄的空气膜。在工艺过程中，气膜的动力学对液滴冲击的结果至关重要，并影响传热效率和阻力。对于光滑基底，本文详细研究了撞击液滴下的薄膜动力学。然而，在粗糙或纳米孔表面，对空气膜的稳定性和动力学的基本理解仍然是缺失的，因为它是液滴和衬底性质的函数。在非润湿纳米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在环境条件下的撞击滴下。我们假设封闭孔隙中的空气与撞击液滴和基体之间的空气耦合。这种相声有助于新型气膜的形成。撞击过程中，表面结构、液体性质和周围环境都会影响气膜的形成和动力学。本项目重点研究这种新型空气膜在纳米孔表面的形成机理和动力学，阐明其影响因素的影响，解决新型空气膜对液滴冲击动力学的贡献。我们打算使用不同的纳米孔表面（开孔或闭孔），并系统地改变孔径和孔长。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。使用高速摄像机的侧面和顶部成像以及底部高速共聚焦成像揭示了撞击过程的动力学。跌落冲击实验是在控制良好的条件下进行的，系统地改变了环境气压、表面温度、表面倾角、液体表面张力和粘度。这些参数的变化使我们能够揭示气膜的起源和动力学，给出最大水滴半径、水面接触面积半径、气膜的半径和厚度、气膜的动力学和寿命，以及水滴弹跳和飞溅的临界冲击速度的定量数据。加热的基板导致液滴的蒸发增强，并在液滴和基板之间添加蒸气源。较低的环境压力减少了液滴和基材之间的气体量。此外，由于空气流过孔隙，具有开放孔隙的表面减少了空气膜。总之，我们的目标是定量了解液滴撞击纳米孔表面时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功能表面，以控制气膜的动力学，实现特殊的不润湿性能，并控制传热和流体阻力。