The Role of Interstitial Air Layer in Drop Impact on Liquid-infused Surfaces

近年来，液体注入表面由于其在跌落冲击时的超滑特性以及在自清洁和防污应用中的潜在益处而引起了人们的极大兴趣，而无需对底层物理学有太多了解。拟议的项目旨在解决这个问题，通过检查润滑和液体注入的表面上的下降冲击动力学，重点是探测的作用，夹带的空气层之前的滴膜接触和滴膜相互作用。了解液滴下方的空气膜动力学可以更好地调节润滑层中的液膜特性，设计更坚固的纳米结构，并最终提高液体注入表面的性能，用于从防污和防结冰到相分离和热管理的应用。由于气膜消散是一种普遍现象，因此本研究的结果也与许多工业过程相关，例如功能材料的多层喷墨打印，润湿表面上的喷涂，内燃机中的燃料燃烧，蜡质植物叶片上的农药喷洒以及微电子设备的冷却。因此，该项目与流体力学的几个研究前沿有关，并与表面科学，纳米制造和传热相结合。该项目还将为PI最近开发的两门关于界面传输现象和纳米制造的课程提供新的课程材料，并为美国物理学会流体动力学会议部的流体运动画廊提供原始贡献。除了培训研究生，PI还将积极指导本科研究人员，并招募女性和代表性不足的少数民族学生。通过费城材料日、费城科学节和德雷克塞尔大学K-12研究生研究员和本科生研究项目，社区外展计划延伸到市中心的K-12教师和学生。本研究的主要目标是促进对润滑和液体注入表面的跌落冲击的基本理解，重点是在液-液接触之前和在液-液相互作用期间探测液滴和膜之间的夹带空气层的作用。与固体表面上的跌落冲击相比，即使是最小的粗糙度也会导致夹带空气膜的随机破裂，润滑表面上的空气膜失效机制预计将更加可控。此外，破裂后的液-液接触可能导致界面不稳定和微气泡的形成，其机理尚不清楚。通过整合全内反射显微镜，反射干涉显微镜，和高速成像，空气膜的演变和破裂后的液-液接触动力学将直接可视化不同的冲击条件和液滴/膜/基板属性与纳米空间和微秒时间分辨率。这种独特的能力使得能够直接探测在液-液分子间相互作用变得重要的厚度处的空气膜轮廓。从这个项目中获得的知识将弥合固体表面与深液池的跌落冲击物理学之间的差距，并使从表面结构设计到增强润滑剂稳定性的技术发现成为可能。具体的项目目标是解决以下问题的基本润滑和液体注入表面上的跌落冲击：（1）如何演变的空气膜和破裂与不同的冲击条件，膜的属性，和环境压力？(2)气膜破裂后液-液接触的前沿增长动力学是什么？(3)在增长的液-液前沿的指状不稳定性的机制是什么？(4)表面结构和倾斜度如何影响空气夹带和跌落冲击动力学？

Liquid-infused surfaces have attracted great interest in recent years due to their super slippery properties upon drop impact and potential benefits in self-cleaning and anti-fouling applications without much understanding of the underlying physics. The proposed project seeks to address this issue by examining the drop impact dynamics on lubricated and liquid-infused surfaces, with a focus on probing the role of the entrained air layer prior to drop-film contact and during drop-film interactions. Understanding the air film dynamics underneath the droplet allows for the better tuning of the liquid film properties in the lubricating layer, designing more robust nanostructures, and ultimately enhancing the performance of liquid-infused surfaces for applications ranging from anti-fouling and anti-icing to phase separation and thermal management. Since the air film dissipation is a universal phenomenon, the outcome of this study is also relevant to many industrial processes such as multilayered inkjet printing of functional materials, spray coating on wetted surfaces, combustion of fuel in internal combustion engines, pesticide spraying on waxy plant leaves as well as cooling of microelectronics. The project is hence relevant to several research frontiers of fluid mechanics in conjunction with surface sciences, nanomanufacturing, and heat transfer. This project will also enable new course materials for PI's two recently developed courses on Interfacial Transport Phenomena and Nanomanufacturing, as well as original contributions to the Gallery of Fluid Motion at the American Physical Society Division of Fluids Dynamics Meetings. In addition to training graduate students, the PI will actively mentor undergraduate researchers and recruit women and under-represented minority students. The community outreach programs extend to inner-city K-12 teachers and students through the Philly Materials Day, Philly Science Festival, and Drexel Graduate Fellows in K-12 and Research for Undergraduate programs.The key objective of this study is to advance the fundamental understanding of drop impact on lubricated and liquid-infused surfaces, with a focus on probing the role of the entrained air layer between the drop and film both prior to liquid-liquid contact and during liquid-liquid interaction. In contrast to drop impact on solid surfaces where even the smallest asperities cause random breakup of the entraining air film, the air film failure mechanisms on lubricated surfaces are expected to be much more controllable. Moreover, the post-rupture liquid-liquid contact may lead to interfacial instability and microbubble formation whose mechanisms are unclear. By integrating the total internal reflection microscopy, reflection interference microscopy, and high-speed imaging, the air film evolution and post-rupture liquid-liquid contact dynamics will be directly visualized for different impact conditions and drop/film/substrate properties with nanometer spatial and microsecond temporal resolution. This unique capability enables direct probing of air film profile at thickness where the liquid-liquid intermolecular interactions become important. The knowledge obtained from this project will bridge the gap between the drop impact physics of solid surfaces to deep liquid pools and enable technological discoveries from surface structural design to enhanced lubricant stability. The specific project aims are to address the following issues as fundamental to drop impact on lubricated and liquid-infused surfaces: (1) How does the air film evolve and rupture with varying impact conditions, film properties, and ambient pressure? (2) What is the frontal growth dynamics of liquid-liquid contact after air film ruptures? (3) What is the mechanism of finger-like instability at the growing liquid-liquid front? (4) How do surface structures and inclination affect air entrainment and drop impact dynamics?