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无需太多了解基本物理的情况下在自清洁和防污垢应用中的潜在好处而引起了人们的极大兴趣。拟议的项目旨在通过研究润滑和液体注入表面上的液滴冲击动力学来解决这一问题，重点是探索液滴-膜接触之前和液滴-膜相互作用期间携带的空气层的作用。了解液滴下的气膜动力学有助于更好地调节润滑层中的液膜特性，设计更坚固的纳米结构，并最终增强从防污、防冰到相分离和热管理等各种应用的注液表面的性能。由于气膜消散是一种普遍现象，本研究的结果也适用于许多工业过程，如功能材料的多层喷墨打印、润湿表面的喷涂、内燃机燃料的燃烧、蜡质植物叶片上的农药喷洒以及微电子产品的冷却。因此，该项目涉及流体力学与表面科学、纳米制造和传热学相结合的几个研究前沿。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，以及在美国物理学会流体动力学分部会议上对流体运动画廊的原创贡献。除了培养研究生外，PI还将积极指导本科生研究人员，并招募女性和代表性不足的少数民族学生。社区外展计划通过费城材料日、费城科学节和德雷克塞尔研究生院的K-12和本科生研究项目扩展到市中心的K-12教师和学生。这项研究的主要目标是促进对液滴冲击润滑和液体注入表面的基本理解，重点是探索液液接触之前和液液相互作用之前液滴和薄膜之间的夹带空气层的作用。与固体表面上的液滴冲击不同，在固体表面上，即使是最小的粗糙度也会导致携带气膜的随机破裂，而润滑表面上的气膜破坏机制预计要容易控制得多。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。通过将全内反射显微镜、反射干涉显微镜和高速成像相结合，可以以纳米级和微秒级的时间分辨率直观地显示不同撞击条件和液滴/膜/衬底性质下的气膜演化和破裂后的液-液接触动力学。这种独特的能力使得能够直接探测液液分子间相互作用变得重要的厚度处的气膜轮廓。从该项目中获得的知识将弥合固体表面的液滴撞击物理与深液池之间的差距，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。该具体项目旨在解决以下问题作为液滴冲击润滑和注入表面的基本问题：(1)在不同的冲击条件、液膜性质和环境压力下，气膜如何演化和破裂？(2)气膜破裂后液-液接触的前沿增长动力学是什么？(3)液液前沿增长的指状不稳定性的机制是什么？(4)表面结构和倾角如何影响空气卷吸和液滴冲击动力学？

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?