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

近年来，注入液体的表面由于其在水滴冲击下的超滑特性以及在不了解潜在物理原理的情况下在自清洁和防污应用方面的潜在优势而引起了人们的极大兴趣。该项目旨在通过研究润滑和液体注入表面上的液滴撞击动力学来解决这一问题，重点是在液滴-膜接触之前和液滴-膜相互作用期间探测夹带空气层的作用。了解液滴下的气膜动力学，可以更好地调整润滑层中的液膜特性，设计出更坚固的纳米结构，并最终提高液体注入表面的性能，用于防污、防冰、相分离和热管理等应用。由于气膜耗散是一种普遍现象，因此本研究的结果也与许多工业过程有关，如功能材料的多层喷墨打印、湿表面喷涂、内燃机燃料燃烧、植物蜡叶农药喷洒以及微电子冷却等。因此，该项目涉及流体力学与表面科学、纳米制造和传热相关的几个研究前沿。该项目还将为PI最近开发的两门关于界面输运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分会会议的流体运动画廊提供原始贡献。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。社区外展项目通过费城材料日、费城科学节、德雷克塞尔大学K-12研究生奖学金和本科项目研究扩展到市中心的K-12教师和学生。本研究的主要目的是促进对液滴对润滑和液体注入表面的影响的基本理解，重点是探索液滴和膜之间的夹带空气层在液-液接触之前和液-液相互作用期间的作用。在固体表面上，即使是最小的凹凸不平也会导致夹带气膜的随机破裂，与之相反，润滑表面上的气膜破坏机制有望更加可控。此外，破裂后的液液接触可能导致界面不稳定和微泡的形成，其机制尚不清楚。通过集成全内反射显微镜、反射干涉显微镜和高速成像技术，将以纳米级空间和微秒级时间分辨率直接可视化不同冲击条件下的气膜演化和破裂后液液接触动力学。这种独特的能力可以直接探测厚度处的气膜轮廓，其中液-液分子间相互作用变得重要。从该项目中获得的知识将弥合固体表面与深层液体池的水滴撞击物理之间的差距，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，作为对润滑和液体注入表面的影响的基本问题：(1)在不同的冲击条件、膜性质和环境压力下，空气膜是如何演变和破裂的？(2)气膜破裂后液液接触前缘生长动力学是怎样的？(3)生长液-液前缘手指状不稳定性的机理是什么？(4)地表结构和倾斜度如何影响空气夹带和下落冲击动力学？

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?