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

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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?