CAREER: Drop impact dynamics and fingering on thin liquid films

Engineers, physicists, and the general public have been fascinated by the beautiful images of the crown that forms when a drop impacts a rigid surface. Besides the mesmerizing images, drop impact, spreading, retraction, and breakup has broad technological implications in agriculture, inkjet and microelectronics printing, spray coating, combustion, energy generation, and forensic science. Despite the many utilities, drop impact dynamics on thin liquid films is not fully understood to date due to (a) the complex interplay between surface tension, viscosity, inertia, and gravity and (b) the challenge of coating surfaces with precisely controlled sub micrometer liquid layers. Recent advances in micro/nanofabrication have enabled this by manipulating fluid-structure interaction. The principal aim of the research program is to provide fundamental understanding of drop impact on thin liquid films. The project will encompass closely integrated education and outreach programs including multi-year student mentoring at the undergraduate and graduate levels and a community outreach to motivate, inspire, and enrich the educational experience of K-12 students through two major public venues: University of Michigan Museum of Natural History and Xplore Engineering. The goal of the research program is to use experiments and theoretical modeling to develop a comprehensive understanding of drop impact dynamics on thin liquid films where the liquid layer thickness is less than one micron. Using state-of-the-art micro/nanoengineered surfaces coated with a lubricant film of known thickness, the research program investigates drop impact dynamics with three principal objectives: 1) understanding the role of air entrapment when a drop impacts a smooth and deformable liquid film featuring liquid-liquid contact between the drop and the lubricant coating, 2) identifying the role of the annular wetting ridge on fingering and drop breakup, and 3) understanding the root cause of the hydrodynamic instability that ensues impact and the number of satellite drops that bifurcate from the radially expanding liquid lamella. The research aims to capture the effects of density and viscosity mismatch between the drop and the lubricant layer on the breakup mechanism. By combining high-speed visualization, fluorescence microscopy, white light interferometry, reflection interference contrast microscopy, and planar laser-induced fluorescence, the proposed research is expected to produce new knowledge and guide future research in soft matter physics and fluid mechanics. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.