Droplet-Surface Interaction on Micro/Nanoengineered Surfaces

Droplet–surface interaction is a phenomenon of fundamental scientific interest and broad technological applications ranging from spray cooling of electronics, inkjet printing, and anti-icing to self-cleaning surfaces and medical devices. Despite more than a century of study, many aspects of droplet dynamics on engineered surfaces remain poorly understood due to the short timescale of these processes and the complex interfacial phenomena that involve surface tension, viscosity, and inertia. This dissertation investigates droplet–surface interaction on state-of-the-art micro/nanoengineered surfaces, focusing on droplet impact dynamics, impact-driven liquid patterning, air bubble entrapment, and droplet friction. In the first study, we demonstrate that droplet impact on a hydrophobic micropillar array at velocities sufficient to induce a local Cassie-to-Wenzel transition produces distinct polygonal liquid patterns, such as squares, rectangles, octagons, and hexagons, whose shape is determined by pillar arrangement, pillar density, and the intrinsic advancing contact angle of the droplet. High-speed imaging reveals that the contact line advances through a stepwise depinning-zipping mechanism, generating different apparent advancing contact angles in the axial and diagonal directions of the pillar lattice. A closed-form analytical model based on contact line pinning physics is derived and validated by designing experiments over a wide range of pillar densities and liquid contact angles. The second study extends the study on air bubble entrapment on micropillar arrays using high-speed, high-magnification imaging. The results show that sparse pillars allow the air film to retract into a single central bubble, while dense pillars fragment the air film into microbubbles pinned to pillar tops. We also find that increasing the Weber number reduces the bubble size in a manner consistent with inertial-lubrication scaling, wherein higher impact inertia compresses the intervening air film more strongly. In addition to the microbubbles on pillar tops, we observe localized entrapment in the wake region behind individual pillars as the liquid front advances through each obstacle. In the third study, we examine the impact dynamics of water droplets on lubricant-infused micro/nanostructured surfaces. Experiments over a wide range of Weber numbers, lubricant viscosities, lubricant layer thicknesses, and lubricant-to-droplet density ratios show that the maximum spreading diameter obeys the same We^(1/4) scaling law derived for flat surfaces. We find that increasing the lubricant viscosity suppresses splashing, while increasing the density difference between the droplet and the lubricant enhances corona splashing, implicating Rayleigh-Taylor instability as a contributing mechanism. Importantly, we show that splashing is maximized at an optimal lubricant overlayer thickness of ≈3–5 µm. The last part of this thesis investigates droplet friction on lubricant-infused micropillars by combining friction force measurements, lubricant film thickness measurements, and direct visualization of the intercalated lubricant film. We identify a critical solid fraction above which tall pillars support an oil film above their tops and exhibit friction behavior indistinguishable from short pillars. Below this threshold, droplets graze over the pillar tops, and friction increases with the pillar density. The overlaying film thickness on tall dense pillars scales with the capillary number Ca as Ca^(2/3), in agreement with the Landau-Levich-Derjaguin (LLD) law. Reflection interference contrast microscopy provides independent visual confirmation of these findings. Collectively, these studies improve understanding of droplet–surface interaction on engineered surfaces and identify the key parameters governing the interaction dynamics. The results provide design guidelines for developing next-generation lubricant-infused and micro/nanoengineered surfaces for applications in thermal management, microfluidics, printed electronics, and anti-icing.