Date of Award
Doctor of Philosophy (PhD)
Inspired by pitcher plants, a micro/nano- structured solid surface infused with low-surface-tension lubricant constitutes a novel class of biomimetic surfaces—lubricant-infused surfaces (LISs). Unlike solid hydrophobic and superhydrophobic surfaces, a LIS is “hemi-solid and hemi-liquid”. The thin lubricating film can extraordinarily decrease adhesion and avoid contact line pinning between the working fluid and the underlying solid, displaying many desirable properties, such as excellent liquid repellency, suppression of fouling, and self-healing. Hence, LISs are ideal for applications where liquids or contaminants must be efficiently removed from surfaces in harsh environments. For example, LISs can promote stable dropwise condensation of water or low-surface-tension liquids and improve heat transfer rates due to a low nucleation energy barrier, high droplet mobility, and high sweeping rates. In the past decade, a plethora of work has been devoted to studying static or dynamic wettings of droplets, lubricant film dynamics and depletion, and surface design. Unfortunately, most of these studies targeted droplets at (sub-)millimetric scale. Oil menisci naturally form surrounding droplets or solid spheres that protrude the oil film of a LIS. The meniscus surrounding a millimetric droplet on a LIS is relatively small compared to the droplet size, but the one surrounding a microdroplet could be of comparable size to the droplet itself and hence considerably influence the droplet dynamics. Due to the unique existence of multiple phases in a LIS system—air, working liquid, infused lubricant, and solid surface—microdroplet dynamics can be complicated, notwithstanding the coupled phase change and heat transfer during condensation. Furthermore, interactions of multiple droplets are very common and of great significance. For instance, millions of microdroplets can be created within one second on a 1-cm2 surface during water dropwise condensation. Understanding the dynamics of these microdroplets is challenging, but crucial to understand to better design surfaces to achieve superior condensation performances. The first part of this dissertation aims at fundamentally understanding the microdroplet dynamics on such a thin lubricating film of LISs, where capillary forces are expected to dominate. The second part investigates the influence of microdroplet mobility on droplet nucleation and growth during dropwise condensation, since microdroplets account for most of the total heat transfer. Coupling high-speed imaging, optical microscopy, and interferometry, I show that the initial uniform lubricant film tends to redistribute during condensation, and eventually turns into oil-rich and oil-poor regions. Condensate microdroplets display high mobility in the oil-rich region and often move long distances (3 - 6 times their diameters) towards larger droplets, independent of gravity. The overlapping oil menisci between microdroplets can create an anisotropic oil profile around the droplets which leads to unbalanced lateral components of capillary forces at the apparent triple-phase contact line. Based on lateral force balance on the moving droplet, a mathematical model describing the long-distance movements is proposed and shows good agreements with the experimental results. The sliding velocity depends on the droplet diameters, the distance between droplets, and the lubricant viscosity. This novel and non-traditional droplet movement is expected to significantly enhance the sweeping efficiency during dropwise condensation, leading to higher nucleation and heat transfer rates. I subsequently introduce that the meniscus-climbing movements of microdroplets can be reversed via thermal regulation of the substrate. Upon cooling the underlying substrate, droplets of different sizes concurrently ascend and descend the oil meniscus on LISs. To solve this mystery, I utilize fluorescence confocal micro particle image velocimetry and numerical simulation to fully uncover microscopic Marangoni convection cells within the oil menisci on cooled or heated LISs. While dynamics of relatively larger water microdroplets are still dominated by unbalanced capillary forces and hence ascend the meniscus, smaller droplets are carried by the surface flow and consequently descend the meniscus. I further demonstrate that the magnitude and direction of the convection cells depend on the meniscus geometry and the substrate temperature, and introduce a modified Marangoni number that well predicts the flow strength. With a better understanding of the interplay of droplet dynamics and oil film, I experimentally investigate droplet nucleation and growth during water condensation on LISs and solid hydrophobic surfaces. Using high-speed microscopy, I reveal that the smallest visible droplets (diameter ∼1 μm, qualitatively representing nucleation) predominantly emerge in oil-poor regions of LISs due to a lower nucleation free-energy barrier. I study the apparent nucleation rate density (NRD) and water collection rate for LISs with different viscosity oils and hydrophobic surfaces at a wide range of subcooling temperatures. On LISs with a lower lubricant viscosity, microdroplets self-propel further and more frequently, leading to a higher surface refreshing frequency and thereby a higher NRD. Interestingly and unexpectedly, hydrophobic surfaces outperform high-viscosity LISs at high subcooling temperatures, but are generally inferior to any of the tested LISs at low temperature differences. To explain this, I introduce two dominant regimes that affect the condensation efficiency: mobility-limited and coalescence-limited, and compare these regimes based on droplet growth rates and water collection rates. The findings of this thesis not only advance the understanding of the interactions of microdroplets and thin lubricating film on LISs but also provide new design rationales for choosing surfaces for enhanced dropwise condensation and water collection efficiencies. Furthermore, the size-based bi-directional movements can also pave new pathways for other applications.
Ramesh Agarwal, Richard Axelbaum, Mark Meacham, Vijay Ramani,