Droplet interactions with micro- and nanostructured surfaces for advanced heat transfer applications

Droplets. Droplets are omnipresent: from rain droplets, over ink-jet printers, to advanced heat exchangers and thermal management systems. But in order to use droplets to our advantage, we need to study and understand how they interact with surfaces. Throughout this dissertation, I use optical photography and high speed imaging to characterize droplet-solid interactions. When liquid water comes into contact with a hydrophobic surface, such as Teflon, it forms individual droplets. The contact angle that the droplet develops with the surface is well understood in an air environment. However, when placed in a pure water vapor environment, I show that contact angles can decrease by up to 10% as compared to those in air. At the same time, on micro- and nanostructured surfaces, the vapor environment has little effect on the static contact angles. Based on Young’s equation and Fowke’s concept of the additivity of surface tensions, I propose that the decrease in contact angle on flat hydrophobic Teflon arises from molecular water vapor adsorption to the Teflon surface. In many engineering applications, the use of metals, as opposed to silicon and polymers, is desired to render surfaces water and oil repellent. I introduce micro electrical discharge machining (mEDM) as a viable tool to fabricate scalable micro-mushrooms (~ 100 µm) on steel blocks (~ 1 cm). I show that narrow micro-mushrooms with wide spacing give the highest contact angles (θA/θR = 170°/151°) and droplet mobility with water, while microstructures with flat tops, strong re-entrant curvature and smaller gap widths are necessary to support non-wetting droplets with liquids with a low surface tension, such as oils and alcohols (θA/θR = 148°/74° with isopropanol). After studying static and quasi-static droplet-surface interactions, I continued characterizing droplet dynamics during impact on micro- and nanostructured surfaces. Contact times during impact on rigid surfaces are constant over a wide range of impact speeds, and are thus difficult to control. I show that contact times of water droplets impacting elastic superhydrophobic surfaces can be reduced by up to 50% when compared to impact on rigid surfaces due to a springboard effect, during which droplet lifts off the surface prior to fully recoiling. Upon impact, the droplet excites the substrate to oscillate, while during liquid retraction, the substrate imparts vertical momentum back to the droplet, causing early droplet lift-off with reduced contact time. Through detailed experimental and theoretical analysis, I show that this novel springboarding phenomenon is achieved for a specific range of Weber numbers (We > 40) and droplet Froude numbers during spreading (Fr > 1). For droplets impacting vibrating superhydrophobic surfaces (60-320 Hz), I show that vibration frequency and phase at impact strongly influence the contact time of the bouncing droplets. I introduce the concept of a frequency-dependent critical impact phase at which contact times transition from a minimum (tc ≈ 0.5 tc,th) to a maximum (tc ≈ 1.6 tc,th). Through semi-empirical modeling I show that average contact times can be actively controlled and varied by controlling the substrate vibration frequency. Finally, I studied the distribution of droplet sizes during dropwise condensation on liquid infused surfaces (LIS, or SLIPS) with a wide range of lubricant viscosities (12 – 2717 cSt). Through analysis of >1000 individual images I show that the steady-state droplet size distribution is independent of lubricant viscosity. I further developed a numerical model to estimate the effect of sweeping and sweeping frequency on the average heat transfer on a large vertical plate, and conclude that only uncommonly high sweeping rates would affect heat transfer rates significantly. I estimate average heat transfer rates during dropwise condensation on SLIPS to be 10-15 times greater than during traditional filmwise condensation, and provide a design framework for optimal heat transfer rates based on surface solid fraction and coating thickness. Overall, this dissertation presents new insights into droplet-solid interactions during traditional wetting, droplet impact, and dropwise condensation, and provides a base line for future research and the development of industrial applications for droplet-based thermal management systems