An Analysis of the Wickability and Profilometry of Samples

Many different factors influence a material\u27s heat flux, or its ability to transfer heat throughout the sample. Throughout the semester, I investigated two of these most prominent factors: wickability and profilometry. Using various experimental techniques, I quantified values for these two properties and assigned them to each of multiple samples, which were being further investigated in a boiling project. Although more research is needed to be done to evaluate any claims, the initial projects I\u27ve done this semester have given me a platform to continue next semester with more efficiency and better experimental methods

Marangoni-Induced Reversal of Meniscus-Climbing Microdroplets

Small water droplets or particles located at an oil meniscus typically climb the meniscus due to unbalanced capillary forces. Here, we introduce a size-dependent reversal of this meniscus-climbing behavior, where upon cooling of the underlying substrate, droplets of different sizes concurrently ascend and descend the meniscus. We show that microscopic Marangoni convection cells within the oil meniscus are responsible for this phenomenon. 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. We 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 their strength. Our findings provide a new approach to manipulating droplets on a liquid meniscus that could have applications in material self-assembly, biological sensitive sensing and testing, or phase change heat transfer.Comment: submitted to Soft Matte

Water Droplet Condensation on Lubricant-Infused Surfaces in a Vacuum Chamber

The infused lubricant on lubricant infused surfaces (LISs) creates the ideal properties for heat transfer condensation due to its chemically homogenous and atomically flat surface. The purpose of this independent study project was to continue the research on lubricant-infused surfaces to find the optimal oil film thickness for condensation. The optimal oil film thickness is determined by its water transfer rate, with reasonable thermal resistance and high droplet mobility to achieve high heat transfer performance. A lot of time was spent designing, building, and testing the vacuum chamber where this experiment will be conducted. The vacuum chamber will take in water vapor from the vapor generator and the vacuum pump will remove any noncondensable gases from the chamber and also decrease the saturation pressure and temperature within the vacuum chamber, causing condensation at lower temperatures. Cooling water will be transferred into the chamber from an external source through the cold plate that will hold the LIS samples, and the condensation on the LIS samples will be observed through the observation window using a camera placed outside of the chamber. We finished all the setup and initial testing, but due to time constraints, the actual experiment itself has been delayed. Next steps include making final modifications on the vacuum chamber and conducting condensation experiments within the vacuum chamber to find the optimal oil film thickness. This experiment will help optimize the conditions for the condensation of water droplets

Acoustic Droplet Manipulation on Lubricant-Infused Surfaces

Both lubricant-infused surfaces and surface acoustic waves have been studied for their applicability to the field of microfluidics. However, combining the use of the two technologies has not been thoroughly explored. Specifically, this research aims to find an empirical relationship between the size of a droplet (characterized by its diameter) and the frequency required to induce motion. By placing droplets of various sizes on a lubricant-infused surface and testing the effects of surface acoustic waves of different frequencies, it has been determined that the frequency required to initiate movement of the droplet increases as the size of the droplet decreases. Experimental results indicate a logarithmic decaying relationship between frequency and droplet diameter, but more research needs to be done on droplets of diameters smaller than one millimeter. Even still, this research shows promise, as further developing an understanding of this relationship could allow greater control in microfluidic applications

Droplet Condensation and Actuation via Surface Acoustic Waves on Lubricant Infused Surfaces

Both lubricant-infused surfaces and surface acoustic waves have been studied for their applicability to the field of microfluidics. However, combining the use of the two technologies has not been thoroughly explored. Specifically, this research aims to build off of the work done with single droplets last semester to characterize the way surface acoustic waves cause condensed droplets to behave over a range of wave frequencies and amplitudes, as well as droplet sizes, for possible heat transfer applications. From this study, it is clear that there are four distinct modes of droplet actuation for a given frequency, depending on their size. Large droplets (high diameter) actuate freely, while a range of smaller droplets will not move at all. Droplets that are smaller still will start to move again to a point, while the smallest droplets observed did not move, but did evaporate more quickly when exposed to surface acoustic waves. The exact mechanisms driving this behavior are not currently understood, but still this research shows promise, as further developing an understanding of these phases of droplet movement could allow for greater control in microfluidic applications

Heat transfer and melt dynamics of spherical ice particles impacting a heated water bath

In metallic additive manufacturing using direct energy deposition, particles and melt pool undergo complex interactions, including particle impact, penetration, and melting. The spatio-temporal evolution of these processes dictates the final material properties and workpiece quality. However, due to the opaqueness of metallic melt pools, in-situ visualization is nearly impossible. To model this system, we use high-speed imaging to investigate the heat transfer and melting dynamics of spherical ice particles (D ≈ 2 mm) impacting heated water baths of varying temperatures (23 – 70°C) with velocities ranging from 0.8 to 2.1 m/s. To visualize the outflow of molten ice, representative of mixing and material homogeneity, the particles were colored with food dye. We show that after impact, molten liquid forms an annular plume travelling downwards in the bath, until hitting the bottom of the enclosure and expanding radially. Due to positive buoyancy forces, unmolten ice particles rise to the top of the water bath, where they fully melt. As temperatures increase, we observe random particle movement, indicating the presence of convective currents. Through video analysis, we examine the relationships between bath temperature, impact velocity, and heat transfer. As expected, increasing the bath temperature decreases the total melt time of the ice particle. Interestingly, the impact velocity has only a minor effect on the melting time. Using non-dimensional analysis, we derive an expression for the correlation between Nusselt and Jakob numbers. Outcomes from this work can be used to match characteristic time scales during additive manufacturing to tailor material properties

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