The present investigation can be divided into two parts: (a) experiments made to examine the mechanism of dropwise condensation of steam with particular reference to the stability of drop promoting surfaces as affected by the material of cooled surface, the drop promoter, the surface finish, the rate of heat transmission, and the presence of non-condensable gas. and (b) a theoretical analysis of the beat transmission through individual droplets, the transient heat transfer through exposed areas, the statistical study of drop size distribution, and the estimation of steam side coefficient. An apparatus was developed to examine qualitatively,the behaviour of drop promoting surfaces on a small scale. It is considered that sufficient evidence was found to show that steam in contact with a cooled surtace condenses as a thin liquid film which later breaks into droplets. surfaces treated to give dropwise condensation deteriorate into mixed condensation in due time, and the duration tor which a treated surface maintains dropwise condensation varies between a few hours to several days, depending on many factors among which· the presence of non-condensable gas must not be overlooked. An approximation to the heat transmission through individual droplets has been worked out with assumed heat flow lines. The result, checked by the relaxation method. is correct within . + 10%. An analysis Of the transient heat transfer through exposed areas was made neglecting the increasing resistance of any accumulating liquid. The drop size distribution was analyzed tor one drop promoting surface at three different heat transmission rates. Based on this drop size distribution, the heat transmission through the drops was estimated by assuming they were held at rest on a cooled surface conducting heat under a steady state. . The estimated coefficient comes within the range or experimental results of many investigators.

Droplet evaporation and condensation are two important topics of interest, since these two phase-change phenomena not only occur in the cycle of global water, e.g., the formation of rain, fog, dew, and snow in nature, but also play a critical role in a variety of applications including phase-change heat transfer enhancement, surface chemistry and energy system optimization. Especially, in the past two decades, the rapid development of the nature-inspired non-wetting surfaces has promoted the applications of droplet-based phase change phenomena in various scenarios. However, most previous studies focused on the sessile droplets on one flat surface in the open space, and the effects of surface topography, i.e., surface curvature or configurations, on droplet evaporation and dropwise condensation are still elusive. This dissertation aims to explore droplet-based evaporation and condensation in more complex spaces and to elucidate how the surface topography affects the evaporating or coalescing droplet dynamics during these phase-change processes. The coalescence-induced jumping of nanodroplet on curved superhydrophobic surface is modeled via molecular dynamic simulations. As the surface curvature increases from 0 to 2, the corresponding energy conversion efficiency of jumping droplet during the coalescence process could be significantly improved about 20 times. To explain this curvature-enhanced jumping effect, the contact line dissipation, i.e., an important source of energy dissipation in nanoscale, is considered in our scaling energy analysis. And this energy-effective jumping of coalesced droplet could be mainly attributed to the reduction of contact line dissipation due to the decrease of contact line length and contact time on curved surface. As the droplets are confined between two parallel or non-parallel low-energy surfaces, i.e., hydrophobic or superhydrophobic surfaces, with a narrow gap, the total evaporation time of the squeezed droplets would be dramatically prolonged about two times. An ellipsoidal segment diffusion-driven model is established to successfully predict the evolution of contact radius and volume of the squeezed droplets during the evaporation process and to clarify it is the vapor enrichment inside the confined space giving rise to the mitigated evaporation. If two hydrophobic surfaces are configured as non-parallel, the confined droplet inside the V-shaped grooves would be self-transported towards the cusp/corner during the evaporation. Based on our energy and force analyses, the asymmetrically confined droplet would move towards an equilibrium location le, where the Laplace pressure induced force is balanced with normal adhesion force, to minimize its Gibbs surface energy. As le decreases during the evaporation, this equilibrium location would directionally shift towards the cusp, which could be regarded as the origin of this evaporation-triggered unidirectional motion. For the first time, the solvent transport and colloidal extraction could be accurately controlled in a combined manner. / Doctor of Philosophy / Droplet evaporation and condensation are two important topics of interest, since these two phase-change phenomena not only occur in the global cycle of water including the formation of rain, fog, dew, and snow in nature, but also play a critical role in a variety of applications including heat transfer enhancement, surface chemistry, and the energy system optimization. Generally, the droplets in these scenarios are deposited on one flat surface opened to the atmosphere. and the effects of surface topography on droplet evaporation and dropwise condensation are still elusive. This dissertation aims to explore droplet-based evaporation and condensation in more complex spaces and to clarify how the surface curvature or configurations affects evaporating or condensing droplet dynamics accompanying these phase change processes. As the coalesced droplet jumps off the curved superhydrophobic surfaces during dropwise condensation, the corresponding energy conversion efficiency would be significantly improved about 20 times due to the increases of curvature. It is demonstrated that the decrease of contact line length and contact time would give rise to the reduction of contact line dissipation, which should be the main factor driving this energy-effective jumping of the coalesced droplets. As the droplets are confined between two parallel or non-parallel low-energy surfaces, i.e., hydrophobic or superhydrophobic surfaces, with a narrow gap, the total evaporation time of the squeezed droplets would be dramatically prolonged about two times in the small space. An ellipsoidal segment diffusion-driven model is established to successfully predict the evolution of contact radius and volume of the squeezed droplets during the evaporation and to clarify it is the vapor enrichment in the confined space giving rise to the mitigated evaporation. If two hydrophobic surfaces are configured as non-parallel, the confined droplet inside the V-shaped grooves would be self-transported towards the cusp/corner of the structure during evaporation. Based on our energy and force analyses, the asymmetrically confined droplet would move towards an equilibrium location le, where the Laplace pressure induced force is balanced with normal adhesion force, to minimize its Gibbs surface energy. As le decreases in the scale of during the evaporation, this equilibrium location would directionally shift towards the cusp, which could be regarded as the origin of this evaporation-triggered unidirectional motion.

An experimental study has been performed to investigate the condensation on the inner surface of open cavities located on horizontal and vertical surfaces of a refrigerator to simulate ice dispensers, water dispensers, electronic display slots, and door handles, etc. Cavity size, cavity depth and insulation thickness were variable parameters. Cavities were located on fresh food and freezer compartments to study two different boundary conditions. Level of condensation was put into a scale based on visual observation for condensed water droplets on the cavity surface. As a practical tool for design engineers, insulation thickness was plotted as a function of cavity depth indicating the level of condensation. Similar cavity geometries were tested on both freezer and fresh food compartments. Aluminum foil attached behind the inner surface of a cavity located on the fresh food door helped to decrease the level of condensation significantly. However, condensation can not be avoided for the cavities located on the freezer compartment deeper than critical values. Forming radius at the corners and on the edges of cavities decreased condensation on these regions. The effects of air circulation on condensation inside and outside the refrigerator were also investigated for some cavities. Particle Image Velocimetry (PIV) method was used to visualize non-disturbed air flow pattern over the cavity and at the cavity opening. High convective heat transfer at the cavity center was strongly associated with relatively high air velocity. Temperature distribution and flow pattern were analyzed by a CFD Programme. Condensation due to relatively low temperatures at the corners and on the edges was associated with conductive heat transfer in multiple directions and insufficient air circulation on these regions.

No description available.

Surface tension is an essential force for the functioning of the world and life. Centuries of study, and still, new applications and limits of surface tension are being explored. Water has always drawn attention for its high surface tension value, 72mN/m compared to ethanol's 20mN/m. The high surface tension allows for numerous applications, superhydrophobic surfaces being one that takes heavy advantage of that value. Superhydrophobicsurfaceshave a high surface energy cost with water, resulting in small contact areas with high advancing and receding contact angles and low contact angle hysteresis. This results in very low adhesion on the surfaces. Here we study the ability of superhydrophobic surfaces with their low adhesion to shed meltwater from frost, showing a decrease in frost thickness to below 3mm for the meltwater to shed. We then take another approach to removing water from a surface, rather than increasing the surface energy cost, we introduce a difference in surface energy cost. Introducing a porous surface across from a solid one, droplets transfer from the solid to the porous, removing over 90% of the volume of the droplet from the solid surface. We thoroughly examine and model the hydrodynamics of the transfer process, varying the solid surface, the donor surface, and the liquid. This bridging between surfaces is then applied to fog harps, examining the efficiencies of large-form fog harps. Fog harps have shown a 3 to 5 times increase in water collection compared to the industry-standard mesh collector. However, droplets from fog collected on the wires eventually grow large enough to touch neighboring wires. Tominimizetheirsurfaceenergy, they begin pulling wires together, "tangling" them. This can potentially reduce efficiency, but has not been applied to large-scale harps until here. Another application of surface tension is then examined, using lower surface tension oils, but trapping them in microstructures to make slippery liquid-infused porous surfaces (SLIPS). The oil coats the microstructure, due to its lower surface tension. This creates a lubricating layer on the surface, along with potential air pockets reducing friction further. These surfaces have been studied extensively with liquids being placed on them, but here we begin to examine them when solids are used instead, showing some interesting cases where increasing the viscosity of the oil actually decreases the friction force. / Doctor of Philosophy / Sponges are something everyone has used, and most people can tell you that they work using surface tension. And for most people, that's enough. It's actually more useful to know to squeeze your sponge dry when you're done to prevent mold than it is to know that it holds onto liquids because of surface tension. But the point here was to take the study of sponges and surface tension to the extreme. To the point that some knowledge is going to be gained solely for the sake of gaining knowledge. Not all knowledge will have immediate uses, but this doesn't take value away from the knowledge, or any eventual uses it might have. So we start this by looking at the building of scientific knowledge and noticing that a brick is missing. Superhydrophobic surfaces, surfaces that water doesn't want to touch, have been studied very extensively and their properties have been thoroughly explored. However, a direct comparison of the defrosting behaviors, the process of frost melting on a surface, between superhydrophobic and hydrophobic surfaces had not been done. Water does prefer to be on a hydrophobic surface compared to a superhydrophobic one, but it's still uncomfortable. A plate was treated so that half was hydrophobic and the other half was superhydrophobic. Frost was grown across the surface and then melted simultaneously, allowing us to characterize the differences in the behaviors, highlighting the ability of the superhydrophobic surface to shed water droplets at smaller sizes than other surfaces. Next is a pure fluid mechanics work supporting a heat transfer application. Evaporation, for enhanced heat transfer, and a hydrophilic wick, essentially a sponge, are paired to create a plate with one-way heat transfer. Heating side A can heat side B, but heating side B can't heat side A. Water in the wick gets heated, evaporates from side A and then condenses on side B, carrying heat with it. The condensation grows until it touches the wick, which then pulls it in, allowing it to be evaporated again and cycling more heat. When side B, the smooth surface, is heated, the water can evaporate off it and condense in the wick, but then it has no way to return, preventing further heat transfer. The process of droplets being pulled from side B to the wick in side A is key to the process. It's a sponge pulling water in using surface tension. However, all the smaller pieces have been taken for granted. The second piece is a systematic study of this capture mechanism, exploring the effects of changing liquids, donor surfaces, and receiving porous wicks. The third is a continuation of the lab's previous work on Fog Harps, arrays of vertical fibers held in place to let fog run into them. The droplets grow until they slide down and can be collected. The wires of the harp are close enough that the water can actually start to tangle them together. This tangling can increase the water needed for sliding and collection to begin. Tensioning the wires can help mitigate the tangling. Here we show harps on around 1,$text{m}^2$, using optimal wire size and spacing that is possible for mass manufacturing. The harps were tested in the lab using humidifiers to generate fog for the harps to collect. Finally, an initial study of solid objects being pulled across oil-infused microstructured surfaces. The microstructure helps keep the oil on the surface thanks to the surface energy of the oil. These oil-infused surfaces have been studied extensively when liquids are placed on them, but not with solid objects. Solid objects can exert significantly more pressure than liquids, which naturally want to spread when they reach a certain thickness. Experiments were performed with a variety of oil viscosities, microstructures, and oil excess thicknesses. This work is not entirely complete but a significant portion of it is presented here.

Wetting, which addresses either spontaneous or forced spreading of liquids on a solid surface, is a ubiquitous phenomenon in nature and can be observed by us on a daily basis, e.g., rain drops falling on a windshield and lubricants protecting our corneas. The study of wetting phenomena can be traced back to the observation of water rising in a capillary tube by Hauksbee in 1706 and still remains as a hot topic, since it lays the foundation for a wide spectrum of applications, such as fluid mechanics, surface chemistry, micro/nanofluidic devices, and phase change heat transfer enhancement. Generally, wetting is governed by the dynamic and static behaviors of the three-phase contact line. Therefore, a deep insight into the dynamics and statics of three-phase contact line at nanoscale is necessary for the technological advancement in nanotechnology and nanoscience. This dissertation aims to understand the dynamic wetting under a molecular kinetic framework and resolve the reconfiguration of liquid molecules at the molecular region of contact line. Water spreading on polytetrafluoroethylene surfaces is selected as a classical example to study the dynamic behaviors of three-phase contact line. To accommodate the moving contact line paradox, the excess free energy is considered to be dissipated in the form of molecular dissipation. As-formed contact line friction/dissipation coefficient is calculated for water interacting with PTFE surfaces with varying structures and is found to be on the same order of magnitude with dynamic viscosity. From an ab initio perspective, contact line friction is decomposed into contributions from solid-liquid retarding and viscous damping. A mathematical model is established to generalize the overall friction between a droplet and a solid surface, which is able to clarify the static-to-kinetic transition of solid-liquid friction without introducing contact angle hysteresis. Moreover, drag reduction on lotus-leaf-like surface is accounted for as well. For the first time, the concept of contact line friction is used in the rational design of a superhydrophobic condenser surface for continuous dropwise condensation. We focus on the transport and reconfiguration of liquid molecules confined by a solid wall to shed light on the morphology of the molecular region of a three-phase contact line. A governing equation, which originates from the free energy analysis of a nonuniform monocomponent system, is derived to describe the patterned oscillations of liquid density. By comparing to the Reynolds transport theorem, we find that the oscillatory profiles of interfacial liquids are indeed governed in a combined manner by self-diffusion, surface-induced convection and shifted glass transition. Particularly for interfacial water, the solid confining effects give rise to a bifurcating configuration of hydrogen bonds. Such unique configuration consists of repetitive layer-by-layer water sheets with intra-layer hydrogen bonds and inter-layer defects. Molecular dynamics simulations on the interfacial configuration of water on solid surfaces reveal a quadratic dependence of adhesion on solid-liquid affinity, which bridges the gap between macroscopic interfacial properties and microscopic parameters. / Doctor of Philosophy / The study of wetting phenomena can be traced back to the observation of water rising in a capillary tube by Hauksbee in 1706 and still remains as a hot topic, since it lays the foundation for a wide spectrum of applications, such as fluid mechanics, surface chemistry, micro/nanofluidic devices, and phase change heat transfer enhancement. The conventional hydrodynamic analysis with no slip boundary condition predicts a diverging shear stress at the contact line as well as an unbounded shear force exerted on the solid surface. To accommodate this paradox, different mechanism and models have been proposed to clarify the slip between a moving contact line and a solid surface. Although almost all models yield reasonable agreement with experimental observations or numerical simulations, it is still difficult to pick up a specific model using only macroscopic properties and experiment-accessible quantities, because the energy dissipation mechanism during dynamic wetting is not identified and the contact line deforms over different length scales. In this dissertation, we ascribe the energy dissipation in dynamic wetting to contact line friction/dissipation under the framework of molecular kinetic theory, as it is assumed that the contact line is constantly oscillating around its equilibrium position. By decomposing contact line friction into two contributions: solid-liquid retarding and viscous damping, we are able to derive a universal model for the contact line friction. This model predicts a decaying solid-liquid friction on superhydrophobic surfaces, corresponding to the lotus effect. In the meantime, this model is able to clarify the recently-discovered static-to-kinetic transition of frictional force between a sessile drop and a solid surface. Later, we used the concept of contact line friction in the droplet growth process in dropwise condensation so as to promote the rational design of superhydrophobic condenser surfaces for sustainable dropwise condensation. As the morphology of a contact line is dependent on the length scale of interest, we focus on the molecular region of contact line. We study the transport and structural change of liquid molecules that are several molecular layers away from the solid surface. It is found that liquid molecules in this region experience patterned density oscillations, which cannot be simply attributed to the random deviations from continuum limit. By combining free energy analysis and Reynolds transport theorem, it is demonstrated that the omnipresent density oscillations arise from the collective effects of self-diffusion, surface-induced convection and shifted glass transition. For liquid water, we propose a bifurcating hydrogen bonding network in contrast to its tetrahedron configuration in bulk water.

Condensation of water vapor is an everyday phenomenon which plays an important role in power generation schemes, desalination applications and high-heat flux cooling of power electronic devices. Continuous dropwise condensation is a desirable mode of condensation in which small, highly-spherical droplets regularly form and shed off the surface before a thick liquid is formed, thereby minimizing the thermal resistance to heat transfer across the condensate layer. While difficult to induce and sustain, dropwise condensation has been shown to achieve heat and mass transfer coefficients over an order of magnitude higher than its filmwise counterpart. Superhydrophobic surfaces have been extensively studied to promote dropwise condensation with mixed results; often surfaces that are superhydrophobic to deposited droplets formed in the gas phase above the surface do not retain this behavior with condensed droplets nucleated and grown on the surface. Recently, nanostructured superhydrophobic surfaces have been developed that are robust to vapor condensation; however, these surfaces still are not ideal for condensation heat transfer due to the high thermal resistance of the vapor layer trapped underneath the droplets and the reduced footprint of direct contact between the highly-spherical droplets and the underlying substrate. This work has two main objectives. First, a comprehensive free energy based thermodynamic model is developed to better understand why traditional superhydrophobic surfaces often lose their properties when exposed to condensed droplets. The model is first validated using data from the existing literature and then extended to analyze the suitability of amphiphilic (e.g. part hydrophobic and part hydrophilic) nanostructured surfaces for condensation applications. Secondly, one of the promising amphiphilic surfaces identified by the thermodynamic model is fabricated and tested to observe condensation dynamic behavior. Two complementary visualization techniques, environmental scanning electron microscopy (ESEM) and optical (light) microscopy, are used to probe the condensation behavior and compare the performance to that of a traditional superhydrophobic surface. Observations from the condensation experiments are used to propose a new mechanism of coalescence that governs the temporal droplet size distribution on the amphiphilic nanostructured surface and continually generates fresh sites for the droplet nucleation and growth cycle that is most efficient at heat transfer.

The purpose of this study was to analyze the dropwise condensation on a cylindrical surface including the sweeping effect theoretically. For this purpose, first the problem of the equilibrium shape and departure size of drops on the outer surface of a cylinder was formulated. The equations of the surface of the drop were obtained by minimizing (for a given volume) the total energy of the drop which consists of surface and gravitational energy by using the techniques of variational calculus. The departure size of the droplets on a surface at varies angle of inclinations were also determined experimentally. Drop departure size is observed to decrease up to as the surface inclination was decreased up to 90 degree and then it increased up to 180 degree. Mean base heat flux, drop departure rate, sweeping frequency, fraction of covered area, sweeping period, local heat flux and average heat flux for the dropwise condensation on a cylindrical surface including the sweeping effect is formulated and the resulting integral equation was solved by using the finite difference techniques. The results show that drop departure rate and sweeping frequency was strongly affected by the angular position and reached asymptotic value at large angular positions. Comparing the results of the average heat flux values at different diameters show that at larger diameters the average heat flux becomes larger. This is due to the increased sweeping effect at larger diameters.

A Theory of Dropwise Condensation

In the past decade, the condensation on superhydrophobic surfaces has been investigated abundantly to achieve dropwise condensation. There is not a specific approach in choosing the size of the roughness of the superhydrophobic surfaces and it was mostly selected arbitrarily to investigate the behavior of condensates on these surfaces. In this research, we are optimizing the size of the roughness of the superhydrophobic surface in order to achieve dropwise condensation. By minimizing the resistances toward the transition of the tails of droplets from the cavities of the roughness to the top of the roughness, the size of the roughness is optimized. It is shown that by decreasing the size of the roughness of the superhydrophobic surface, the resistances toward the transition of the tails of droplets from Wenzel state to Cassie state decrease and consequently dropwise condensation becomes more likely.