<p> Water drops at their boiling point were projected through a steam atmosphere to strike a surface which was varied in temperature from 300 to 900 degrees Fahrenheit. A high-speed motion picture study of the collision process showed that, except at low surface temperatures, the drop flattened out on impact and rebounded in a state of oscillation. Measurements of the change in drop diameter on--collision indicated that the amount of evaporation due to heat transfer from the surface was extremely small except when the drop extensively wetted the surface. Solution of a mathematical model of the initial impact dynamics and models of heat transfer through a vapour film beneath the drop and by direct liquid-surface contact confirmed experimental observations. </p> / Thesis / Doctor of Philosophy (PhD)

Ion contaminants from Printed Circuit Board (PCB) assembly processes pose a high reliability risk because they result in damaged circuits. Therefore, it is essential to understand the level of ionic species on the electronic circuitry as well as the reliability risks caused by these contaminants. There are a number of approaches available in the industry to assess the reliability risks ; for example, the water drop test (WDT) is one of the techniques used to determine the propensity of an ionic contaminant to cause electrical short failures by dendrite formation. The objective of this research is to determine the time to cause the failures, known as electrochemical migration (ECM) failures. A test vehicle was developed for the WDT to obtain the time to cause ECM failure in presence of different anions. The time to form dendritic bridges that cause short circuits was determined as a function of the different anions and the spacings between PCB pads. The experimental method involved dispensing aqueous solutions containing common inorganic and organic acid anions onto test vehicles, applying electrical bias voltages and measuring the time to form dendrites. Specially designed test structures cells were created to contain the test solutions. At each of the test cells, a cavity held the solution and constant current was applied through different metal geometries. To be representative of popular board finishes, test vehicle boards incorporated both Sn-Pb Hot Air Soldering Level (HASL) and Pb free HASL surface finishes.

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<p> Water drops at their boiling point were projected through a steam atmosphere to strike a surface which was varied in temperature from 300 to 900 degrees Fahrenheit. A high-speed motion picture study of the collision process showed that, except at low surface temperatures, the drop flattened out on impact and rebounded in a state of oscillation. Measurements of the change in drop diameter on collision indicated that the amount of evaporation due to heat transfer from the surface was extremely small except when the drop extensively wetted the surface. Solution of a mathematical model of the initial impact dynamics and models of heat transfer through a vapour film beneath the drop and by direct liquid-surface contact confirmed experimental observations. </p> / Thesis / Doctor of Philosophy (PhD)

The degradation of high voltage electrical insulation is a prime factor that can significantly influence the reliability performance and the costs of maintaining high voltage electricity networks. Little information is known about the system of localized degradation from corona discharges on the relatively new silicone rubber sheathed composite insulators that are now being widely used in high voltage applications. This current work focuses on the fundamental principles of electrical corona discharge phenomena to provide further insights to where damaging surface discharges may localize and examines how these discharges may degrade the silicone rubber material. Although water drop corona has been identified by many authors as a major cause of deterioration of silicone rubber high voltage insulation until now no thorough studies have been made of this phenomenon. Results from systematic measurements taken using modern digital instrumentation to simultaneously record the discharge current pulses and visible images associated with corona discharges from between metal electrodes, metal electrodes and water drops, and between waters drops on the surface of silicone rubber insulation, using a range of 50 Hz voltages are inter compared. Visual images of wet electrodes show how water drops can play a part in encouraging flashover, and the first reproducible visual images of water drop corona at the triple junction of water air and silicone rubber insulation are presented. A study of the atomic emission spectra of the corona produced by the discharge from its onset up to and including spark-over, using a high resolution digital spectrometer with a fiber optic probe, provides further understanding of the roles of the active species of atoms and molecules produced by the discharge that may be responsible for not only for chemical changes of insulator surfaces, but may also contribute to the degradation of the metal fittings that support the high voltage insulators. Examples of real insulators and further work specific to the electrical power industry are discussed. A new design concept to prevent/reduce the damaging effects of water drop corona is also presented.

The interaction of a liquid drop with a solid surface is being actively studied to understand practically encountered scenarios such as the impact of fuel spray droplets onto the walls of engine combustion chamber, the formation of thermal barrier coating on the surfaces of turbine blades, the process of ink-jet printing, etc. The surface topography of solid surface is one of the major parameters influencing the dynamics of drop-surface interaction process. Understanding the precise role of surface topography features such as micro asperities and grooves on the spreading and receding processes of impacting liquid drops is crucial for the improvement in abovementioned applications. Recent developments in the fabrication of micro- and nano-structures on solid surfaces provide fabulous opportunities to investigate the role of single/multiple micro asperities and grooves on the dynamics of impacting drops. The thesis deals with an experimental work on the impact of water drops on stainless steel surfaces comprising unidirectional parallel grooves. A group of six target grooved surfaces covering a wide range of surface wettability were considered. The target surfaces were prepared using the techniques of photolithography, electro discharge machining, and laser machining. Scanning electron microscope and optical surface profilometer were used to characterize the groove texture geometrical parameters of the target grooved surfaces. The experiments of drop impact were carried out in an experimental apparatus consisting of a liquid drop generator, a substrate table, and a digital video imaging system. Free-falling distilled water drops released from a certain height were allowed to impact normally on the target surfaces. The image sequences of drop impact dynamics were constructed from the images captured using the digital video imaging system. Majority of the drop impact experiments were captured using a high speed video camera operating with frame speed ranging from 3000 to 10000 fps. For the target grooved surfaces, the impact dynamics was analyzed for the impacting drop liquid oriented both in the direction perpendicular to the grooves ( ) and in the direction parallel to the grooves (||) via independent test runs. The captured digital frames were used to deduce the temporal variation of impacting drop parameters such as drop contact diameter, drop contact angle, and drop height at the center of impacting drop with the aid of image processing software. The impacting drops were characterized in terms of Weber number, We expressed in terms of drop impact velocity and drop diameter measured just before the start of impact process. The study covered We ranging from 1.8 to 170. In general, the groove texture on the solid surface influences the drop impact process for all We examined in the study. The effect is more pronounced for the receding of impacting drops. For high We drops, the groove texture enhances the perturbations seen at the periphery of spreading lamella. The study showed quantitatively that the drop impact process on a target grooved surface comprising unidirectional parallel grooves develops a non-axisymmetric drop flow on the grooved surface exhibiting different spreading and receding processes of impacting drop liquid in the directions perpendicular ( ) and parallel (||) to the grooves. The maximum spreading diameter reached immediately after the completion of early inertia-dominated spreading in is less than that obtained in || due to the loss of drop kinetic energy caused by the pinned motion of drop liquid in . The non- axisymmetric drop flow on the target grooved surface develops a difference between the frequencies of contact angle oscillation of impacting drop liquid in  and ||. The frequency difference in contact angle oscillation causes the beating phenomenon in the temporal variation of the contact angle anisotropy, Δθ and drop height at the center of impacting drop, Z. For a given target grooved surface, the experimental measurements suggested that the beat frequency is almost independent of We. The temporal variation of Δθ and Z do not show the traces of beating phenomenon for the impact of high We drops. Owing to the non-axisymmetric drop flow, the final equilibrium drop shape is eccentric for the impact of low We drops and approaches a circular shape for the impact of high We drops. The role of groove texture geometrical parameters is seen in the drop impact process via the surface wettability especially for the impact of low We drops. Larger surface roughness factor makes the target grooved surface to exhibit hydrophobic characteristics.

Dynamic behavior of impacting water drops on superhydrophobic solid surfaces provides important details on the stability/durability of such solid surfaces. Multi-scale surface roughness combined with a layer of low energy chemical is an essential surface modification process followed to create superhydrophobic capabilities on solid surfaces. The present work aims at studying the effect of low energy surface coating on droplet impact dynamics by carrying out experiments of water drop impacts on rough solid surfaces with and without chemical modification. A group of six aluminium alloy (Al6061) surfaces (three pairs) are prepared. Roughness, characterized in terms mean surface roughness, Ra, is introduced to these metallic surfaces using sand-paper polishing, electric discharge machining (EDM), and chemical based surface etching process. Low energy surface layer is laid on the rough surfaces by coating NeverWet hydrophobic solution, octadecyl-trichloro-silane (OTS), and perfluorodecyltricholorosilane (FAS-17). The impact dynamics of water drops is analyzed by capturing high speed videos for a range of drop Weber number from 1 to 570 and the salient features of drop impact process on the coated rough surfaces are compared with the corresponding uncoated rough surfaces. A one-to-one comparison on the spreading, fingering, receding, and final equilibrium of impacting drops on the coated and uncoated target surfaces is presented. Upon coating NeverWet, the original surface features of the base aluminium surface are completely covered by the hydrophobic coating material resulting in a fresh top surface layer. The outcomes as well as the bounce-off characteristics of impacting water drops on the coated surface are comparable to those observed on lotus leaf. The surface morphology features of rough aluminium surfaces coated with OTS and FAS-17 are comparable to those of the corresponding uncoated surfaces. The quantitative measurements on primary spreading and maximum spread factor of impacting drops are largely unaffected by the presence of low energy chemical coating. The dominant effect of surface coating is seen on the receding of impacting drops and hence the final drop configuration. This behavior is more prominently seen on EDM fabricated rough surface (larger Ra) combined with OTS coating than that on etching based rough surface (smaller Ra) combined with FAS-17 coating highlighting the dependence of coating effect with roughness features.

Dynamic soil water repellency is an important soil phenomenon in the vadose zone as it is now recognised that most soils in the world are likely to express some degree of reduced wettability and/or long term hydrophobicity. Fractional wettability and contact angles are, however, rarely discussed or quantified for natural systems. This is particularly the case in the presence of dynamic contact angles. Soil water repellency remains a persistent impediment and challenge to accurate conceptual and numerical models of flow and storage in the vadose zone. This dissertation addresses the opportunity and pressing need for contributions that develop better quantifiable definitions, descriptions, and understanding of soil water repellency. Using materials collected from post wildfire sites, this work employs water and ethanol to identify, isolate, and quantify contact angle dynamics and fractional wettability effects during infiltration. Varied concentrations of water and ethanol solutions were applied to soils and observed through X-ray microtomography, tension infiltration experiments, and moisture content measurements in the laboratory and field. Several analyses from lab and field investigations showed that applications of ethanol and specifically, water-ethanol aqueous solutions provide unique additional insights into proportions of media that remain non-wettable and how those proportions affect overall hydrologic processes, which are not readily observable through water infiltrations alone. Observations include the wetting up of microporous structures, reduced storage, and changes in unsaturated hydraulic conductivities. Challenges which develop as a consequence of variable fluid properties including changes to operational pore assemblages, slow down of wetting fronts, and non-uniqueness relative to infiltration responses are addressed. Important insights and contributions were developed through this approach and water-ethanol mixtures are valuable tools for developing greater quantification and mechanistic data to better inform our models and understanding of dynamic soil water repellency. / Dissertation / Doctor of Philosophy (PhD) / Quantifying fluid behaviours in soils is important for a host of environmental, social, and economic reasons. Over the last 25+ years, one soil phenomenon has garnered increased attention because it interferes with our ability to carry out this work. Soils that are or become water repellent develop all over the world and where hydrophobic or non-wetting substances can enter soil and remain in pore spaces or as coatings on particles. To assist in the tracking and management of its complex effects on water storage and infiltration, the goals of this work were to develop fundamental insights into the manifestation and effects of this variable soil property on key hydrologic properties and processes. This work tests a new conceptual model for understanding these systems through both field and laboratory work and using a number of different technologies. These include X-ray microtomography (μXCT), tension infiltrometry, and more regularly applied techniques which are sensitive to changes in repellency. The works shows how combining fractional wettability and contact angle dynamics generates a stereoscopic conceptual framework which facilitates increased capacity for quantifying and understanding of soil systems expressing dynamic soil water repellency.