The present thesis deals with two-phase electrohydrodynamic simulations of bubble and droplet dynamics under externally applied electric fields. We used the Coupled Level-Set and Volume-of-fluid method (CLSVOF) and two different electrohydrody-namic formulations to study the process of bubble and drop formation from orifices and needles, the interactions of two conducting drops immersed in a dielectric medium, and the oscillations of sessile drops under two different ways of applying external elec-tric field. For the process of bubble formation in dielectric liquids due to the injection of air from submerged orifices and needles, we show that a non-uniform electric field pro-duces smaller bubbles while a uniform electric field changes only the bubble shape. We further explain the reason behind the bubble volume reduction under a non-uniform electric field. We show that the distribution of the electric stresses on the bubble inter-face is such that very high electric stresses act on the bubble base due to a non-uniform electric field. This causes a premature neck formation and bubble detachment lead-ing to the formation of smaller bubbles. We also observe that the non-uniform elec-tric stresses pull the bubble interface contact line inside the needle. With oscillatory electric fields, we show that a further reduction in bubble sizes is possible, but only at certain electric field oscillation frequencies. At other frequencies, bubbles bigger than those under a constant electric field of strength equal to the amplitude of the AC electric field, are produced. We further study the bubble oscillation modes under an oscillatory electric field. We implemented a Volume-of-fluid method based charge advection scheme which is charge conservative and non-diffusive. With the help of this scheme, we were able to simulate the electrohydrodynamic interactions of conducting-dielectric fluid pairs. For two conducting drops inside a dielectric fluid, we observe that they fail to coalesce when the strength of the applied electric field is beyond a critical value. We observe that the non-coalescence between the two drops occur due to the charge transfer upon drop-drop contact. The electric forces which initially bring the two drops closer, switch direction upon charge transfer and pull the drops away from each other. The factors governing the non-coalescence are the electric conductivity of the drop’s liquid which governs the time scale of charge transfer relative to the capillary time scale and the magnitude of the electric forces relative to the capillary and the viscous forces. Similar observations are recorded for the interactions of a charged conducting drop with an interface between a dielectric fluid and a conducting fluid which is the same as the drop’s liquid. For the case of a pendant conducting drop attached to a capillary and without any influx of liquid from the capillary, we observed that the drop undergoes oscillations at lower values of electric potential when subjected to a step change in the applied electric potential. At higher values of electric potential, we observed the phenomenon of cone-jet formation which results due to the accumulation of the electric charges and thus the electric forces at the drop tip. For the formation of a pendant conducting drops from a charged capillary due to liquid injection, we observed that the drops are elongated in presence of an electric field. This happens because the free charge which appears at the drop tip is attracted towards the grounded electrode. This also leads to the formation of elongated liquid threads which connect the drop to the capillary during drop detachment. We plotted the variation of total electric charge inside the drops with respect to time and found the charge increases steeply as the drop becomes elongated and moves towards the grounded electrode. For sessile drop oscillations under an alternating electric field, two different modes of operations are studied. In the so called ‘Contact mode’ case, the droplet is placed on a dielectric coated grounded electrode and the charged needle electrode remains in direct contact with the drop as it oscillates. In the ‘Non-contact mode’ case, the drop is placed directly on the grounded electrode and electric potential is applied to a needle electrode which now remains far from the drop. We show that the drop oscillations in the contact mode are caused by concentration of electric forces near the three phase contact line where the electric charge accumulates because of the repulsion from the needle. For the non-contact mode, we observe that the electric charge is attracted by the needle towards the drop apex resulting in a concentration of the electric forces in that region. So the drop oscillates due to the electric forces acting on a region near the drop tip. We also present the variation of the total electric charge inside the drop with respect to time for the two cases studied.

Thesis (PhD (Physics))--Stellenbosch University, 2008. / The advent of high intensity short pulse lasers has opened the door to investigating buried solid-solid interfaces through the technique of optical second harmonic generation (SHG). This has led to extensive study of technologically important systems such as the Si/SiO2 interface. In this study, SHG is employed to study the interface between highly boron doped p+-type Si and its native oxide layer (SiO2). Previous studies from this laboratory have extensively investigated the photo-induced charge transfer process across the Si/SiO2 interface in the case of undoped natively oxidized Si by means of SHG, with initial SHG measurements being performed on boron doped p+-type Si. The natively oxidized p+-type Si/SiO2 sample was placed on a computer controlled positioning system which allowed for translation of the sample and rotation around the azimuth. The laser system employed was characterized in terms of spectral composition, pulse duration, pulse repetition rate, spatial pro le and pulse energy in order to ensure quantitative measurements. The SHG signal generated from the sample interface was recorded in re ection. Under the applied irradiation conditions, defects are created at the interface by the near infra red (NIR) femtosecond radiation from the laser. These defects are then populated via multi-photon processes by electrons and to a lesser extent holes. The charge transfer across the interface induces an interfacial electric eld. This photo-induced electric eld is in addition to the built-in interfacial electric eld caused by positive ionization of naturally occurring interfacial defects due to the strong doping of the bulk Si. It is this interfacial electric eld, consisting of the built-in doping induced eld and the photo-induced electron and hole elds, that is probed by SHG. The SHG signal is strongly dependent on the magnitude of this interfacial electric eld as the electric eld induced second harmonic (EFISH) signal dominates all other contributions to the observed SHG signal in the case of the Si/SiO2 system. The temporal evolution of the SHG signal is recorded for di erent intensities from virgin as well as the pre-irradiated samples. This yields information about the time scales on which the charge separation occurs as well as the in- uence of existing photo-induced trap sites on the charge separation process, since the strength of the SHG signal is an indirect measure of the interfacial electric eld strength. The angular dependence of the SHG signal (SH rotational anisotropy measurements) for both the initial signal (when the doping induced electric eld dominates) and the saturated signal (when the electron induced electric eld dominates) is measured. Both these measurements show a four fold symmetry but with a relative 45 phase shift between them. This iii is taken as con rmation of the reversal of the interfacial electric eld direction. The initial SHG signal as a function of intensity is also recorded for di erent incident wavelengths. The variation in the non-quadratic dependence of the initial SHG signal on the incident intensity is attributed to a resonant enhancement of two-photon absorption and subsequent screening of the interfacial electric eld by charge carriers. The measurement performed and the results obtained contribute to the understanding of the photo-induced charge separation process across buried solid-solid interfaces, speci cally as it applies to the important Si/SiO2 interface.

In the present PhD work, three sophisticated models based on the "Engineering" modeling approach have been utilized to conveniently describe and thoroughly analyze details of Lightning events at the CN Tower. Both the CN Tower and the Lightning Channel are represented by a number of connected in series Transmission Line sections in order to account for the variations in the shape of the tower and for plasma processes that take place within the Lightning Channel. A sum of two Heidler functions is used to describe the "uncontaminated" Return Stroke current, which is injected at the attachment point between the CN Tower and the Lightning Channel. Reflections and refractions at all points of mismatched impedances are considered until their contribution becomes less than 1% of the originally injected current wave. In the proposed models, the problem with the current discontinuity at the Lightning Channel front, commonly taken care of by introducing a "turn-on" term when computing radiation fields, is uniquely treated by introducing reflected and transmitted components. For the first time, variable speed of propagation of the Return Stroke current front has been considered and its influence upon the predicted current distributions along the whole Lightning Channel path and upon the radiated distant fields analyzed. Furthermore, as another novelty, computation of the electromagnetic field is accomplished in Cartesian Coordinates. This fact permits to relax the requirement on the verticality of the Lightning Channel, normally imposed in Cylindrical Coordinates. Therefore, it becomes possible to study without difficulty the influence of a slanted Lightning Channel upon the surrounding electromagnetic field. Since the proposed sophisticated Five-Section Model has the capability to represent very closely the structure of the CN Tower and to emulate faithfully the shape of, as well as physical processes within the Lightning Channel, it is believed to have the potential of truthfully reproducing observed fields. The developed modeling approach can be easily adapted to study the anticipated radiated fields at tall structures even before construction.

Arc Flash events occur due to faults in electrical equipment combined with a significant release of energy across an electrical arc. Due to the large energy release, plasma is generated, pressures increase, and the plasma expands. Under these conditions the plasma becomes excited enough to liquefy metal causing physical damage to equipment and any humans in the vicinity. This thesis investigates the state of art for detection of arc flash events and investigates a method of improving detection reliability, and speed by monitoring the high frequency voltage / current patterns utilizing methods similar to arc flash circuit interrupters (AFCI). A second alternative detection approach is determined through analysis of the physics of plasma development. The current state of art is based upon light detection. However this thesis experimentally investigates what happens before the arc event emits visible light. The results show that current flows to ground during an arc event slightly prior to the production of light. Further it shows through analysis of the physics of plasma that a high speed plasma detector has the potential to identify an arc event before the presence of visible light. Through the design and construction of experimental test setups, and physics analysis, this thesis provides new paths for detecting arc events that present opportunities to improve detection time.

Metal and oxide interactions are of broad scientific and technological interest in areas such as heterogeneous catalysis, microelectronics, composite materials, and corrosion. In the real world, such interactions are often complicated by the presence of interfacial impurities and/or high electric fields that may change the thermodynamic and kinetic behaviors of the metal/oxide interfaces. This research includes: (1) the surface hydroxylation effects on the aluminum oxide interactions with copper adlayers, and (2) effects of high electric fields on the interface of thin aluminum oxide films and Ni3Al substrate. X-ray photoelectron spectroscopy (XPS) studies and first principles calculations have been carried out to compare copper adsorption on heavily hydroxylated a- Al2O3(0001) with dehydroxylated surfaces produced by Argon ion sputtering followed by annealing in oxygen. For a heavily hydroxylated surface with OH coverage of 0.47 monolayer (ML), sputter deposition of copper at 300 K results in a maximum Cu(I) coverage of ~0.35 ML, in agreement with theoretical predictions. Maximum Cu(I) coverage at 300 K decreases with decreasing surface hydroxylation. Exposure of a partially dehydroxylated a-Al2O3(0001) surface to either air or 2 Torr water vapor results in recovery of surface hydroxylation, which in turn increases the maximum Cu(I) coverage. The ability of surface hydroxyl groups to enhance copper binding suggests a reason for contradictory experimental results reported in the literature for copper wetting of aluminum oxide. Scanning tunneling microscopy (STM) was used to study the high electric field effects on thermally grown ultrathin Al2O3 and the interface of Al2O3 and Ni3Al substrate. Under STM induced high electric fields, dielectric breakdown of thin Al2O3 occurs at 12.3 } 1.0 MV/cm. At lower electric fields, small voids that are 2-8 A deep are initiated at the oxide/metal interface and grow wider and deeper into the metal substrate, which eventually leads to either physical collapse or dielectric breakdown of the oxide film on top.

Understanding, predicting, and controlling electromagnetic field interactions on and between complex RF platforms requires high fidelity computational electromagnetic (CEM) simulation. The primary CEM tool within NASA is GEMINI, an integral equation based method-of-moments (MoM) code for frequency domain electromagnetic modeling. However, GEMINI is currently limited in the size and complexity of problems that can be effectively handled. To extend GEMINI’S CEM capabilities beyond those currently available, primary research is devoted to integrating the MFDlib library developed at the University of Kentucky with GEMINI for efficient filling, factorization, and solution of large electromagnetic problems formulated using integral equation methods. A secondary research project involves the hybrid parallelization of GEMINI for the efficient speedup of the impedance matrix filling process. This thesis discusses the research, development, and testing of the secondary research project on the High Performance Computing DLX Linux supercomputer cluster. Initial testing of GEMINI’s existing MPI parallelization establishes the benchmark for speedup and reveals performance issues subsequently solved by the NASA CEM Lab. Implementation of hybrid parallelization incorporates GEMINI’s existing course level MPI parallelization with Open MP fine level parallel threading. Simple and nested Open MP threading are compared. Final testing documents the improvements realized by hybrid parallelization.

The limiting current technique has widely been used to study liquid-solid mass transfer in various reactor configurations. In the present contribution several underlying physical aspects have been investigated in order to improve the design of mass transfer experiments. Experimentally, the significant influence of electrolyte composition and hydrodynamic conditions have been studied and quantified to ensure conditions of high reproducibility. In the course of single phase COMSOL simulations, different electrode configurations have been examined with emphasis on concentration fields and electric current distribution showing a large sensitivity of the experimental configuration on the absolute current values.

Electroporation affects the dielectric properties of cells. Dielectric measurement techniques can provide a label-free and non-invasive modality to study this phenomenon. In this thesis we introduce a dielectrophoresis (DEP) based technique to study changes in the cytoplasm conductivity of single Chinese hamster ovary (CHO) cells immediately after electroporation. Using a microfluidic chip, we study changes in the DEP response of single CHO cells a few seconds after electroporation. First, in order to quantify our DEP measurement results and relate them to the cells internal conductivity, we introduce a dielectric model for CHO cells. This is achieved by measuring the DEP response of many individual cells in the β-dispersion frequency region and curve fitting to the measured data. Second, we present quantitative results for changes in the cytoplasm conductivity of single cells subjected to pulsed electric fields with various intensities. We observe that when electroporation is performed in media with lower ionic concentration than cells cytoplasm, their internal conductivity decreases after electroporation depending on the intensity of applied pulses. We also observe that with reversible electroporation there is a limit on the decrease in the cells’ internal conductivity. We hypothesize the reason is the presence of large and relatively immobile negative ions inside the cell which attract mobile positive ions (mainly sodium and potassium) to maintain cell electrical neutrality. We monitor the temporal response of cells after electroporation to measure the time constant of changes due to ion transport and observe this ranges from seconds to tens of seconds depending on the applied pulse intensity. This result can be used to infer information about the density and resealing time of very small pores (not measurable with conventional marker molecules). Lastly, we measure the electroporation of cells in media with different conductivities. Our results show that electroporation in very low conductivity media requires stronger pulses to achieve a similar poration extent as in high conductivity media. The outcome of this thesis can be used to improve our understanding of the dynamics of electroporation as well as its modelling in order to make more accurate predictions or optimize the process for specific applications. / February 2017

Theoretical and experimental plasmonics is a growing field as a method to create near fields at sub-wavelength distances. In this thesis, a finite-difference time-domain method is used to simulate electromagnetic waves onto a thin film that present of nickel nanorods with sharp apexes. The absorbed, transmitted, and reflected fields were shown to depend linearly on silver film thickness and nanotip length. The electric field is visualized along the tip to show strong charge density along the base of the tip’s apex and how that density changes for wavelength, metal, and source tilt. Lastly, the study shows gold film on the nanotip apex provides the largest enhancement of the electric field for the wavelengths 532, 572, and 633 nm.

This research is divided into three major parts. In part I, the critical supersaturations required for the homogeneous nucleation of 2,2,2-trifluorothanol (TFE) vapor have been measured over a temperature range (266-296 K) using an upward thermal diffusion cloud chamber (DCC). The measured supersaturations are in agreement with the predictions of both the classical and the scaled theory of nucleation. Moreover, the condensation of supersaturated TFE vapor on laser-vaporized magnesium nanoparticles has been studied under different experimental conditions, such as the supersaturation, the pressure and the electric field. In part II, the laser vaporization controlled condensation (LVCC) technique was used to prepare Au-Ag alloy nanoparticles in the vapor phase using designed targets of compressed Au and Ag micron-sized powder mixtures of selected composition. The results showed that the optical properties of these nanoparticles could be tuned depending on the alloy composition and the laser wavelength. Different intermetallic nanoparticles (FeAl and NiAl) from the vapor phase has also been prepared, using the same approach.In this work, the fraction of the charged particles generated during the laser vaporization process was used to prepare a new class of nanoparticle assemblies in the LVCC chamber under the influence of an electric field. The results showed that the electric field required to induce the formation of these nanoassemblies is material and field dependent. By coupling the LVCC chamber with the differential mobility analyzer, size-selected nanoparticles have been prepared in the vapor phase. The prepared nanoparticles were characterized by different techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV-visible spectroscopy. In part III, new methods were developed to prepare nanoparticle-polymer composites from the vapor phase. In the first method, the LVCC method was used to prepare a carbonaceous cross-linked resin, with different nanoparticles (Ni, Pt and FeAl) embedded inside. In the second method, free radical-thermally initiated polymerization was used to polymerize a monomer vapor of styrene on the surfaces of activated Ni nanoparticles.