"The importance of two-phase heat transfer for thermal management of aerospace avionic systems has become increasingly important as these systems have become miniaturized. Embedded active cooling systems are used to remove heat from processors and other electronic components and transferring this heat to radiators or other heat exchangers. As the characteristic dimension of flow channels for two-phase flow becomes comparable to bubble size, the mini-channels (< 3 mm) used to direct the cooling fluid can complicate nucleate boiling heat transfer. Bubbles can encounter other heated walls, rapidly expanding and greatly reducing heat transfer as well as causing pressure oscillations and flow instabilities. The use of eletrohydrodynamic (EHD) effects, through the introduction of non-uniform electric fields, can help mitigate this problem by altering the behavior of nucleating bubbles. A combined experimental and computational study was undertaken using HFE-7100, an engineered fluid used in heat transfer applications, to investigate the potential for enhancement of nucleate boiling using EHD effects induced by applying a non-uniform electric field. In the experimental study, a minichannel was constructed consisting of an upper and lower copper electrode and glass side walls to allow visualization. The channel height and width were 3mm and 4.76 mm respectively, representative of the minichannel regime. The upper electrode was grounded while the lower electrode was heated and biased to high voltage. Optical imaging combined with post-processing and statistical analysis was used to quantify the effect of EHD on the bubble behavior. Bubbles were found to form preferentially on nucleation sites resulting from imperfections in the heated copper surface over artificially created nucleation sites. When a high voltage is applied across the electrodes, the electric field enhancement along the rim of the nucleation site is believed to influence the force balance on the forming bubble and thereby influence the bubble departure size and frequency. EHD forces also act on the bubble surface as a result of the variation in permittivity between the liquid and vapor phases, altering its shape as has been previously reported in the literature. Test results are presented that demonstrate that the application of EHD increases the nucleation site density on the heated surface and increase the bubble departure frequency from individual sites. In addition, test results are presented to show that EHD forces alter the shape of bubbles during growth and the vertical position of the detached bubbles as they are carried along in the cross flow. To better understand the underlying phenomena affecting the bubble shape and departure frequency, a numerical simulation of the bubble growth and departure was performed using COMSOL multiphysics software customized to incorporate a user-defined body force based on the Maxwell Stress Tensor. Tracking of the bubble surface, including coalescence and breakup was incorporated using the phase field variable method in which the Navier-Stokes and heat transfer equations are solved for each phase of the fluid. Results from the simulations confirmed the sensitivity of the bubble elongation and neck formation to the nucleation site geometry, specifically the angle along the rim where field enhancement occurs. The enhanced constriction of the bubble neck resulted in early detachment of bubbles when compared to simulations in which EHD was not applied. This finding provides some insight into the higher bubble departure frequency and nucleation site density observed in the experiment. The results from the combined experimental and numerical study suggest that EHD enhancement may provide a mechanism for extending the use of nucleate heat transfer to minichannels, thereby enabling additional options for cooling in compact, embedded systems. "

The goal of this doctoral work was to observe and analyze the stability and dynamics of phospholipid-encapsulated microbubbles, and in particular the reaction to sudden submersion in a multi-gas medium. To accomplish this goal, first an experimental technique was developed to observe a microbubble in a single-gas environment suddenly immersed in a multi-gas environment, without perturbing the microbubble position. A modified Epstein-Plesset model was concurrently developed to account for the multiple gas species in the bulk solution. The model was used to analyze previous data for the effect of anesthesia carrier gas on microbubble ultrasound contrast agent in vivo circulation persistence. The focus of the experiments then shifted to microbubbles of different sizes encapsulated with a homologous series of saturated diacyl-chain lipid surfactants and emulsifiers. Constitutive models for the elastic and gas permeation properties of the lipid encapsulation were developed to elucidate the unique behaviors observed during the experiments. The experimental techniques employed were: (1) transmission bright field optical microscopy to obtain real-time, digital videos of microbubbles growing and dissolving in response to perturbations in the local gas environment and (2) the Langmuir trough film balance to determine the elasticity of the phospholipid monolayers during compression, expansion, and expansive relaxation. The modeling techniques employed was (1) a forward-wind finite difference method to discretize a series of non-linear differential equations and (2) a Newton-Raphson method to solve the diameter of a microbubble from the mechanical stress balance. These modeling techniques were used to determine the behavior of a microbubble a priori, whereas the fitting models implemented the iterative methods to solve for parameters without a Newton-Raphson method. Results showed that microbubbles coated with soluble surfactants and dissolving in a single gas solution could be predicted by the original Epstein-Plesset model. When subjected to a multi-gas medium, the modified Epstein-Plesset model accurately predicted microbubble growth and dissolution. The model was used to analyze the increase in microbubble circulation lifetime observed by others in anesthetized rats inhaling air rather than oxygen as the anesthesia carrier gas. The predictive capabilities of the model broke down, however, if the gas-core was encapsulated with a phospholipid monolayer. A typical, large (>40 µm diameter) lipid-coated microbubble displayed stunted growth, followed by three anomalous dissolution regimes: (1) rapid dissolution back to the initial resting diameter followed by (2) slow, steady dissolution and finally (3) stabilization, where the apparent surface tension approached a near-zero value. The model was modified to allow fitting of the radius-time curve by varying the surface tension. The analysis showed that the surface tension is dynamic, and suggested that a "break up" tension allowed for rapid expansion of the microbubble beyond the initial resting diameter. Lipid jamming was proposed as the mechanism eventually halting dissolution. Further observations of smaller microbubbles (<20 µm diameter) coated with a homologous series of saturated diacyl chain lipids gave significantly different results. Initially the microbubbles grew, but growth was severely subdued, if not eliminated, for more solid encapsulations below a threshold size (~10 µm diameter). Following growth, most microbubbles rapidly dissolved back to their original size. The microbubbles then experienced an anomalous lag time before spontaneously dissolving again. The lag times were highly variable and shown to correlate to the reduced temperature of the encapsulation, rather than the initial microbubble size. Most of the microbubbles stabilized again at a diameter of 1-2 µm, and this "stable diameter" appeared to be universal and independent of both the initial microbubble size and the rigidness of the encapsulation. Constitutive models were developed to describe these physical phenomena in the early growth and dissolution stages which were verified with independent monolayer relaxation studies.

Chapter 1 briefly introduces the background, problems, applications as well as recent progress of the nanobubbles research. The relationship between the formation/stabilization of nanobubbles and the long-rang structures of water molecules, particularly the restructuring of water molecules at the water/gas interface, are emphasized. / Chapter 2 introduces the theories of static and dynamic light scattering and Zeta-potential measurements as well as the details of the instrument set-up. In this chapter, the fundamental equations of the scattering theory are figured out basis on the quasi-classical electrodynamics and combination of the statistical mechanics as well as molecular dynamic theory. Finally, the statistical properties of photon counting are discussed. / In chapter 3, aqueous solutions of tetrahydrofuran, ethanol, urea and alpha-cyclodextrin were studied by a combination of static and dynamic laser light scattering (LLS). In textbooks, these small organic molecules are soluble in water so that there should be no observable large structures or density fluctuation in either static or dynamic LLS. However, a slow mode has been consistently observed in these aqueous solutions in dynamic LLS. Such a slow mode was previously attributed to some large complexes or supramolecular structures formed between water and these small organic molecules, Our current study reveals that it is actually due to the existence of small bubbles (&sim;100 nm in diameter) formed inside these solutions. Our direct evidence comes from the fact that it can be removed by repeated filtration and regenerated by air purging. Our results also indicate that the formation of such nanobubbles in small organic molecules aqueous solutions is a universal phenomenon. Such formed nanobubbles are rather stable. The measurement of isothermal compressibility confirms the existence of a low density micro-phase, presumably nanobubbles, in these aqueous solutions. Using a proposed structural model, i.e., each bubble is stabilized by small organic molecules adsorbed at the gas/water interface, we have, for the first time, estimated the pressure inside these nanobubbles. / In chapter 4, by using a combination of laser light scattering (LLS) and zeta-potential measurements, we investigated effects of salt concentration and pH on stability of the nanobubbles in alpha-cyclodextrin (alpha-CD) aqueous solutions. Our LLS results reveal that the nanobubbles are unstable in solutions with a higher ionic strength, just like colloidal particles in an aqueous dispersion, but become more stable in alkaline solutions. The zeta-potential measurement shows that the nanobubbles are negatively charged with an electric double layer, presumably due to the adsorption of negative OTT ions at the gas/water interface. It is this double layer that plays dual roles in the formation of stable nanobubbles in aqueous solutions of water-soluble organic molecules; namely, it not only provides a repulsive force to prevent the inter-bubble aggregation and coalescence, but also reduces the surface tension at the gas/water interface to decreases the internal pressure inside each bubble. / In chapter 5, the addition of salt can induce slow coalescence of nanobubbles (&sim;100 nm) in an aqueous solution of alpha-cyclodextrin (alpha-CD). A combination of static and dynamic laser light scattering was used to follow the coalescence. Our results reveal that its kinetic and structural properties follow some scaling laws; namely, the average size (&lt;zeta>) of nanobubbles is related to their average mass (&lt;M>) and the coalescence time (t) as &lt;M> &lt;zeta>dr and &lt;zeta> &sim; tgamma with two salt-concentration dependent scaling exponents (df and gamma) For a lower sodium chloride concentration (C NaCl = 40 mM), gamma = 0.13 +/- 0.01 and df = 1.71 +/- 0.02. The increase of CNaCl to 80 mM results in gamma = 0.32 +/- 0.01 and df = 1.99 +/- 0.01. The whole process has two main stages: the aggregation and the coalescence. At the lower C NaCl, the process essentially stops in the aggregation stage with some limited coalescence. At higher CNaCl leads the coalescence after the aggregation and results in large bubbles. / In this thesis, the nanobubbles in the aqueous solutions have been studied by using combination of static and dynamic laser light scattering (LLS), isothermal compressibility measurements and Zeta-potential measurements. We found that the nanobubbles extensively exist in aqueous solutions and the interface of each nanobubble is negatively charged. The addition of electrolytes can destabilize such interface to induce the coalescence of nanobubbles. / Jin, Fan. / "Aug 2007." / Adviser: Chi Wu. / Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1030. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2007. / Includes bibliographical references (p. 108). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract in English and Chinese. / School code: 1307.

This dissertation begins with a review of Calabi-Yau manifolds and their moduli spaces, flux compactification largely tailored to the case of type IIb supergravity, and Coleman-De Luccia vacuum decay. The three chapters that follow present the results of novel research conducted as a graduate student. Our first project is concerned with bubble collisions in single scalar field theories with multiple vacua. Lorentz boosted solitons traveling in one spatial dimension are used as a proxy to the colliding 3-dimensional spherical bubble walls. Recent work found that at sufficiently high impact velocities collisions between such bubble vacua are governed by "free passage" dynamics in which field interactions can be ignored during the collision, providing a systematic process for populating local minima without quantum nucleation. We focus on the time period that follows the bubble collision and provide evidence that, for certain potentials, interactions can drive significant deviations from the free passage bubble profile, thwarting the production of a new patch with different field value. However, for simple polynomial potentials a fine-tuning of vacuum locations is required to reverse the free passage kick enough that the field in the collision region returns to the original bubble vacuum. Hence we deem classical transitions mediated by free passage robust. Our second project continues with soliton collisions in the limit of relativistic impact velocity, but with the new feature of nontrivial field space curvature. We establish a simple geometrical interpretation of such collisions in terms of a double family of field profiles whose tangent vector fields stand in mutual parallel transport. This provides a generalization of the well-known limit in flat field space (free passage). We investigate the limits of this approximation and illustrate our analytical results with numerical simulations. In our third and final project we investigate the distribution of field theories that arise from the low energy limit of flux vacua built on type IIb string theory compactified on the mirror quintic. For a large collection of these models, we numerically determine the distribution of Taylor coefficients in a polynomial expansion of each model's scalar potential to fourth order. We provide an analytic explanation of the proncounced hierarchies exhibited by the random sample of masses and couplings generated numerically. The analytic argument is based on the structure of masses in no scale supergravity and the divergence of the Yukawa coupling at the conifold point in the moduli space of the mirror quintic. Our results cast the superpotential vev as a random element whose capacity to cloud structure vanishes as the conifold is approached.

Deuterons might be trapped in a bubble embryo which occur s due to statistical fluctuation in heavy water. The size of the bubble embryo is expected to be an order of a small molecule. The ground state energy level which the deuteron may occupy in the bubble is calculated by solving the Schroedinger equation, and by considering the interaction between the trapped deuteron by a spherical bubble and the surrounding polarized liquid medium (heavy water). From the dependence of the energy eigenvalue of the ground state on the bubble radius, the pressure exerted on the bubble wall is obtained. It is found that the pressure is negatively very large if the bubble radius is about the molecular size (3 to 7 Å). From extrapolating this result to larger sizes, we expect that a bubble would quickly collapse if enough energy is supplied and never grows to a stable bubble when the deuteron is trapped in the ground state.

No description available.

Unwanted gas bubbles are a challenge for microfluidic-based systems, as adherence to channel networks can disrupt fluid delivery. This is especially true for devices with biological applications, as the presence of a single bubble creates thin fluid films with extremely high shear stresses, which can damage biological samples. Current strategies to remove bubbles require complicated fabrication or off-chip components. This thesis describes an on-chip microfluidic strategy utilizing permeation for in-plane trapping and removal of occasional gas bubbles. The trap was demonstrated with nitrogen bubbles, which were consistently removed at a rate of 0.14 µL/min for a single trap, and shown to have long-term operation capability by removing approximately 4,000 bubbles during one day without failure. The trap was integrated with a microfluidic system for the study of small blood vessels. Experiments were complemented with analytical and numerical models to characterize the bubble removal process.

Unwanted gas bubbles are a challenge for microfluidic-based systems, as adherence to channel networks can disrupt fluid delivery. This is especially true for devices with biological applications, as the presence of a single bubble creates thin fluid films with extremely high shear stresses, which can damage biological samples. Current strategies to remove bubbles require complicated fabrication or off-chip components. This thesis describes an on-chip microfluidic strategy utilizing permeation for in-plane trapping and removal of occasional gas bubbles. The trap was demonstrated with nitrogen bubbles, which were consistently removed at a rate of 0.14 µL/min for a single trap, and shown to have long-term operation capability by removing approximately 4,000 bubbles during one day without failure. The trap was integrated with a microfluidic system for the study of small blood vessels. Experiments were complemented with analytical and numerical models to characterize the bubble removal process.

Hydrodynamic effects play important roles in fluidized bed combustion processes. Since the motion of "bubbles" is an important influence on fluidized bed heat transfer, a better understanding of their behavior is necessary for improving the design of fluidized bed boilers. Using a two-dimensional bed, silica sand particles were fluidized with air at room conditions. The bubbling bed was videotaped, and both qualitative and quantitative information were gathered. Bubble characteristics such as size, rise velocity and frequency were studied while particle size and superficial gas velocity were varied. Results were compared with some existing theories and other similar research. The effect of internal surfaces at several heights in the bed was also studied. General bubble behavior agreed well with descriptions from previous research, and the expected spherical-cap bubble shape was observed. Both bubble size and rise velocity increased with particle size and with fluid velocity. Bubble frequency increased with fluid velocity, but decreased with increasing particle size and height in the bed. These results agree with previous work done using optical probes to measure bubble characteristics. Comparisons of data with empirical models showed general agreement. The presence of internal surfaces had the effect of reducing the bubble size, rise velocity, and frequency, and also of reducing the influence of changing particle size and superficial velocity on the bed behavior. / Graduation date: 1992

In this thesis the critical parameters involved in the dislodgement and deformation of microbubbles in laminar channel flow, are determined and evaluated. Experimentally the effects of surface tension, viscosity, fluid flow rate, density, and bubble diameter on bubble dislodgement were evaluated. A theoretical scale analysis was performed which provided a general relationship between the parameters. Experimental results provided reasonable comparisons with values calculated from the scale analysis. Non-dimensional plots were generated of Weber number, at bubble dislodgement, versus Reynolds number and Weber number as a function of a non-dimensional bubble diameter. A calculated velocity detachment equation was also produced. This work is applicable to many areas of science and industry, particularly in the field of ink-jet printing. / Graduation date: 1992