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The work contained in this thesis presents four research manuscripts concerning the flow and motion of drops and bubbles in different geometries. The first project explores the geometry of a totally wetting droplet on a conical fiber. A droplet on a fiber undergoes spontaneous motion toward the base of the fiber due to capillary forces, and viscous dissipation opposes the motion. In the first paper (Chapter 3), it was found that balancing the viscous shear force with the driving capillary force describes the motion of the droplet along the fiber. However, in nature, if fibers are coated with a liquid, there is rarely one droplet present; the second paper (Chapter 4) studies a conical fiber coated with multiple droplets. A liquid film coating a fiber will break up into droplets and it is found that the spacing of droplets depends on the shape of the fiber. The merging of droplets was studied and the dynamics well matches numerical simulations. The third paper (Chapter 5) studies the fluid film that a droplet will leave behind as it moves along the fiber. Using asymptotic matching to film deposition theory, this study found that the film thickness is affected by the curvature of the droplet. These studies show that the conical geometry and droplet curvature play an important role in droplet motion and film deposition. The last project (Chapter 6) in this thesis concerns a chain of uniform sticky bubbles that rise through an aqueous bath. It is found that the chain of bubbles will buckle regularly as it moves through a liquid bath, much like a solid rope will buckle when impacting a surface. As the bubble chain rises through the bath, a compressive force develops due to an imbalance between the buoyancy of the chain and the viscous drag of the liquid surrounding it. Unlike solid ropes, there is no bending to stabilize the bubble chain and the regular buckling pattern is unex- pected. Using scaling arguments, it is found that the viscous bath both stabilizes the chain and introduces the compressive force. The geometry of the buckling can be described from a force balance between the compressive and stabilizing forces. Drops and bubbles prove to be useful experimental tools to probe driven flow in different geometries and provide valuable insight into fundamental and applied physics systems. / Thesis / Doctor of Science (PhD)

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The MIGHTI airglow imager onboard the ICON spacecraft in LEO was built to make remote thermospheric windspeed measurements at low latitudes. The MIGHTI team, when reviewing the data, observed variations in day-to-day brightness potentially indicative of plasma bubbles: regions of low-density E-region plasma which rise through the F-region and cause radio scintillation that interferes with communications and GPS performance. Here, we explore the possibility of MIGHTI observing plasma bubbles by using its red-line airglow measurements to attempt to detect this phenomenon. Small-scale structuring indicative of plasma bubbles is searched for by comparing measurements between MIGHTI's two identical imagers, which make remote airglow measurements at the same region from perpendicular directions. The usability of the two imagers for this purpose is assessed, given they are not calibrated to measure absolute airglow brightness, and it is determined that the level of disagreement between them does not prevent these comparisons. The evolution of the ionosphere in the time between the two instruments' measurements is accounted for using seasonal medians of expected behavior. Co-located measurements where the two MIGHTI imagers disagreed significantly were found, filtering out disagreements in measurement not likely to have a significant underlying ionospheric cause, although none were indicative of plasma bubble observations. These significantly differing measurements were most common shortly after dusk and in regions near the equator, especially between -30 to 70 degrees longitude. Simulations show the lack of definitive plasma bubble detections is likely due to MIGHTI's long image exposure time averaging out the effect of plasma bubbles as ICON orbits. More is now known about the potential for making comparative red-line airglow measurements between MIGHTI's imagers, and this information could be used in future work to explore larger-scale ionospheric structuring within the MIGHTI dataset. / Master of Science / The ionosphere is a region of Earth's atmosphere from an altitude of 70 to 500 km that plays an important role in radio communications, GPS, and spacecraft operation. Instabilities called plasma bubbles can develop within this region, causing interference in radio communications and degrading GPS accuracy. Therefore, understanding this region is important and has become a priority for NASA. The Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) is an instrument onboard the Ionospheric Connection Explorer (ICON) spacecraft, designed to measure winds and temperatures from 90 to 300 km altitude. MIGHTI accomplishes this by observing red-line airglow: a phenomenon where the gasses in the upper atmosphere emit red light due to photochemical reactions. Because of these reactions, red-line airglow is brighter where plasma density is higher, and dimmer inside plasma bubbles, where plasma density is lower. In this thesis, we explore the potential for using MIGHTI to detect plasma bubbles in the ionosphere by imaging red-line airglow. We find that plasma bubbles are relatively small enough that MIGHTI cannot conclusively detect them because of its long exposure time as its imager sweeps over an area of about 460 km.

Raw produce has frequently been identified as the source of bacterial pathogens that can cause human illnesses, including listeriosis and salmonellosis. Microbial pathogens may attach and form biofilms on raw fruit surfaces and can be difficult to remove. A cavitation process (formation of bubbles in water) was studied for its effectiveness for removal and inactivation of Listeria monocytogenes and Salmonella Newport from the surfaces of fresh Roma tomatoes and cantaloupes. Individual fruit were separately inoculated with each pathogen, then submerged in a water tank and treated with a bubble flow through an air stone using one airflow rate (0 – 14 liters/min.) for up to 60 sec. As air flow increased, pathogen reduction increased up to 1.2 log CFU/fruit greater than with water alone (no bubbles). Additional pathogen reduction in the tank water (organisms detached from the fruit) was observed with the bubble treatments. Therefore, these bubble streams can be used to enhance the detachment of bacteria from fruit surfaces and to inactivate a proportion of these detached microorganisms. Additionally, recoveries of Salmonella from inoculated Roma tomatoes and cantaloupe were determined for treatment water that contained 50 or 150 ppm sodium hypochlorite. The combination of cavitation and chlorine resulted in greater efficacy of inactivating the pathogen in treatment water, but not in removing this organism from the fruit surfaces. The physical force of a bubble stream on raw produce can effectively reduce and inactivate surface bacteria, and has the potential to reduce antimicrobial chemical and water use in post-harvest packing operations. / Master of Science in Life Sciences / Every year, one in six Americans will have been affected by a foodborne illness, many of which are caused by bacteria found on the surface of fresh fruits and vegetables. Most of these bacteria are removed with the help of a water wash with or without chlorine added. Nevertheless, microorganisms, including bacterial pathogens, may attach and form biofilms on raw fruit surfaces and can be difficult to remove. For this research, a cavitation process (formation of bubbles in water) was studied for its effectiveness for removal and inactivation of <i>Listeria monocytogenes</i> and <i>Salmonella</i> Newport from the surfaces of fresh Roma tomatoes and cantaloupes. Individual fruit were separately spiked with each pathogen, then submerged in a water tank and treated with a bubble flow through an air stone using one airflow rate (up to 14 liters air per minute) for 30 or 60 seconds. As air flow increased, the number of bacteria was reduced by up to 94% more bacteria per fruit than when using water alone (no bubbles). Additional bacteria reduction in the tank water (organisms detached from the fruit) was observed with the bubble treatments. Therefore, these bubble streams can be used to enhance the detachment of bacteria from fruit surfaces and to kill or injure some of these detached microorganisms. Additionally, recoveries of <i>Salmonella</i> from inoculated Roma tomatoes and cantaloupe were determined for treatment water that contained 50 or 150 parts per million sodium hypochlorite (chlorine solution). The combination of cavitation bubbles and chlorine showed a greater ability for inactivating these bacteria in the tank water, but not in removing this organism from the fruit surfaces. The physical force of a bubble stream on raw produce can effectively reduce and inactivate surface bacteria, and this process could reduce the amount of water or chemicals used to process fresh fruits and vegetables, while ensuring that these foods will not cause people to get sick upon eating.

Inert gases in metals can occur due to ion implantation, from a plasma in a magnetron device or as a result of being by-products of nuclear reactions. Mainly because of the nuclear applications, the properties of the inert gases, helium, argon and xenon in the body centred cubic (bcc) iron crystal are examined theoretically using a combination of molecular dynamics, static energy minimisation and long time scale techniques using empirical potential functions. The same techniques are also used to investigate argon and xenon in aluminium. The primary interest of the work occurred because of He produced in nuclear fission and its effect on the structural materials of a fission reactor. This structure is modelled with perfectly crystalline bcc Fe. In bcc iron, helium is shown to diffuse rapidly forming small bubbles over picosecond time scales, which reach a certain optimum size. In the initial phase of He accumulation, Fe interstitials are ejected. This occurs instantaneously for bubbles containing 5 He atoms and as the more He accumulates, more Fe interstitials are ejected. The most energetically favourable He to vacancy ratios at 0 K, vary from 1 : 1 for 5 vacancies up to about 4 : 1 for larger numbers of vacancies. An existing He bubble can be enlarged by a nearby collision cascade through the ejection of Fe interstitials, allowing more He to be trapped. Ar and Xe in bcc Fe prefer to be substitutional rather than interstitial and there are large barriers to be overcome for the inert gas atoms to diffuse from a substitutional site. Bubbles that form can again be enlarged by the presence of a nearby collision cascade or at very high temperatures. In this case the most energetically favourable vacancy ratios in the bubbles is 1: 1 for Ar and from 0.6: 1 to 0.8: 1 for Xe. For Ar and Xe, bubble formation is more likely as a direct result of radiation or radiation enhanced diffusion rather than diffusion from a substitutional site. Ar in aluminium is also studied. Ar atoms in fcc Al prefer to be substitutional rather than interstitial and evolution into substitutional occurs over picosecond time scales at room temperature. Bubble formation can occur more easily than in bcc iron, mainly because the barriers for vacancy diffusion are much lower but the time scales for bubble accumulation are much longer than those for He. A vacancy assisted mechanism is found which allows Ar to diffuse through the lattice. Finally some preliminary results on the energetics of different geometrical structures of larger Xe bubbles in Al are investigated since experiment has indicated that these can become facetted.

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