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Yes / There has been little study on the microscopic bubble entrainment and diffusion process on the high-speed self-aerated flows although the problem under investigation is theoretically important and has important engineering application. This study presents an experimental investigation on visual processes of free surface air entrainment and single bubble diffusion in supercritical open channel flows. The typical surface deformation, single air bubble rising and penetration are recorded using a high-speed camera system. Results show that for a single bubble formation process, surface entrapment development and bubble entrainment through a deformation evolution underneath the free surface are the two main features. The shape variation of local surface deformation with time follows an identical power law for different bubble size generations. The entrained bubble size depends on both size scale and shape of entrapped free surface. As the single bubble moves downstream, its longitudinal velocity is approximately the same as that of water flow surrounded it, while its vertical velocity for rising and penetration increases with the increase of the water flow velocity. An empirical-linear relationship for the bubble rising and penetration velocity with water flow velocity is obtained. This study demonstrates that the microscopic bubble movement can improve the self-aeration prediction in the open channel flow and advance the knowledge of our understanding of the macroscopic and microscopic air–water properties in hydraulic engineering. / National Natural Science Foundation of China (Grant number 51609162), Sichuan Science and Technology Program (Grant number 2019JDTD0007) and the Open funding of the State Key Laboratory of Hydraulics and Mountain River Engineering of Sichuan University (Project No: Skhl1809).

Between two neighboring air bubbles in a froth (or foam), a thin liquid film (TLF) is formed. As the bubbles rise upwards, the TLFs thin initially due to the capillary pressure created by curvature changes. As the film thicknesses (H) reach approximately 200 nm, the disjoining pressure created by surface forces in the films also begins to control the film drainage rate and affect the waves motions at the air/water interfaces. If the disjoining pressure is negative, both the film drainage and the capillary wave motion accelerate. When the TLF thins to a critical film thickness (Hcr), the amplitude of the wave motion grows suddenly and the two air/water interfaces touch each other, causing the TLF to rupture and bubbles to coalesce. In the present work, a new model that can predict Hcr has been developed by considering the film drainage due to both viscous film thinning and capillary wave motion. Based on the Hcr model, bubble-coarsening in a dynamic foam has been predicted by deriving the geometric relation between the thickness of the lamella film, which controls bubble-coalescence rate, and the Plateau border area, which controls liquid drainage rate. Furthermore, a model for predicting bubble-coarsening in froth (3-phase foam) has been developed by developing a film drainage model quantifying the effect of particles on pc. The parameter pc is affected by the number of particles and the local capillary pressure around particles, which in turn vary with the hydrophobicity and size of the particles in the film. Assuming that films rupture at free films, the pc corrected for the particles in lamella films has been used to determine the critical rupture time (tcr), at which the film thickness reaches Hcr, using the Reynolds equation. Assuming that the number of bubbles decrease exponentially with froth height, and knowing that bubbles coalesce when film drains to a thickness Hcr, a bubble coarsening model has been developed. The model predictions are in agreement with the experimental data obtained using particle of varying hydrophobicity and size. / Ph. D.

In the oil sand industry, bitumen is separated from sands by aerating the heavy oil so that it can float out of a flotation vessel, leaving the unaerated sands behind. A bubble-against-plate apparatus equipped with a high-speed camera has been developed to record the optical interference patterns of the wetting films formed on a flat surface and subsequently obtain the temporal and spatial profiles of the films offline using the Reynolds lubrication theory. The technique has been used to study the interaction mechanisms between air bubbles and bitumen. It has been found that the film thinning kinetics increases in the order of asphaltene, bitumen, and maltene, and that the kinetics increases sharply with increasing temperature. In addition to obtaining kinetic information, the temporal and spatial profiles of the wetting films have been used to derive appropriate hydrodynamic information that can be used to determine the disjoining pressures (∏) in the wetting films. The results obtained in the present work show that ∏ < 0 for maltene and bitumen, while ∏ > 0 for asphaltene at temperatures in the range of 22 to 80 °C. The disjoining pressure data have been analyzed by considering the contributions from the hydrophobic and steric forces in addition to the classical DLVO forces. It has been found that the hydrophobic force increases with increasing temperature, which corroborates well with contact angle data. Dynamic contact angle measurements show that air bubbles attach on bitumen with relatively small contact angles initially but increase sharply to >90° . The extent and the kinetics of contact angle change increase sharply with increasing temperature. These findings suggest that the primary role of temperature may be to increase iii bitumen hydrophobicity and hence hydrophobic force, which is the driving force for bubblebitumen interaction. A thermodynamic analysis carried out on the basis of the Frumkin-Derjaguin isotherm suggests that the disjoining pressure will remain positive (and hence no flotation) until the hydrophobic force becomes strong enough (due to temperature increase) to overcome the positive disjoining pressure created during the course of bitumen liberation. / Ph. D.

Mosquitoes drink using a pair of in-line muscular pumps in the head that draw liquid food through a long drinking channel termed as proboscis. Experimental investigations of mosquito drinking using synchrotron x-ray indicate two modes of drinking, a predominantly occurring continuous mode in which the anterior cibarial and posterior pharyngeal pumps expand cyclically at a constant phase difference and an isolated burst mode in which the pharyngeal pump expansion is several orders of magnitude larger than in the continuous mode. The objective of this thesis is to explain the mechanics and functional implication of this two-pump dual mode drinking of a mosquito. A reduced order mathematical model suggests that the primary role of the pharyngeal pump is in the burst mode. Since the precise geometry of the pump during drinking is yet not known, the drinking mechanism is modeled using different pump geometries based on morphological constraints in the animal. The model shows the continuous mode as being more effective in terms of energy expenditure, while the burst mode creates a large pressure difference across the proboscis which might be used to clear an obstruction in the channel or prime the channel. The hypothesis regarding the ability of a mosquito to self-clear an obstruction is analyzed by modeling the presence of an air bubble inside the system. The model indicates that air bubbles maybe able to stop flow during continuous mode drinking, and these same bubbles can be cleared by switching temporarily to burst mode drinking. / Master of Science

Air bubbles are a promising means of controlling fouling for a range of applications, particularly delaying fouling in marine environments. This work investigates the mechanism by which the collision of an air bubble with a solid removes adsorbed bacteria. A key feature of the work is that the numbers of bacteria were monitored via video microscopy throughout the collision, so we were able to explore the mechanism of bacteria removal. When a bubble collides with a solid, an air–water interface crosses the solid twice, and we were able to distinguish the effects of the first and second air–water interface. The bacterium Pseudomonas aeruginosa was allowed to adhere to smooth polydimethylsiloxane (PDMS) and then a collision with a bubble was investigated for one of three different approach geometries: perpendicular, parallel, and oscillating parallel to the solid surface. Other factors examined were the speed of the bubble, the duration of bacterial adhesion on the solid surface, and the wettability of the solid. Surface wettability was identified as the most significant factor. When the solid dewets, almost all bacteria were removed from hydrophobic surfaces upon the passage of the first air–liquid interface. In contrast, when a thin liquid film remained between the solid and the bubble (a hydrophilic solid), variable amounts of bacteria remained. Although almost all bacteria were initially removed from hydrophobic solids, many bacteria were redeposited on hydrophobic surfaces upon the passage of the second air–liquid interface, especially when the first and second air–liquid interfaces moved in opposite directions. As described previously, a lower velocity of the bubble allows more time for the thin liquid film to drain, and improved removal efficiency on hydrophilic solids. A rougher solid (8 µm diameter hemispherical protrusions) decreased the detachment efficiency because bacteria and liquid were able to shelter in concavities. Air bubbles are capable of removing bacteria over a range of conditions and are a potentially efficient means of combating biofilm growth for practical applications. / Master of Science / A major problem for equipment submerged in seawater is their eventual coverage in marine organisms including bacteria, barnacles, seaweed, and algae. This work investigates how effectively an air bubble removes bacteria adhered to a submerged solid. Adhered bacteria were observed and counted throughout the interaction of a bubble with a solid. When a bubble collides with a solid and is then removed, the bubble edge passes over the solid twice. The edge of the bubble is referred to as an air–liquid interface. The effects on adhered bacteria removal of the first and second passes of the bubble air–liquid interface were observed. Pseudomonas aeruginosa, a bacterial species common to both marine and medical environments, was allowed to adhere to flat solids made up of the polymer polydimethylsiloxane (PDMS) prior to a collision with an air bubble. The air bubble was collided with the solid in three distinct ways: directly from above, across the solid surface in one direction, and across the solid surface in one direction before being pulled back in the other direction. The speed of the bubble, the amount of time bacteria were adhered to the solid prior to bubble collision, and the extent to which the solid could be wet were all also examined for their effects on adhered bacteria removal. The extent to which a solid surface could be dewetted was identified as the most significant factor. For solids that are easily dewetted, almost all adhered bacteria were removed with the passage of the first air–liquid interface. Many bacteria were then redeposited back onto the solid surface upon the passage of the second air–liquid interface, especially when it moved in a direction opposite to the first. In contrast, for solids that are easily wet by water, variable amounts of bacteria remained after the first air–liquid interface swept across its surface. Slower moving air–liquid interfaces were also shown to be more effective at removing adhered bacteria. Solid surfaces with rough patterning made it more difficult to remove bacteria. Air bubbles can be an effective method to combat adhered bacteria and potentially prevent eventual biological growth on different types of underwater applications.

The purposes of this research were to implement the circular plume model developed by Wuest et al. (1992) and to develop and verify a linear plume model based on the circular model. The linear model developed is the first that models a bubble plume generated by a linear source in thermally stratified water and considers the effects of gas transfer between the bubbles and surrounding water. The basis for both models is eight differential flux equations which are solved numerically using Euler’s method. Knowledge of ambient temperature, dissolved solids, dissolved oxygen, and dissolved nitrogen profiles as well as gas input rate, diffuser dimensions, and initial bubble size are required to implement the models. The implementation of the circular model was successful as the results obtained corresponded with those reported by Wuest et al. (1992). The linear model made predictions very similar to those made by the circular model and, therefore, was also considered to perform well. Comparisons of the linear model with available data met with limited success. Initially, the linear model’s predictions of laboratory scale plume velocity data resulted in overpredictions of 40 to 50 percent when compared to actual data. Error in predictions of laboratory scale oxygen transfer data were greater than 100 percent. The model fared better when its predictions were compared to full scale data; the predicted temperature was within 7 percent of that measured at three depths and the predicted oxygen concentration was within 4, 20, and 38 percent for the three depths. Some of the discrepancies in the data likely result from the fact that the Froude number used in the model to calculate initial velocity was derived for a circular, rather than a linear, source. Determination of the appropriate linear Froude number would likely improve the model’s predictions. / Master of Science

Bubble columns and airlift reactors were modeled numerically to better understand the hydrodynamics and analyze the mixing characteristics for each configuration. An Eulerian-Eulerian approach was used to model air as the dispersed phase within a continuous phase of water using the commercial software FLUENT. The Schiller-Naumann drag model was employed along with virtual mass and the standard k-e turbulence model. The equations were discretized using the QUICK scheme and solved with the SIMPLE coupling algorithm. The flow regimes of a bubble column were investigated by varying the column diameter and the inlet gas velocity using two-dimensional simulations. The typical characteristics of a homogeneous, slug, and heterogeneous flow were shown by examining gas holdup. The flow field predicted using two-dimensional simulations of the airlift reactor showed a regular oscillation of the gas flow due to recirculation from the downcomer and connectors, whereas the bubble column oscillations were random and resulted in gas flow through the center of the column. The profiles of gas holdup, gas velocity, and liquid velocity showed that the airlift reactor flow was asymmetric and the bubble column flow was symmetric about the vertical axis of the column. The average gas holdup in a 10.2 cm diameter bubble column was calculated and the results for the two-dimensional simulation of varying inlet gas velocities were similar to published experimental results. The average gas holdup in the airlift reactor for the three-dimensional simulations compared well with the experiments, and the two-dimensional simulations underpredicted the average gas holdup. / Master of Science