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4차원 X선 영상법 개발: 미세기포 동역학 연구

4차원 X선 영상법 개발: 미세기포 동역학 연구
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Four-dimensional (4-D) imaging of bubbles, despite providing useful evidences to investigate their interactions, is largely unexplored for microbubbles, mostly due to strong light scattering and shallow depth of field in conventional optical imaging. In this thesis, a 4-D X-ray imaging method for bubble dynamics is developed based on development of high performance X-ray imaging detector. The high spatio-temporal resolution of the developed detector allows 4-D X-ray microtomography of (rising) microbubbles with high temporal resolution. The thesis consists of three parts
1) development of a high performance X-ray detector for 4-D microtomography, 2) development of 4-D X-ray microtomography system, and 3) development of tracking 4-D X-ray microtomography. As the first topic, a high performance X-ray imaging detector was developed to realize high spatial and/or temporal resolution for 4-D microtomography. A high spatial resolution (a measurable feature size of 300 nm) without radiation damage of optical lens and camera was achieved by optimizing the optical configuration with high numerical aperture and long working distance, specifically by miniaturizing the conversion (of X-rays to visible lights) system. Temporal resolution is also significantly enhanced up to 9.8 μs, by enhanced light conversion efficiency of scintillation screen and enlarged solid angle of signal collection. The developed detector was successfully tested not only for the visualization of pinch-off phenomenon of a water droplet from a metal needle, but also for bubble detachment of a microbubble from a nozzle. These tests clearly evidenced the capabilities of the X-ray imaging detector for fast microtomography in microbubble systems.The second part of the thesis was to develop a 4D X-ray microtomography system for visualization of microbubble based on the X-ray imaging detector developed above. The 4-D microtomography system was built up in SPring-8 BL29XU RIKEN beamline with bright monochromatic X-rays. The shortest scan time for the acquisition of a tomographic series was 0.25 s with 1.25 μm spatial resolution. The 4-D visualization with the microtomography system was successfully tested by observing quasi-static bubbles in a viscous bubbly flow. The last part of the thesis was to develop ‘Tracking 4-D X-ray microtomography’ for 4-D visualization of rising microbubbles. By counterbalancing their rise, time-dependent information of rising bubbles such as sizes, shapes, position and velocities of rising microbubbles (< 500 μm) were clearly visualized in 3-D geometry. Rising speed of individual bubbles, relative distance and angles could be acquired simultaneously. In addition, high resolution imaging of multiple rising microbubbles could be successfully visualized regardless of horizontal overlap. The tracking X-ray microtomography affords new opportunities for understanding bubble-bubble (or bubble-particle) interactions at micro scales, which is important in various fields such as microfluidics, biomechanics, and floatation.
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