| Summary: | 博士 === 國立臺灣科技大學 === 機械工程系 === 102 === In this study, microscale schlieren and microscale background-oriented schlieren system (?嫊OS) systems are developed to facilitate measurement of inhomogeneity in a microfluidic. By mixing aqueous ethanol solution and water in a T-microchannel, calibration procedures is carried out to obtain the relations between the grayscale readouts and gradients of mass fraction for microscale schlieren method, and between pattern shift and gradients of mass fraction for ?嫊OS method. Once the relations are available, the full-field gradients of mass fraction in a microfluidic can be acquired by mapping the grayscale readouts or pattern shift data in the microfluidic device. Comparing to ?嫊OS, microscale schlieren system can offer a more accurate result with a better spatial resolution. Maximum detectable gradient of refractive index and sensitivity can be controlled more flexibly with microscale schlieren method. Comparing to ?嫊OS method, microscale schlieren method can observe transient schlieren phenomena, and it’s more suitable to use for visualization of refractive index gradient field. However, ?嫊OS system is easier to setup and can be used in an open space. ?嫊OS can measure full-field light deflection in both x- and y- directions simultaneously, whereas microscale schlieren system is restricted by the orientation and design of the knife-edge. Hence, ?嫊OS system can offer superior quantification result in a more complex microfluidic.
For both systems, the calibration curves are independent of mass fractions of the working fluid and Reynolds numbers. Microscale schlieren system can obtain better sensitivity by decreasing aperture size or increasing cutoff of the knife-edge. Comparing to 50% cutoff, increasing or decreasing cutoff will both decearse the comparable measuring ranges in both directions of concentration gradients as well as increase analytical error. Hence, modifying aperture size is a better solution to increase sensitivity instead of altering the cutoff. The optimal background illumination level is achieved at a grayscale readout of 121, and it maximizes the detectable range, while minimizing errors in the analysis. For ?嫊OS, increasing the distance between the background and the object leads to better sensitivity without increasing the uncertainty. Chrome-on-glass photomask with patterns printed by a laser pattern generator is used to obtain high resolution background of ?嫊OS for a accurate measurement. Three parameters are considered: pattern configuration (random-dot, random-grid, and grid), dot diameter, and area fraction of dot coverage. The performance of each background design is evaluated by the corresponding uncertainty of the calibration curve. For the grid design, accurate results can be obtained when there is about one dot in each interrogation window. For both random-dot and random-grid design, the lowest error level is achieved with a dot diameter of 6 ?慆, which corresponds to a dot-image diameter of 2.8 pixels. Because a sparse distribution leads to vacant interrogation windows, the optimal random-dot design has the highest area fraction of 0.196 (design value). In contrast, the random grid design with too many dots becomes comparable to the grid design and has higher analytical error. As a result, the best random-grid background has an area fraction of 0.098.
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