本研究運用微機電製程技術，以矽晶圓及派瑞克斯玻璃經陽極接合，製作微型霧化器，用以快速大量製造用於粒子影像測速儀（Particle Image Velocimetry，PIV）之螢光性微米級粒子，而PIV流場觀測又有氣體流場及液體流場之分別，其個別適用之螢光粒子一般則以不同之粒徑範圍區分。吾人選用液體輔助方式進行霧化造粒，配合添加界面活性劑，以及適當之溶質溶劑之重量百分比濃度，並以不同設計之霧化氣進行操作，以期能製造出分別適用於液體或氣體流場觀測之螢光粒子，最後以霧化器Chip A在輔助液體氣源壓力6 ，工作流體流量0.25 下，可製作出粒徑分布小於10μm，適用於氣體流場PIV觀測之螢光粒子，並且發現以不同分子量大小之苯乙烯聚合物混和溶液為工作流體，其粒子表面形貌較使用單一分子量苯乙烯聚合物更為均勻平整。而粒子於螢光顯微鏡下亦觀測出其有良好之螢光性質。

SUMMARY

The study applies MEMS-based micro atomizer to manufacture micro fluorescent particles for Particle Image Velocimetry（PIV）application. In this thesis, micro atomizer is fabricated by using anodic bonding of Si wafer and pyrex glass 7740. Micro atomizers are operated in water-assisted spray which leads particles to form in spherical surface morphology. To avoid the formation of aggregates, the surfactants were added in assisted-water. In PIV flow field observations, tracer particles in different size range are used in gas-flow field and liquid-flow field respectively. Expecting to meet this situation, we designed different micro atomizer chips, and each one was operated in appropriate parameters. As the results, the size distribution of the fluorescent particles produced by atomizer Chip A is below 10μm in diameter, which means the particles are suitable for use in gas-flow field of PIV. Additionally, it is found that the surface morphology of the particles is more smooth and uniform by using two different molecular weight of styrene polymers than using one styrene polymer only. The fluorescence properties are determined with fluorescence microscope.
Key words: micro atomizer, water-assisted spray, fluorescent particles.

INTRODUCTION

Spray technology has been developed for a long time. It is widely used in our daily life, such as sprinklers, inkjet printers, internal combustion engine, electronic cooling technology, powder metallurgy and so on. To manufacture micro fluorescent particles for Particle Image Velocimetry（PIV）application, we use MEMS process to fabricate micro atomizers which is made by silicon wafer. In fluorescent PIV experiments, These so called tracer particles are usually made by polymers with fluorescent dying. In this study we designed different structure of atomizer nozzles. With appropriate operating parameters, we expect to manufacture the particles which can respectively be used in gas-flow field or liquid-flow field of PIV.

CHIP DESIGN AND FABRICATION

We designed six kinds of channel of atomizers, and each of them is made by silicon wafer in MEMS process. First, depositing SiO2 on the surface of the silicon wafer, and next, coating photoresist on top. After exposure and development, the pattern of flow channel is made on the photoresist. We can use photoresist layer as a mask to etch SiO2 layer, and then use SiO2 layer as a mask to etch silicon wafer. Last, using anodic bonding of silicon wafer and pyrex glass 7740, the atomizer chip is fabricated.

MATERIALS AND METHODS

Chip test
While finished the chip fabrication, a spray experiment was set to test the chips. We used glycerol solution in air-assisted spray under RT-Sizer to measure droplets size distribution. As the result, Chip A and Chip B are more suitable for manufacturing micro fluorescent particles in ideal size.
Materials
We use Poly(styrene-co-maleic acid)(PSMA; Mw = 120000) solution as working fluid, which is dissolved in Ethyl acetate, and add fluorescent dye, Rhodamine B. The weight percentage of working fluid is 2wt%. The assisting fluid is DI water with surfactants PVA and Tween 20, which weight percentage is 2wt% and 0.5wt%.
Methods
The whole spray process was in progress in a closed chamber to isolate the influence outside. The chip was fixed at the top of the chamber, and below it was a container filled with DI water and surfactants (2wt% PVA and 0.5wt% Tween 20 ) to collect the particles. While spraying, a magnetic stirrer was used to mix the particles uniformly in the water. Finally we take particles into Coulter-Sizer to measure the particle size information.

RESULTS AND DISCUSSION

Chip A
The pressure source of assisting water is fixed at 6kgf/cm2. Figure 1 shows the results of measurements. The horizontal axis is flow rate of working fluid, the vertical axis is particles diameter. The three curves are results of Dv90, Dv50 and Dv10, we can see that all three measurements have minimum value when the chip worked at lowest flow rate, 0.25 c.c./min. There are about 90% in volume of particles which diameter is less than 10μm, as shown in Figure 2. The morphology of particles produced by Chip A is shown in Figure 3, we can see some particles have porous surface.
Chip B
The atomization mechanism of Chip B is different from Chip A. Particles atomized with Chip B did not form spherical. The SEM picture is shown in figure 4.
Fluorescent properties
Figure 5 shows particles fluorescence, we can see the fluorescence image obviously.
Surface morphology improving
Particles produced by Chip A have porous surface. To improve this problem, we tried different recipes of working fluid. We add 1wt% polystyrene (Mw = 32000) and 1wt% PSMA in Ethyl acetate as new working fluid, and atomized it at assisting water pressure 6kgf/cm2 and flow rate 0.25c.c./min. The result shows that particles made by new working fluid have smooth surface instead of porous surface. The SEM picture is shown as figure 6.

CONCLUSION

By using atomizer Chip A, we can manufacture micro fluorescent particles which can be used in gas-flow field of PIV (90% of particles size less than 10μm). We can also make particles smooth surface by using two different molecular weight polymer (PSMA and polystyrene). Because different molecular weight means different length of train structure and short-train molecular can easily fill voids between long-train molecular, the surface of particles will be smooth.

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