1.Introduction

A number of microfluidic devices have been developed recently for chemical analysis applications in the form of μ-TAS (Micro-Total Analysis Systems) and labs-on-a-chip [1]. Microfluidic device is fluid machinery, which consists of several microchannels. These μ-TAS and labs-on-a-chip have many advantages such as increased speed, efficiency, portability, and reduced consumption. In order to utilize these advantages, chemical processes such as mixing, reaction, and solvent extraction were integrated onto a microchip using a multiphase microflow network [2]. The investigation of the fluid mechanics and mass transport is therefore very important for effective design of microfluidic devices.

In this thesis, I have developed the in situ mass transport measurement systems including a micro-LIF system, a high-speed micro-PIV system, and a 3D scanning micro-PIV system. Furthermore, utilizing the findings, I have applied microfluidic devices in order to explore material science and crystal engineering: A Fullerene C60 crystallization reactor system was developed by utilizing such a microfluidic liquid/liquid interface.

2. Development of Mass Transport Measurement System: 3D scanning micro-PIV

To obtain three-dimensional velocity distribution in microscopic scale, a novel particle image velocimetry system was developed (Figure 1a). To validate the measurement accuracy of the system, the developed 3D scanning micro-PIV technique was applied to a microtube flow measurement (Figure 1b). The flow in a micro-round tube with 95 μm diameter was visualized using fluorescent particles, which absorbed green light (~535 nm) and emitted orange light (~575 nm). In the 3D scanning micro-PIV technique, 2D (x,y)-2C (u,v) velocity was calculated using a cross correlation method, which is most popular for digital particle image velocimerty. In order to obtain information on the vertical axis (z), the objective lens was equipped with a piezo actuator. The piezo actuator was displaced by the input current signal from a function generator. The amplitude and frequency of displacement were determined by the voltage and frequency of the input current signal, respectively. Thus, when the piezo actuator was displaced in z direction, the objective lens also moved and one can obtain information on the vertical axis. In addition, Out-of-plane velocity (w) was calculated using a dual-plane PIV technique [3].

2.2 Microscopic 3Dimensional and 3 component flow velocity distribution

Figure 2a shows optical micrographs of the test section. The microtube was made of FEP (Fluorinated Ethylene Polymer). The refractive index 1.338 was almost similar to that of water (1.330). Since the mictotube was set in a water bath shown in Figure 1b, the near wall regions were clearly observed inside the microtube flow. Figure 2b shows the three-dimensional velocity and three-component distribution of the microtube flow. The working fluid of water was injected at the constant flow rate of 4.0 μL/min, corresponding Reynolds number was 0.91. The velocity distribution was obtained at a spatial resolution of 5.4 x 2.7 x 4.2 μm. The microtube was set parallel to the x-axis and the test fluids flowed from back to front. While the velocity component was almost zero at the near wall region, the maximum velocity was observed at the center of the microtube. The velocity distribution had the appearance of typical, fully developed laminar flow. The obtained velocity profiles agreed with theoretical solution of Hagen-Poiseilles equation. On the measurement accuracy of the 3D scanning micro-PIV system, the maximum temporal variance was RMS = 1.6 mm/s in streamwise velocity component. In other hand, the depthwise velocity (w) of the microtube flow was negligible small. Thus, this was not suitable test section for the validation of the out of plane velocity.

3.Development of Crystallization Reactor System: A Microfluidic Synthesis of C(60) Single Crystals

3.1 Screening of Fullerene C(60) crystals

Since their initial discovery in 1985, Fullerenes C(60) have attracted significant attention for their unique physical and chemical properties, and have resulted in the creation of a new research field. Fullerene C(60) is also excellent candidate of new medicine. Since the metastable crystals showed different solubility and bioavailability from those of stable crystals, screening of metastable or stable phases of crystals is critical for development of new medicine. In terms of protein crystals, efficient screening techniques using microfluidic chips had already been presented. In this thesis, I have developed a screening method of C60 crystallization using a microfluidic device. The test solutions were toluene with C(60) and alcohol, including isopropanol, ethanol, and methanol. In all the experiments, the concentration of C60 dissolved in toluene was a quarter of the saturation concentration at 25 oC. Two solutions were kept at a low temperature of 0 oC by a temperature controller until just before introduction into the channels. Toluene and alcohol were introduced into a Y-shaped glass microchannel with 100 μm width, 40 μm depth and 40 mm length independently with a flow rate of 1.0 μL/min (Fig. 3a). The C60 crystals precipitated at the toluene/alcohol interface [4]. The microchannel was immersed in a water bath and a water immersion objective lens (M = 100, NA = 1.0) was used for in situ observations (Fig. 3b). The room temperature was kept at 20 oC using an air conditioner. The starting water temperature was 5 oC as set by the temperature controller until just before introduction of the test solutions. Subsequently, the water bath was left out at room temperature. Therefore, the temperature of the water bath increased and approached 20 oC gradually during the experiments. The water bath temperature was constantly checked by a thermometer.

3.2 Structural and optoelectonic properties of C60 crystals

In the first experiment, I introduced toluene with C(60) and IPA into the channels. Within 5 minutes after the fluid flow was stopped, at the bath temperature of 6 oC, C(60) crystals were formed in Fig. 4a-b. The lengths of the crystals were approximately 30 μm. The center axis areas of the crystals were semi-transparent in left inset Fig. 4a-b. In addition, several crystals had crosswise cut edges in right inset Fig. 4a-b. SEM (Scanning Electron Microscopy) revealed the reason for the formation of the semi-transparent whiskers in Fig. 4b-a. Several "macaroni"-like crystals with 7 μm diameter were observed. Their cross-sections were hexagonal. Another type of tube which was cut crosswise at the edge was also observed in Fig. 4b-b. The edge outline was not parallel to the center axis, indicating that the cross-sections of these "penne"-like crystals were also hexagonal. The characteristics of the cross-sections of these crystals were emphasized in another magnified SEM image in Fig. 4b-c. Within the void area, multi-layers of the crystal were observed. The cross-section of the void was not circular but was hexagonal. Some spherical crystals shown in Fig. 4b-d were also observed on the same microgrid. Within 15 minutes after the flow was stopped, at a bath temperature of 9 oC, tree-like dendrites appeared in Fig. 4a-c. A number of branches were observed to sprout from a main trunk. The main trunks and the branches were approximately 50 μm and 10 μm in length, respectively. The main trunks originated at the upper channel wall, indicating that nucleation of the crystals occurred at the wall on the toluene side. Thirty-two minutes after the flow was stopped, at a bath temperature of 16 oC, short rhombus-shaped columns with 10 μm length were formed in Fig. 4b-e. Forty minutes after the flow was stopped, at a bath temperature of 18 oC, the typical C(60) nanowhiskers with high aspect ratios appeared in Fig. 4b-f. Their lengths and diameters were more than 20 μm and less than 5 μm, respectively.

To change the solvated state and the supersaturation of C(60) from the conditions of the previous studies, in the second experiment, we attempted to use ethanol instead of IPA. One minute after the flow was stopped, at a bath temperature of 5 oC, crystals with hollow ends appeared on the toluene side in Fig. 4a-d. The size was about 4 μm in diameter and 30 μm in length. A TEM (Transmission Electron Microscope) micrograph showed the outline of the open side void in Fig. 4c-a. Five minutes after the flow was stopped, at a bath temperature of 7 oC, many particles of a few micrometers size were formed on the toluene side in Fig. 4a-e. Several sprouts from the particles were also observed (right arrow in Fig. 4a-e). The sprouts grew in one direction. On the other hand, a Y-shaped branch crystal was also observed (left arrow). The Y-shaped crystal had two (Fig. 4c-b) or three (Fig. 4c-c) of the small separation branches. Seven minutes after the flow was stopped, at a bath temperature of 8 °C, several crystals with sharp edges appeared in Fig. 4a-f. The crystals radiated in all directions and became multiple pods. The crystals had 6-8 columns with two- (Fig. 4c-d) or three-dimensional structures (Fig. 4c-e). Some crystals had acute edges in Fig. 4c-f. The size of each crystal was about 20 μm, and interestingly, each other column was positioned at a regular angle of 60o.

SAED (Selected Area Electron Diffraction) revealed that the C(60) crystals synthesized in the microfluidic device had a hexagonal close-packed structure (hcp) or a face centered cubic (fcc) structure in Fig. 5a. The lattice constants were a = 19.2 Å and c = 9.7 Å (Fig. 5a-a) and a = 23.3 Å (Fig. 5a-b). They were found to be single crystallines. Overall, the lattice constants of these crystals were larger than those of pristine C(60): a = 14.2 Å. This indicated that the C(60) crystals synthesized in the microfluidic device may be van der Waals crystals including the toluene molecules in the structure. All the Raman spectrum of the C(60) crystals synthesized under different flow rate conditions during nucleation showed 8 Hg and 2 Ag modes, which corresponds to characteristic active modes of C(60) single crystals (Fig.5b). Especially, Ag(2) pentagonal pinch mode 1468 cm-1 showed the C(60) crystals consisted of Fullerene monomers, indicating that any polymerization of C(60) crystal did not occur in the microfluidic device during crystallization. In other hand, PL (Photoluminescence) spectra of the C(60) crystals were unique (Fig.5c). The PL intensity of the rod-shaped crystal was 1 order of magnitude higher than that of bulk crystals (Fig.5c-a), suggesting that the doping by toluene molecules enhanced the luminescence of C(60). The difference of PL intensity was resulted from different lattice structure and proportion of included toluene molecules. In addition, it was obvious that 0.05 eV blueshift of PL spectra of the C(60) rods was observed compared with those of the raw C(60) materials (Fig. 5c-b). This indicated that distortion of fcc structure and the strong influence of toluene molecules on electronic state of the C(60) rods.

4. Conclusions

In order to investigate in situ microscopic scale fluid mechanics and mass transport, a micro-LIF system, a high-speed micro PIV system, and a 3D scanning micro-PIV system were developed. By using the systems, concentration and velocity fields inside the microfluidic devices were measured at previously unachieved high spatiotemporal resolutions and dimensions. In addition, by utilizing the insights obtained above, the Fullerene C(60) crystallization reactor was developed. The screening of metastable phases of C(60) crystals were carried out and their structural and optoelectronic properties were investigated. The C(60) crystals showed unique physical properties resulted from specific microfluidic environment. The present works provide physicochemical properties of microscopic fluid mechanics, mass transport, and crystallization. These insights will advance research fields of μ-TAS, lab-on-a chip, material science and crystal engineering.

Figure 1 (a) Schematic of experimental setup and (b) 3D scanning micro-PIV system

Figure 2 (a) Optical micrograph of the microtube and (b) 3D-3C velocity distribution.

Figure 3 (a) Schematic of microchannel and (b) temperature control system

Figure 4 (a) Optical micrographs and SEM images of C(60) crystals synthesized in (b) toluene-IPA and (c) toluene-ethanol systems

Figure 5 (a) SAED, (b) micro-Raman and (c) micro-photoluminescence spectra of C(60) crystals

本論文は、集積化学プロセスなどに用いられるマイクロ流体デバイス対して、その内部を流動する流動場の新しい計測手法を開発するとともに、マイクロ流体デバイス内部での結晶合成を行い、様々な結晶を生成するとともに、その物理的な特性に関する評価について論じたものである。本論文は4章で構成されている。

第1章では、本研究の動機と目的について述べている。集積化学プロセスなどに用いられるマイクロ流体デバイスの現状をまとめるとともに、流動場の計測手法に関する研究の現状や材料合成に関する研究の現状をまとめている。これを踏まえて、本研究で目的とする、マイクロ流体デバイスに対する流動計測手法開発と結晶合成応用の課題を示すと共に、本研究の位置づけを明確化している。

第2章は本研究で開発された3種類のマイクロ流体デバイス流動計測手法について記している。まず、マイクロ流体デバイス中の化学反応をpH分布として捕らえるため、マクロスケールの流体計測手法であるレーザー誘起蛍光法(LIF)をマイクロ流動に応用する事を提案している。その手法の開発について記した後、100μmチャネルにおける塩酸・水酸化ナトリウム水溶液による中和反応を計測し、化学反応によって拡散が大きくなる現象を定量的に明らかにしている。2番目の手法として、マイクロ粒子画像流速測定法(マイクロPIV)手法に、高解像度高速度ビデオカメラを応用することを提案し、マイクロ流動デバイス内部の速度分布変動を時系列で計測する手法を開発した。この手法を、水・油対向流の界面近傍流動挙動を計測し、その有効性を確認している。この手法は空間方向の解像度を保ったまま、時間方向への計測量の拡大するものである。3番目の手法は、2番目の手法の時間方向への拡張を空間方向(奥行き方向)へ計測に拡大し、平均量ではあるが、3次元3成分速度分布計測手法を提案している。対物レンズをピエゾで移動させることによって奥行き方向の多断面を時系列で取得し、その情報から水平方向の情報に加えて、奥行き方向の速度分布を計測する手法を提案している。本手法を95μmの円管内速度分布計測に応用し、ポアズイユフローが正しく計測できることを確認している。

第3章ではマイクロ流路の特性を生かし、微小空間におけるフラーレン(C60)の結晶合成について記している。従来、フラスコで行われている貧溶媒・富溶媒界面に析出するC(60)結晶生成をマイクロ流動場に応用し、マイクロチャネル内で結晶生成を行うことに成功している。温度や貧溶媒を変化させることで、様々な結晶を生成できる事を示し、これらをある種の相図としてまとめている。また、条件によっては、二流体の混合部からの結晶成長をする状況を実時間で観測し、二流体の速度の関数として結晶成長速度や結晶幅などの情報を得ている。さらに、得られた結晶について、ラマン分光、蛍光発光挙動などを計測することで、生成された結晶中に貧溶媒分子がトラップされている可能性について示している。

第4章は結論であり、本論文で得られた成果をまとめている。

以上のように、本論文は、マイクロ流体デバイスにおける、流体挙動計測法として、3種類の新しい計測手法を提案し、その有効性を確認するとともに、マイクロ流体デバイスを用いたフラーレン結晶合成を行い、その特性について評価を行った研究であり、システム量子工学、特にマイクロ流体工学の発展に寄与することが少なくない。よって、本論文は博士(工学)の学位請求論文として合格と認められる。