Fluid mixing enhancement through chaotic advection in mini micro channel 2

Chapter Fabrication and Experimental Testing Chapter Fabrication and Experimental Testing 4.1 Introduction on fabrication of microfluidic devices The fabrication technique is an important aspect of microfluidics technology. In some previous studies on microfluidic mixer, they were prototyped using traditional photolithography method (Liu et al., 2000; Park et al., 2004; Hong et al., 2004; Kim et al., 2005), which involves mask making, deposition, exposure, chemical etching processes, etc. Silicon wafer are usually used as the substrate. While this technique has been developed for many years and is easy to implement, it is time-consuming and expensive. In recent years, alternative fabrication methods using polymers such as polycarbonate (PC), poly(methylmethacrylate) (PMMA), and poly(dimenthylsiloxane) (PDMS), etc., have been reported (Boone et al., 2002; Kim & Xu, 2003). Polymers are increasingly used due to their good properties such as biocompatibility, and great flexibility for fabrication. Compared with silicon wafer, polymer materials are also cheaper and thus reduce the cost. Relevant fabrication methods include template imprinting (Martynova et al., 1997; Xu et al., 2000; McDonald et al., 2002), injection molding (McCormick et al., 1997), laser mask patterning (Roberts et al., 1997; Zimmer et al., 2000; Wan et al., 2005), and laser direct writing (Lade et al., 1999; Lim et al., 2003; Hauer et al., 2003). The first three methods need the fabrication of a mask, template or mold. Comparatively, the laser direct writing method is more flexible. Lasers have been used for micromachining of various materials including polymers for many years. The discussions about the laser ablation mechanism and its — 64 — Chapter Fabrication and Experimental Testing effects on the material properties can be found elsewhere (Pugmire et al., 2002; Siew et al., 2005; Moreno et al., 2006). While scanning the laser beams on the substrate surface with the laser intensity above the ablation threshold fluence, the material can be removed from the target through photochemical reaction. It may also involve photothermal effects which directly melt and evaporate the material. This process usually leaves debris on the surface which affects the quality of the cut. Among the various laser systems, nanosecond excimer lasers have been widely used for machining of microstructures below 100 µm . Femtosecond laser involves less photothermal effects; clean ablation and precision micromachining can be achieved. CO2 laser is less costly and suitable for micro-machining of polymeric materials, especially for PMMA (Klank et al., 2002; Bowden et al., 2003; Jensen et al., 2003). In contrast to photochemical ablation, CO2 laser machining mainly involves the photothermal process. Many microfluidic components have complex 3D structures. It can be realized with gray level and contour mask technique (Zimmer et al., 2000), multi-level photolithography (Anderson et al., 2000), solid-object printing (McDonald et al., 2002), etc. All these methods involve additional work of fabrication of masks or templates, and complex processing strategies. An alternative way for 3D fabrication is to bond the substrates layer by layer. When hard materials such as silicon and glass are used, adhesion failure or stress failure may occur. Comparatively, binding soft polymer materials is much easier. The bonding techniques mainly include: (1) Spinon glass bonding (SOG) that can be applied for silicon wafers (Alexe et al., 2000); (2) Laser bonding that is used to bond a transparent acrylic substrate with an opaque one (Potente et al., 1999); (3) Thermal bonding of polymer substrates in oven or boiling water (Martynova et al., 1997; Kelly & Woolley, 2003), etc. — 65 — Chapter Fabrication and Experimental Testing 4.2 Meso-scale mixer devices for preliminary testing According to the similarity principle in fluid dynamics, the flow characteristics are only affected by the Reynolds number. If the diffusion process in a chaotic mixer is so weak that it can be neglected, the flow pattern will remain invariant at the same Re, despite the dimension of the mixer. Based on this principle, the scaled-up mixer models were fabricated for preliminary experimental testing. At the same Re, they are supposed to provide reliable evaluation on the new design. In this study, all the mixer devices (including the miniature mixers as will be introduced in the following sections) were fabricated in the Singapore Institute of Manufacturing Technology (SIMTech). 4.2.1 Fabrication processes Compared with the miniature micromixer, it is easier to make meso-size models. In this study, they are fabricated with transparent PMMA (polymerthylmethacrylate) plates. Fig. 4.1 illustrates the fabrication process. A Synrad J48-2w CO2 laser (Synrad, Inc.) with a UC-2000 controller is used for cutting. As controlled by a computer, the desired patterns are transferred into the movements of the laser beam. A laser galvanometer scanner is used for scanning application. Projecting the laser beam on the surface of the PMMA plate, the polymer is evaporated and the plate is gradually cut through to form the channels and the inlet/outlet holes in different layers. Then the PMMA plates are bonded up layer by layer using acrylic glue to form the complete mixer. In this way, the channel depth of the mixer is defined by the thickness of the PMMA plate, which is 1.5 mm in the present study. In simulation, the channel depth of the mixer is 150 µm . So the fabricated models are scaled up by 10times. To reduce the size of the mixer device, the channels are bent into three segments. As an example, the picture of a TLCCM mixer is given in Fig. 4.2. — 66 — Chapter Fabrication and Experimental Testing Fig. 4.1 Fabrication of meso-size mixer models for preliminary experimental testing. Fig. 4.2 Picture of TLCCM-B made of PMMA. 4.2.2 Experimental mixing results With the optical method introduced in Section 2.4, the mixing in the channel can be directly observed and recorded. A highly viscous 98% glycerol-2% liquid food dye (red and blue) solution was used. At 23 0C, its kinematic viscosity is about ν ≈ 6.8 × 10 −4 m s −1 . Its diffusivity is estimated according to Nishijima & Oster (1960), D ≈ 0.2 × 10 −10 m s −1 . The weak diffusion will provide a stringent testing on the mixing performance. Some mixing pictures are shown below. Figures 4.3 (a) and (b), respectively, demonstrate the mixing in a simple rectangular channel and a 3D serpentine channel at Re = 1. Apparently in both the models, no mixing is achieved. For the 3D serpentine channel, it does not exhibit much difference from the planar channel. The fluid interface remains rather sharp all through the channel which is 15-mixer-unit length. Since no chaotic advection is produced, the mixing is limited by diffusion. — 67 — Chapter Fabrication and Experimental Testing These two pictures clearly suggest the difficulty of fluid mixing in the low-Re laminar regime. (a) (b) Fig. 4.3 Mixing results at Re = in: (a) a planar rectangular channel, and (b) a 3D serpentine channel. Fig. 4.4 presents the results of TLCCM-A and TLCCM-B at Re=0.01. Due to the perturbations caused by the channel geometry, the fluids are continuously subdivided into thinner and thinner stream layers through splitting and recombination process. The sub-figures show the detailed fluid distribution at different positions along the mixer. The growth of the number of fluid striations is quite clear. The ability of the TLCCM mixers to generate chaotic advection at Re of O(10-2) indicates that they not depend on the fluid inertial effects, and rapid mixing can still be achieved at extremely low Re. This is a very important feature as in microfluidic applications, Re is usually small. — 68 — Chapter Fabrication and Experimental Testing (a) Mixing in TLCCM-A. (b) Mixing in TLCCM-B. Fig. 4.4 Experimental mixing results of the TLCCM mixer at Re=0.01. — 69 — Chapter Fabrication and Experimental Testing From the pictures, the thinning rate of the fluid striation thickness γ can be estimated as a mixing index. It is defined as γ = d wc , where d is the thickness of the individual fluid striation and wc is the channel width. For model A, γ decreases from 0.5 near the inlet to about 0.2~0.3 after 2-mixer-unit length, and further drops below 0.1 after mixer units. Compared with model A, model B exhibits faster mixing. As shown in Fig. 4.4, the number of the fluid striations is nearly doubled after each mixer unit. In the first cycle, striations are observed (in the lower top-layer channel) in the figure). Then, it increases to 4, and around 30 in the 2nd, 3rd and 5th mixer unit. Correspondingly, γ decreases from 0.5 to 0.25, 0.13 and reaches around 0.03. 4.3 Miniature PMMA mixer for further confirmation Though the mixer models of meso-size have exhibited rapid mixing, at micro scales the effects of surface tension become more intense and this may cause some difference in the mixing results. So in the next step, smaller mixer models were fabricated for more convincing evidence. 4.3.1 Direct laser cutting of microchannel The same CO2 laser system was applied for fabrication of the miniature mixer for further confirmation. The microchannel to be machined is in the sub-millimeter level. To achieve a satisfactory accuracy, the laser ablation process must be implemented carefully. The wavelength of the CO2 laser is 10.6 µm . The output power is controlled through adjusting the pulse width modulation (PWM) duty cycle. The dimensions of the microchannel are affected by many parameters, such as the laser fluence, the laser scanning speed, etc. In this study, to simplify the processing — 70 — Chapter Fabrication and Experimental Testing procedures, the PWM frequency is set at kHz and the duty cycle is set at 50%. The microstructure is fabricated by raster scanning of the laser beam. The channel width can be controlled by the number of overlapping laser beams and the spacing between them. While the spacing is adjustable, it must be small enough to avoid insufficient overlapping, which may result in ridges on the bottom surface of the channel. The channel depth is controlled by the times of laser scan and the scanning speed. With suitable parameters, desirable microchannels can be achieved. (Some studies on CO2 laser processing of PMMA material is given in Appendix B.) In our study, the microchannels were fabricated using three overlapping laser beams at a spacing of 75 µm . The number of passes is and the laser scanning speed is 22 cm/s. The cross-sectional profile of a microchannel engraved with the CO2 laser is shown in Fig. 4.5(a). Fig. 4.5(b) shows the top-layer and bottom-layer microchannels of TLCCM mixer. The channel exhibits a Gaussian-like profile, rather than a rectangle as in the original design. Slight rims are also observed at the edges of the channels. These are common in CO2 laser micromachining. It suggests that striking thermal effects occurred in the process, which has caused melting and resolidification of the polymer material (Klank et al., 2002). These findings suggest that the current CO2 laser has limitations for precision fabrication. However, compared with the feature size of the microchannel which is 500~600 microns, the deformation of the channel geometry is acceptable. The following functional testing would also confirm that the deformation does not affect the mixing performance of the design. — 71 — Chapter Fabrication and Experimental Testing (a) (b.1) TLCCM-A. (b.2) TLCCM-B. Fig. 4.5 Microchannels fabricated using CO2 laser. (a) Cross-sectional profile of a microchannel. (b.1) and (b.2) show the top-layer and base-layer structure of the TLCCM mixer. 4.3.2 Thermal bonding of PMMA substrates 4.3.2.1 Solvent-assisted thermal bonding After the channels were fabricated layer by layer, the substrates need to be bonded together to form the whole mixer. As mentioned in the introduction, for polymer materials such as PMMA and PC, this can be realized using thermal bonding technique. During the process, two substrates are placed in contact and a certain pressure is applied. They are then heated to a temperature near their glass transition temperature Tg. This will cause the polymer chain near the interface to inter-diffuse and the substrates will get bonded. To date, there have been many relevant reports. — 72 — Chapter Fabrication and Experimental Testing Kelly & Woolley (2003) introduced a fast, low cost method for thermal bonding of PMMA substrates. In their study, a blank piece of polymer substrate was clamped with another one with imprinted microchannels, and then immersed in boiling water for about hour. Good bonding was achieved. Sun et al. (2006) reported a lowpressure, high-temperature thermal bonding process. A high bonding strength and good structural integrities can be achieved. In some other studies, suitable solvents including acetone (Liu et al., 2004), DMSO-methanol solution (Brown et al., 2006) and epoxy resin were used for surface treatment. The solvent will slightly solubilize the surface of the polymeric material and facilitate the inter-diffusion. In the current work, the debris and the rims on the channel edges induced by the laser machining have raised the difficulty for direct thermal bonding. It requires a higher pressure and this would cause large deformation of the microstructure. To maintain the integrity of the microchannel, we adopted a solvent-assisted thermal bonding method. The applied solvent is an acrylic glue-alcohol solution. The glue is commercially available (Dama, Singapore) and its composition is as follows: aromatic hydrocarbon 70%; fatty acid, 10%; diethanolamine salt, 10%; hexylene glycol, 5% and stabilizer triethanolamine, 5%. This glue cannot be directly used because its viscosity is too high and it may fill up the channel and cause blockage. Usually the viscosity of the adhesive material for micro-fabrication must be very low (less than 200 cps according to Kim & Xu (2003)). For pre-bonding, the acrylic glue is first diluted with alcohol at a best glue-alcohol volume ratio around 3:5. After getting the two PMMA substrates aligned with the surfaces in contact, the gap between the substrates can be filled up with a very thin layer of the solution through capillary effects. The diluted acrylic glue will slightly dissolve the PMMA surface, and keep the integrity of the microstructures. After several minutes, the substrates will be pre— 73 — Chapter Fabrication and Experimental Testing bonded. Next, the substrates are sandwiched with two pieces of glass microscope slices and clamped with common binder clips. By immersing the assembly in the boiling DI water for around hour, the PMMA substrates can be well bonded. According to our testing, during the pre-bonding and thermal bonding processes, nearly no air bubbles are trapped in the gap between the polymer substrates, and the transparency remains good. This will facilitate direct observation of the mixing. Fig. 4.6 shows the photographs of the microchannel structure after bonding, and the miniature TLCCM mixer. First half cycle Second half cycle TLCCM-A (a) Top-layer channel Bottom-layer channel TLCCM-B (b) Fig. 4.6 Photographs of the PMMA micromixers TLCCM-A and TLCCM-B; and the microstructures of the two-layer flow channels after thermal bonding. — 74 — Chapter Fabrication and Experimental Testing 4.3.2.2 Bonding quality test In order to check the performance of the current bonding method, the bonding strength was tested with the Instron Microtester (Instron Corp., USA). The samples were prepared following the same procedures as described previously. Microchannels were first fabricated on 1.5 mm-thick PMMA substrates using CO2 laser. They were then aligned and bonded together. As shown in Fig. 4.7, the bonding area is 12 mm × 40 mm and lies in the middle of the sample. The load is applied at two ends of the sample. The test length is 120 mm. Two tests were conducted and the results are shown in Fig. 4.7. Both the samples failed at around 48 kg force, with a corresponding shear stress of 0.98 MPa (load/bonding area). However, the failure was caused by the breaking up (the tensile stress is around 26 MPa when the cross section is taken as 1.5 mm × 12 mm) of the samples rather than delamination. The bonding area retains intact throughout the testing. This suggests that the bonding strength is quite high and durable. 50 Load (kgf) 40 test test 30 20 10 0.0 0.2 0.4 0.6 0.8 1.0 Extension (mm) Fig. 4.7 Bonding strength test for solvent-assisted thermal bonding. — 75 — Chapter Fabrication and Experimental Testing A leakage test was also performed. Fig. 4.8 shows the test set-up. The micromixer is first filled with water. Food dye is added for identification of any leakage. The outlet tube is then closed, and high pressure is applied from the inlet. The pressure is adjustable and its magnitude is read from a pressure gauge. The pressure starts from 1.0 bar, and after every minutes it is increased by 0.5 bar to see whether leakage occurs. Due to the limitation of current experimental facilities, a maximum pressure of 8.0 bar was applied. Three chips were tested and no leakage was observed even at the maximum pressure. This further confirmed the durability of current solvent-assisted thermal bonding. Fig. 4.8 Schematic of leakage test of thermally bonded microfluidic mixer. 4.3.3 Experimental mixing results For functional testing, the same optical method was applied. The experimental set-ups are similar with that shown in Fig. 2.8. The only difference is that the original Nikkor micro-lens was replaced by a DIN 10X micro objective lens to record the mixing pictures at a micro scale. It is coupled with the digital camera through an adapter. The same 98% glycerol-2% liquid food dye solution was applied. The mixing was recorded at the locations as indicated in Fig. 4.9(a), and they — 76 — Chapter Fabrication and Experimental Testing are presented in Fig. 4.9(b) and (c). For both the mixers, the flow rate is 0.14 ml/min, and the corresponding Reynolds number is around Re ~ 0.01. Results show that although the Gaussian-like cross-sectional profile of the channel is different from our original design which is rectangular; the mixing topology remains nearly the same. In both mixers, the fluids are continuously laminated into thinner fluid striations, and therefore a significant increase in interfacial area. This is consistent with previous observations using meso-size models. (a.1) TLCCM-A. (a.2) TLCCM-B. (b) Mixing in TLCCM-A, from left to right, the first cycles. (c) Mixing in TLCCM-B, from left to right, the 1st, 2nd, 3rd and 5th cycles. Fig. 4.9 Experimental mixing results of the TLCCM mixer at Re ~ 0.01. The dashed lines in subfigures (a.1) and (a.2) indicate the positions of the observation windows. Shadowed regions indicate the top-layer channel. — 77 — Chapter Fabrication and Experimental Testing 4.4 Miniature PDMS mixer Besides the laser fabrication of PMMA mixer, smaller mixers were fabricated with PDMS using soft-lithography technique. The fabrication process is outlined in Fig. 4.10: (a) A layer of SU-8 is first spin-coated onto a silicon substrate. (b) A mask with the channel design is created on a transparent film. Then, the negative photoresist is applied. Through the UV exposure, the illuminated SU-8 is polymerized. (c) Then, the unpolymerized SU-8 is washed away, forming a master for casting PDMS channels. (d) Next, the PDMS solution is poured over the master and it is cured at 40-80 oC for around one hour. (e) Then the PDMS is peeled off from the master. One-layer channel is obtained. (f) Similarly, the other PDMS layer containing the microchannel is fabricated. After oxygen plasma treatment, the substrates were aligned manually under microscope and then bonded together. The microphotographs of a portion of the mould and the microchannel are shown in Fig. 4.11. The depth of the channel is 50 micron, and the width of the channel is 100 micron. Fig. 4.10 Fabrication process of PDMS mixer. — 78 — Chapter Fabrication and Experimental Testing (a) (b) Fig. 4.11 Microphotographs showing a part of (a) the mould, and (b) the cast microchannel of TLCCM-B. For experimental testing, a 80% glycerol -20% food dye solution was used. The previously used 98% aqueous glycerol solution is too viscous ( 6.8 × 10 −4 m s −1 ), and it may cause delamination of the PDMS mixer. For the current solution at 23 0C, its viscosity coefficient is ν ≈ 4.81 × 10 −5 m s −1 . Its diffusivity is D ≈ 1.1 × 10 −10 m s −1 . It will still guarantee a stringent test on mixing performance. The mixing was performed at around Re ≈ 0.05. The corresponding flow rate is 0.01 ml/min. A DIN 20X micro objective lens was used to record the mixing pictures. The results are presented in Fig. 4.12. In both the mixers, the fluid striations are clearly recorded, and their distribution patterns are quite consistent with previous observations using bigger models. It is also found that, the mixing is faster than the scaled-up models as shown in Fig. 4.4. This is because: first, the PDMS mixer is much smaller. Correspondingly, the diffusion length or fluid striation thickness is narrower. Its feature size is only 30 that of the big PMMA models, as the channel width is reduced from mm to 100 µm . Second, as there is less glycerol in the current solution, the diffusion of the food dye becomes comparatively stronger. Consequently, molecular diffusion contributes more to the current mixing, blurring the material interface. — 79 — Chapter Fabrication and Experimental Testing (a) (b) Fig. 4.12 Mixing pictures of PDMS mixer at Re ≈ 0.05. (a) TLCCM-A. The dashed rectangles indicate the positions of the observation windows. (b) TLCCM-B. The sample windows are consistent with that in Fig. 4.9 (a.2). — 80 — Chapter Fabrication and Experimental Testing 4.5 Mixing test using chemical method 4.5.1 Testing method In above mixing test, highly viscous glycerol solution was used. As the diffusion of the food dye in glycerol is very weak, the fluids’ interface remains quite sharp. The thinning rate of the fluid striations can be recorded as an index of the convective mixing. In this section, the performance of the mixer was examined using a chemical method (refer to Cha et al., 2006). The glycerol solution is diluted, and diffusive mixing plays a more important role. A 11.76 WT% NaOH (Aldrich, Germany) aqueous solution and a 1% pH indicator phenolphthalein (Reagent and Fine Chemicals, UK) are used. Both of them are mixed with glycerol at a volume ratio 1:5.7. The concentration of glycerol is about 85 Vol.%. Its kinematic viscosity is ν ≈ 8.37 × 10 −5 m s −1 . Its diffusivity is D ≈ 0.9 × 10 −10 m s −1 . While the NaOH solution is alkaline, the phenolphthalein changes its color from colorless to violet red at pH range 8.3~10. As the mixing continues, the color will become deeper and deeper. The mixing pictures were recorded by a BX51 Research Microscope (Olympus) coupled with the SONY DXC390P 3CCD Camera. Based on the pixel intensity, a revised format of standard deviation (Eq.(2.3)) is calculated to quantify the mixing degree. σI = I ni = Here, N N ∑ (1 − I ni )2 (4.1a) i =1 I i − I unmix I mix − I unmix (4.1b) I ni = normalized pixel intensity; I i = pixel intensity; — 81 — Chapter Fabrication and Experimental Testing I unmix = pixel intensity before mixing; I mix = pixel intensity after complete mixing. The pixel intensities I i , I unmix and I mix are grayscale values. They are converted from the RGB pixel intensity using I = ( I red + I green + I blue ) / . (4.1c) The value of I unmix is taken near the inlet before the solutions come into contact. To obtain the value of I mix , the two solutions are first completely mixed. They are then filled into the mixer, and the pixel intensity is taken under the same working conditions as the mixing test. 4.5.2 Mixing results The mixers of meso-size as described in Section 4.2 were tested. The flow rate of the two fluids is taken as 0.1, 0.4 and 0.8 ml/min, respectively. The corresponding Reynolds number is around 0.009, 0.035 and 0.070. The results are shown in Fig. 4.13 and Fig. 4.14. When the two colorless fluids come into contact, their color changes to red. In Fig. 4.13(b.1) and Fig. 4.14(b.1), the fluids’ interfaces can be distinguished and they grow along the channel. This is consistent with the previous tests using food dye. In Fig. 4.13(b.2) and Fig. 4.14(b.2), the interface is not so clear. But the pixel intensity continuously increases along the channel, showing the progress of mixing. When the flow rate increases from 0.1 to 0.4 ml/min, the Re is still very low, but the resident time is correspondingly reduced by nearly times. As a result, the mixing becomes slower. From model A, at Q& = 0.1 ml/min, the standard deviation decreases from near 1.0 to around 0.10 after 3.5-unit length. At Q& = 0.4 and 0.8 ml/min, the — 82 — Chapter Fabrication and Experimental Testing values are respectively 0.31 and 0.43 after the same mixer length. For model B, the mixing also becomes slower with increase in the flow rate. Apparently, the current mixing attributes more to the diffusion process. Compared with model A, model B exhibits better mixing. In model A, the standard deviation remains above 0.8 at location b4 at all three flow rates. In model B, for a flow rate of 0.8 ml/min, its value is 0.61 after the same mixer length. For a flow rate of 0.1 ml/min, this value is further reduced to 0.26. This is because the stable regions as discussed in Chapter can be penetrated through diffusion, and in the current test, diffusive mixing plays a more important role. 4.6 Conclusions In this chapter, we demonstrated the fabrication and experimental testing of the TLCCM mixers. Scaled up models were first fabricated using PMMA plates for preliminary testing. Subsequently, smaller models (with feature size 300 ~ 400 µm ) were made using laser direct writing and thermal bonding technique for more evidence. Compared with photolithography techniques, the laser fabrication of polymeric microfluidic mixers is faster and cheaper. It is very flexible to allow for design changes and is useful for rapid prototyping. Miniature PDMS mixers were also fabricated using soft-lithograph technology. Compared with laser machining, this replica fabrication is more suitable for mass production. We also demonstrated a simple method to evaluate the mixing performance of a mixer. When transparent materials such as PMMA or PDMS are used, direct optical testing can be applied. It could be either dyed liquids or chemical solutions which change their color upon mixing. Relevant observations are quite consistent with each — 83 — Chapter Fabrication and Experimental Testing other and they are also in line with simulation results, further confirming the effectiveness of the current mixer design. (a) Observation windows (shadow area shows the top layer). (a1) (a2) (a3) (a4) (b.1) (b1) (b2) (b3) (b4) (b5) (b6) (b7) (b8) (b.2) (b) Mixing pictures of TLCCM-A at Q& = 0.1 ml/min. — 84 — Chapter Fabrication and Experimental Testing (a1) (a2) (a3) (a4) (c.1) (b1) (b2) (b3) (b4) (b5) (b6) (b7) (b8) (c.2) (c) Mixing pictures of TLCCM-A at Q& = 0.4 ml/min. 1.0 0.8 σI 0.6 0.4 0.1 ml/min 0.4 ml/min 0.8 ml/min 0.2 0.0 (n) Mixer length (windows bn) (d) Standard deviation σ I along mixer. Fig. 4.13 Mixing results of TLCCM-A using chemical method. — 85 — Chapter Fabrication and Experimental Testing (a) Observation windows (shadow area shows the top layer). (a1) (a2) (a3) (a4) (a5) (a6) (a7) (a8) (b.1) (b1) (b2) (b3) (b4) (b5) (b6) (b7) (b8) (b.2) (b) Mixing pictures of TLCCM-B at Q& = 0.1 ml/min. — 86 — Chapter Fabrication and Experimental Testing (a1) (a2) (a3) (a4) (a5) (a 6) (a7) (a8) (c.1) (b1) (b 2) (b 3) (b 4) (b 5) (b 6) (b 7) (b 8) (c.2) (c) Mixing pictures of TLCCM-B at Q& = 0.4 ml/min. — 87 — Chapter Fabrication and Experimental Testing 1.0 0.1 ml/min 0.4 ml/min 0.8 ml/min 0.8 σI 0.6 0.4 0.2 0.0 (n) Mixer length (windows bn) (d) Standard deviation σ I along mixer. Fig. 4.14 Mixing results of TLCCM-B using chemical method. — 88 — [...]... cross-sectional profile of the channel is different from our original design which is rectangular; the mixing topology remains nearly the same In both mixers, the fluids are continuously laminated into thinner fluid striations, and therefore a significant increase in interfacial area This is consistent with previous observations using meso-size models (a.1) TLCCM-A (a .2) TLCCM-B (b) Mixing in TLCCM-A, from left... coefficient is ν ≈ 4.81 × 10 −5 m 2 s −1 Its diffusivity is D ≈ 1.1 × 10 −10 m 2 s −1 It will still guarantee a stringent test on mixing performance The mixing was performed at around Re ≈ 0.05 The corresponding flow rate is 0.01 ml/min A DIN 20 X micro objective lens was used to record the mixing pictures The results are presented in Fig 4. 12 In both the mixers, the fluid striations are clearly recorded,... method 4.5.1 Testing method In above mixing test, highly viscous glycerol solution was used As the diffusion of the food dye in glycerol is very weak, the fluids’ interface remains quite sharp The thinning rate of the fluid striations can be recorded as an index of the convective mixing In this section, the performance of the mixer was examined using a chemical method (refer to Cha et al., 20 06) The glycerol... the current mixing, blurring the material interface — 79 — Chapter 4 Fabrication and Experimental Testing (a) (b) Fig 4. 12 Mixing pictures of PDMS mixer at Re ≈ 0.05 (a) TLCCM-A The dashed rectangles indicate the positions of the observation windows (b) TLCCM-B The sample windows are consistent with that in Fig 4.9 (a .2) — 80 — Chapter 4 Fabrication and Experimental Testing 4.5 Mixing test using chemical... TLCCM-A at Q = 0.1 ml/min — 84 — Chapter 4 Fabrication and Experimental Testing (a1) (a2) (a3) (a4) (c.1) (b1) (b2) (b3) (b4) (b5) (b6) (b7) (b8) (c .2) & (c) Mixing pictures of TLCCM-A at Q = 0.4 ml/min 1.0 0.8 σI 0.6 0.4 0.1 ml/min 0.4 ml/min 0.8 ml/min 0 .2 0.0 0 1 2 3 4 5 6 7 8 9 (n) Mixer length (windows bn) (d) Standard deviation σ I along mixer Fig 4.13 Mixing results of TLCCM-A using chemical method... thermal bonding — 74 — Chapter 4 Fabrication and Experimental Testing 4.3 .2. 2 Bonding quality test In order to check the performance of the current bonding method, the bonding strength was tested with the Instron Microtester (Instron Corp., USA) The samples were prepared following the same procedures as described previously Microchannels were first fabricated on 1.5 mm-thick PMMA substrates using CO2 laser... right, the first 3 cycles (c) Mixing in TLCCM-B, from left to right, the 1st, 2nd, 3rd and 5th cycles Fig 4.9 Experimental mixing results of the TLCCM mixer at Re ~ 0.01 The dashed lines in subfigures (a.1) and (a .2) indicate the positions of the observation windows Shadowed regions indicate the top-layer channel — 77 — Chapter 4 Fabrication and Experimental Testing 4.4 Miniature PDMS mixer Besides the... thermal bonding Fig 4.8 Schematic of leakage test of thermally bonded microfluidic mixer 4.3.3 Experimental mixing results For functional testing, the same optical method was applied The experimental set-ups are similar with that shown in Fig 2. 8 The only difference is that the original Nikkor micro- lens was replaced by a DIN 10X micro objective lens to record the mixing pictures at a micro scale It... the inlet before the solutions come into contact To obtain the value of I mix , the two solutions are first completely mixed They are then filled into the mixer, and the pixel intensity is taken under the same working conditions as the mixing test 4.5 .2 Mixing results The mixers of meso-size as described in Section 4 .2 were tested The flow rate of the two fluids is taken as 0.1, 0.4 and 0.8 ml/min,... master One-layer channel is obtained (f) Similarly, the other PDMS layer containing the microchannel is fabricated After oxygen plasma treatment, the substrates were aligned manually under microscope and then bonded together The microphotographs of a portion of the mould and the microchannel are shown in Fig 4.11 The depth of the channel is 50 micron, and the width of the channel is 100 micron Fig 4.10 . for preliminary experimental testing. Fig. 4 .2 Picture of TLCCM-B made of PMMA. 4 .2. 2 Experimental mixing results With the optical method introduced in Section 2. 4, the mixing in the channel. micro-machining of polymeric materials, especially for PMMA (Klank et al., 20 02; Bowden et al., 20 03; Jensen et al., 20 03). In contrast to photochemical ablation, CO 2 laser machining mainly involves. different from our original design which is rectangular; the mixing topology remains nearly the same. In both mixers, the fluids are continuously laminated into thinner fluid striations, and