Chaotic vortex micromixer utilizing gas pressure driving force

Chemical Engineering Journal 214 (2013) 1–7

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Chaotic vortex micromixer utilizing gas pressure driving force Lung-Ming Fu a, Wei-Jhong Ju b, Chien-Hsiung Tsai c, Hui-Hsiung Hou b, Ruey-Jen Yang b,⇑, Yao-Nan Wang c,⇑ a

Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan Department of Engineering Science, National Cheng Kung University, Tainan 701, Taiwan c Department of Vehicle Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan b

h i g h l i g h t s " A PMMA-based chaotic vortex passive micromixer is proposed. " An external gas pressure driving force is applied to the samples contained within the open chamber. " The driving force prompts the formation of a chaotic vortex structure. 2

" A gas pressure driving force of 1.2 kg/cm is sufficient to drive the sample fluids. " A reaction ratio of 99.9% is achieved within 6 s.

a r t i c l e

i n f o

Article history: Received 27 July 2012 Received in revised form 20 October 2012 Accepted 22 October 2012 Available online 30 October 2012 Keywords: Micromixer Microfluidic CO2 laser

a b s t r a c t A novel chaotic vortex micromixer is proposed comprising an open mixing chamber, a sealed mixing chamber and a serpentine microchannel. In the proposed approach, the samples are loaded into the open chamber and an external gas pressure driving force is then applied. The driving force generates a chaotic vortex structure in the open chamber and drives the samples through the serpentine channel into the sealed chamber. As the samples fill the sealed chamber, a compression reaction force is produced, which generates a second chaotic vortex structure and drives the samples back to the open chamber. The vortex structures perturb the sample streams and therefore prompt an efficient mixing of the two species. The flow phenomena within the mixing chambers are evaluated by means of numerical simulations. The numerical results are verified by performing flow visualization experiments. A good agreement is found between the two sets of results. It is shown that a reaction ratio of almost 100% is obtained within 6 s given a gas pressure driving force of 1.5 kg/cm2. Overall, the results show that the micromixer proposed in this study provides a simple solution for mixing problems in Lab-on-Chip systems. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In recent years, microfluidic devices have found widespread use in the chemical and biological fields for such applications as food safety inspection, drug discovery, clinical diagnosis, environmental monitoring, bio-separation, micro-reactions, and real time polymerase chain reaction (PCR) [1–18]. Compared with traditional analytical methods, microfluidic devices have many practical advantages, including a lower sample and reagent consumption, an improved analysis speed, a greater sensitivity, a shorter processing time, a reduced power consumption, a greater portability, lower fabrication and operating costs, and the potential for parallel ⇑ Corresponding authors. Tel.: +886 6 2757575 63343; fax: +886 6 2766549 (R.J. Yang), tel.: +886 8 7703202 7456; fax: +886 7740398 (Y.N. Wang). E-mail addresses: [email protected] (R.-J. Yang), [email protected]. edu.tw (Y.-N. Wang). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.10.032

processing and integration with other miniaturized devices. The performance of many microfluidic devices is critically dependent on achieving an efficient mixing of the various samples and reagents. Existing mixing schemes can be broadly categorized as either active or passive, depending on the manner in which the species are mixed. In active microfluidic mixers, an enhanced mixing performance is achieved by disturbing the fluid flow using some form of external driving field or energy source. In such mixers, the species are typically mixed by electrokinetic instabilities, pressure perturbations, acoustic/ultrasonic disturbances, small impellers, dielectrophoretic electro-hydrodynamic flows, magnetic or thermal disturbances, and so on [19–31]. Sun and Sie [19] conducted an experimental investigation into the mixing phenomena within a divergent microchannel perturbed by a custom-built dynamic pressure generator. The results showed that the optimal mixing efficiency was obtained given a large half-angle of the divergent

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section and a phase difference of 0.25–0.5p in the two actuating pressures. Lim and Lam [20] examined the mixing phenomena within a constriction microchannel containing time periodic electroosmotic flow. The effects on the mixing performance of the fluid interface oscillation amplitude and the contact length between the two fluid streams were quantified both experimentally and numerically. In addition, the numerical results were used to identify the values of the AC driving amplitude and frequency which optimized the mixing efficiency. Wen et al. [21,22] developed and characterized an active microfluidic mixer in which the species (DI water and an aqueous ferrofluid) were perturbed by an electromagnet driven by an AC or DC power source. Campisi et al. [23] proposed a micromixer in which the species were electrokinetically displaced by generating rolls through AC electroosmosis on co-planar electrodes. The experimental results showed that a good mixing performance was obtained at AC frequencies in the range of 10– 100 kHz and peak-to-peak voltages of 15–20 V. Wang et al. [25] performed a numerical investigation into the mixing performance of a magnetic particle driven micromixer. The simulations focused specifically on the effects of the magnetic actuation force, the switching frequency and the lateral dimension of the channel. The results showed that the optimal mixing performance was obtained at a relatively high operating frequency for large magnetic actuation forces and narrow microchannels. Xu et al. [26] examined the thermal mixing characteristics of two miscible fluids in a T-shaped microchannel. It was shown that sample mixing in the T-junction region of the channel was the result mainly of thermal diffusion, while sample mixing in the mixing channel occurred as a result of both thermal diffusion and convection. In passive micromixers, a mixing effect is achieved by increasing the contact area and contact time of the species samples by means of specially-designed microchannel configurations. Passive micromixers contain no moving parts and require no energy input other than the pressure head used to drive the fluid through the channel. In such devices, mixing is typically achieved by means of three-dimensional serpentine structures, embedded barriers, twisted channels, surface-chemistry modification, and so on [32– 50]. Tofteberg et al. [32] developed a passive micromixer in which the species were repeatedly split, rotated and then re-combined until the desired degree of mixing was achieved. The initial results suggested that the optimal mixing performance could be obtained by separating the stream into three or four substreams. Chang et al. [34] examined the driving characteristics of an electrowetting-ondielectric (EWOD) device with aluminum oxide as the dielectric layer. It was shown that the threshold voltage required to drive a 2-ll water droplet was just 3 V. Moon and Migler [36] presented a planar polymer micromixer (PPMM) in which molten polymer was driven through mixing chambers fabricated from metal shims. The results showed that complex 3D mixing flows could be generated via an appropriate stacking of the shims. Moreover, it was shown that the minimal sample size was less than 100 mg, i.e., significantly lower than that of traditional micromixers. In recent years, many researchers have demonstrated the feasibility of integrating micromixers with other microfluidic devices in order to realize Lab-on-Chip (LoC) systems for chemical and biological analysis applications [51–63]. Yeh et al. [51] presented a gradient-generating microfluidic chip comprising a microfluidic network and micromixers for establishing a suitable glucose concentration for endothelial cells (ECs). Hong et al. [52] fabricated an integrated microfluidic chip comprising mixing and reaction columns for methanol detection purposes using a novel continuous wave unfocused CO2 laser ablation technique. It was shown that a reaction time of just 30 min was sufficient to complete the methanol detection process. Hou et al. [53] presented a disposable microfluidic device for glucose concentration detection applications comprising a double parallel connection micromixer (DPCM), a

T-type microchannel and a collection chamber. The numerical results indicated that a mixing efficiency of as much as 92.5% could be obtained at low Reynolds numbers (e.g. Re = 12). Wang et al. [54] presented an integrated microfluidic system for counting the CD4+/CD8+ T lymphocytes in a whole blood sample. In the proposed device, the target cells were labeled fluorescently in a vortex-type micromixer and were then counted in a downstream cytometer using a conventional laser-induced fluorescence (LIF) technique. Lin et al. [55,56] presented an integrated microfluidic chip for rapid DNA digestion and time-resolved capillary electrophoresis (CE) analysis. The chip comprised two gel-filled chambers for DNA enrichment and purification, a microfluidic mixer for DNA/ restriction enzyme mixing, a serpentine channel for DNA digestion reaction, and a CE channel for on-line CE analysis. The time-resolved electropherograms showed that the device was capable of concentrating and analyzing Ux-174 DNA samples comprising 11 fragments within 24 min. This paper proposes a rapid passive micromixer in which the species are mixed under the combined effects of an external gas pressure driving force and an internal compression reaction force. In the proposed approach, the species are injected into an open circular chamber and are then pushed through a serpentine channel to a sealed chamber via the application of an external gas pressure pulse. The introduction of the species into the sealed chamber generates a compression reaction force, which pushes the species back toward the open chamber. The external and internal driving forces create vortex structures within the two chambers and prompt an efficient mixing of the two species as a result. The transient mixing phenomena within the open and sealed chambers are examined both numerically and experimentally. Moreover, the mixing efficiency of the proposed device is characterized as a function of the sample viscosity by evaluating the concentration distribution within the two chambers using an image-processing technique.

2. Fabrication and experimental details Fig. 1a and b presents photographs showing the overview and side view of the proposed micromixer, respectively. As shown in Fig. 1c, the mixer comprised a PMMA substrate with a thickness of 6.0 mm sandwiched between two PMMA substrates with a thickness of 1.6 mm. The central substrate was drilled with two via holes with a diameter of 4 mm, while the upper substrate was drilled with a single via hole; also with a diameter of 4 mm. Finally, the lower substrate was patterned with a serpentine channel with a width of 250 lm and a depth of 200 lm in order to connect the two chambers in the middle substrate. The PMMA substrates were patterned using a continuous mode VII-12 CO2 laser system (Giant Technologies Incorporated) with a maximum power output of 12 W and a wavelength of 10,500 nm. As shown in Fig. 2, the PMMA substrates were fixed individually to a stationary working table and the required features (e.g., ports, chambers and serpentine microchannel) were produced by moving the laser source over the substrate surface in accordance with the corresponding AutoCad design. In every case, the ablation process was performed using an output power and laser travel speed of 4.8 W and 30 mm/s, respectively (note that a full description of the ablation process is available in a previous study by the current group [52]). Following the ablation process, the substrates were carefully aligned and sealed using a hot-press bonding technique. Briefly, the substrates were sandwiched between two thick glass sheets and a light contact pressure was applied as the temperature was increased to 100 °C over a period of 10 min. The pressure was then increased to 5 kg/cm2 for a further 10 min with no change in the temperature. Finally, the chip was cooled to room temperature over a period of 10 min.

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Open chamber

φ4

(c)

1.6

First layer

φ4 6

Sealed chamber

Second layer 1.6 Third layer

Unit: mm

Fig. 1. Photographs of chaotic vortex micromixer: (a) overview and (b) side view. (c) Schematic illustration showing configuration of each PMMA substrate in three-layered structure.

Mirror

Mirror

CO2 laser

X-Stage

Mirror

Computer Focusing-Lens

Substrate

Y-Stage Z-Platform

high-precision pneumatic controller pressure (range: 0.5–10 kg/ cm2). The mixing experiments were performed using iodine buffer solutions with viscosities ranging from 5 to 60 cP and DI buffer solutions containing 2 wt.% sodium hydroxide. The chemical reaction between the two samples was follows: 3I2 þ 6OH ! 5I þIO 3 þ 3H2 O. The iodine solutions were brown while the DI-based sample was colorless. Optical images of the mixing process were captured using a CMOS camera (HS10, Fujifilm, Japan) and were then analyzed using a digital imaging technique in order to evaluate the mixing efficiency within the two chambers. 4. Numerical simulations

Fig. 2. Schematic illustration showing experimental setup used to pattern PMMA substrates using CO2 laser system.

3. Materials and methods Fig. 3 presents a schematic illustration of the experimental setup used to evaluate the performance of the proposed micromixer. The driving force required to drive the species from the open chamber to the sealed chamber was generated by regulating the gas pressure developed by an air compressor using a self-built

Mixing Chip Chip Holder

Pneumatic controller Lens

Computer

CMOS Camera

In the present study, there are two kinds of liquid (iodine buffer and 2% sodium hydroxide) in the tanks prepared for mixing with air on the top of liquid surface. So, the flow field inside the present mixer is governed by the incompressible two-phase Navier–Stokes equations. The governing conservation equations of mass, momentum and species are written respectively as:

@ aq qq v qÞ ¼ 0 þ r  ðaq qq~ @t

where aq denotes volume fraction of phase q, qq is the q phase density, ~ v q is velocity of phase q. A single momentum equation is solved throughout the domain, and the resulting velocity field is shared among the phases. The momentum equation, shown below, is dependent on the volume fractions of all phases through the properties q and l.

@ q~ v v~ v Þ ¼ rp þ r  ½lðr~ v þ r~ v T Þ þ q~g þ r  ðq~ @t

ð2Þ

@ qC v CÞ ¼ r  ðqDrCÞ þ r  ðq~ @t

ð3Þ

where p denotes the static pressure, D is the binary diffusion coefficient for the binary mixture of iodine buffer and sodium hydroxide and C is the concentration of iodine buffer. In this calculation, the value of D is 1  1010 m2 s1. For a multi-phase system, the volume-fraction-averaged density and viscosity takes on the following forms:

q ¼ Rqq aq Fig. 3. Schematic illustration showing experimental setup used to evaluate performance of proposed micromixer.

ð1Þ

l ¼ Rl q a q

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Red-line liquid Blue-line air

High pressure gas

(a) Step 1

scheme is recommended by Hardt and Schonfeld [65] in order to minimize the numerical diffusion. In order to provide good resolution, the computational domain is discretized with structure hexahedral meshes, the cells near the wall having sides of 5 lm length. To ensure grid independence, the numbers of grids are 1,100,000. 5. Results and discussion

Sealed chamber Open chamber Inner compressed gas

(b) Step 2

Sealed chamber Open chamber Fig. 4. Three-dimensional flow streamlines within open and sealed chambers in (a) step 1 and (b) step 2 of mixing process. Note that gas pressure driving force is 1.5 kg/cm2.

Since the flow involves two different fluids, the Volume of Fluid (VOF) model is utilized to solve a single set of momentum equations and tracking the volume fraction of each of the fluids throughout the domain. The governing equations are cast into a set of finite difference equations by the third-order QUICK scheme. The technique employed in the present study is the finite volume method. The SIMPLEC algorithm [64] is then used to solve the resulting difference equations. The adoption of the QUICK

Most passive microfluidic mixers enhance the mixing performance by using complex geometry microchannels to increase the contact area and contact time between the two sample fluids. In such devices, mixing is accomplished via diffusive or geometry effects, and thus the mixing channel must have an extended length or special shape in order to achieve a satisfactory mixing result. However, in the device proposed in the present study, the mixing performance is enhanced by using an external gas pressure driving force and an internal compression reaction force to induce 3D chaotic vortex structures within the two circular chambers. In a preliminary investigation, it was found that the critical value of the gas pressure driving force required to drive the samples from the open chamber to the sealed chamber was equal to 1.2 kg/cm2. Therefore, to achieve a satisfactory mixing performance, the present experiments were performed using a gas pressure driving force of 1.5 kg/cm2. The performance of the micromixer was evaluated both numerically and experimentally. Fig. 4 shows the 3D flow streamline distributions within the two circular chambers in the two steps of the mixing process. In step 1, the external gas pressure driving force is applied to the two samples within the open chamber; causing them to be driven through the serpentine microchannel into the sealed chamber. As shown in Fig. 4a, the external driving force prompts the formation of a chaotic vortex as a result of the

Fig. 5. Experimental and numerical results for flow vortex structures in (a) sealed chamber in step 1 and (b) open chamber in step 2. Note that gas pressure driving force is 1.5 kg/cm2.

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Fig. 6. Experimental flow images at various times in interval of 0–30 min given no gas pressure driving force at the open chamber.

self-rotation effect. As the species enter the sealed chamber, a compression reaction force is induced, and thus the species are pushed back toward the open chamber. Fig. 4b shows that the compression reaction force induces a second chaotic vortex within the open chamber. Fig. 5 presents the experimental and numerical results obtained for the flow vortices formed in the two chambers of the micromixer. Note that in capturing the experimental images shown in Fig. 5, the sample comprised DI water containing fluorescent polystyrene beads (2 lm, Duke scientific, USA) and the vortex structures were imaged using a CMOS camera positioned beneath the microfluidic chip. It is seen that a good agreement exists between the experimental and numerical results. Thus, the validity of the numerical model is confirmed. Both sets of results confirm the

Side view

Bottom view

(a)

formation of large vortex structures within the two circular chambers during the two-step mixing process. Fig. 6 shows experimental flow images of the two circular chambers at various times in the interval of t = 0–30 min following the injection of 35 ll of 2 wt.% sodium hydroxide buffer into the open chamber followed by the injection of 35 ll of iodine buffer (viscosity 60 cP). In this case, no gas pressure driving force was applied, and thus species mixing occurred as a result of diffusion alone. Consequently, a time of 30 min was required to achieve a complete reaction (as indicated by the formation of a totally clear solution) Fig. 7 (Movie 1) presents the experimental flow images captured at various times in the interval of 0–4 s following the injection of 35 ll of 2 wt.% sodium hydroxide buffer and 35 ll iodine buffer (viscosity 60 cP) and the application of a gas pressure driving force of 1.5 kg/cm2 (Fig. 7a). As shown in Fig. 7b and c, chaotic vortex structures are formed in the open and sealed chambers as a result of the gas pressure driving force and compression reaction force, respectively. In Fig. 7c, the chemical reaction between the two samples is around 80% complete. To achieve a full reaction, a second gas pressure driving pulse was applied to the open chamber. As shown in Fig. 7d and e, the sample mixture is almost entirely clear. In other words, a full reaction between the two samples is achieved. Fig. 8 compares the experimental and numerical results obtained for the species concentration distributions in (a) sealed chamber in step 1 at 1.5 s and (b) open chamber in step 2 at 6 s with the gas pressure driving force is 1.5 kg/cm2. It is seen that the experimental and simulated flow patterns are also in good qualitative agreement. The mixing performance of the proposed micromixer was evaluated by analyzing the experimental flow images using a digital image processing technique. The captured color images were converted into gray-scale images, in which the gray scale values were assumed to represent the concentration levels of the iodine buffer (brown) and I, IO 3 and H2O reactants (colorless), respectively. Specifically, the gray-scale value for the iodine buffer was set to 1, while that for the I, IO 3 and H2 O reactants was set to 0. Furthermore, a gray-scale value of less than 0.05 was taken to indicate a full reaction between the two samples. The degree of reaction

Experimental

Numerical

(b)

(c)

(a) (d)

(e)

(b) Fig. 7. Experimental flow images at various times in interval of 0–6 s given gas pressure driving force of 1.5 kg/cm2.

Fig. 8. Experimental and numerical results for species concentration distributions in (a) sealed chamber in step 1 at 1.5 s and (b) open chamber in step 2 at 6 s. Note that gas pressure driving force is 1.5 kg/cm2.

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Acknowledgement The financial support provided to this study by the National Science Council of Taiwan under Grant.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2012.10.032. References

Fig. 9. Variation of reaction ratio with mixing time as function of sample viscosity. Note that gas pressure driving force is 1.5 kg/cm2.

(i.e., the reaction ratio) at any moment in the mixing process was evaluated as follows:



r¼ 1

R A

 f ðCÞdA  100%; A

where f ðCÞ ¼



0; C 6 0:05 1; C > 0:05 ð4Þ

where C is the species concentration profile across the width of the mixing chamber and A is the mixing area of the chamber. It is noted that the reaction ratio was quantified by computing the average reaction ratio over an area of 6 mm  4 mm (3000  2000 pixels). Fig. 9 shows the numerical and experimental results for the variation of the reaction ratio over time as a function of the iodine sample viscosity (note that a single gas pressure driving pulse (1.5 kg/cm2) was applied). It is seen that for the sample with a viscosity of 60 cP, the reaction ratio exceeds 99.9% within 6 s. Moreover, it is observed that the mixing performance improves as the viscosity of the iodine buffer reduces. 6. Conclusion This study has presented a PMMA-based chaotic vortex passive micromixer comprising two mixing chambers (one open and one sealed) and an interconnecting serpentine microchannel. In the proposed device, an external gas pressure driving force is applied to the samples contained within the open chamber. The driving force prompts the formation of a chaotic vortex structure in the open chamber and causes the samples to flow through the serpentine channel into the sealed chamber. As the samples fill the sealed chamber, a compression reaction force is produced, which induces a second chaotic vortex structure and drives the samples back through the serpentine channel toward the open chamber. The vortex structures in the two chambers perturb the sample streams, and thus an efficient mixing of the two species occurs. The mixing mechanisms within the micromixer have been investigated both numerically and experimentally. The results have shown that a gas pressure driving force of 1.2 kg/cm2 is sufficient to drive the sample fluids from the open chamber to the sealed chamber. Moreover, it has been shown that given a gas driving pressure of 1.5 kg/ cm2 and an iodine buffer viscosity of 60 cP, a reaction ratio of 99.9% is achieved within 6 s. Overall, the results presented in this study provide a valuable contribution to the ongoing development of micro-total-analysis systems.

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