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Optical measurement of flow field and concentration field inside a moving nanoliter droplet
Sensors and Actuators A 133 (2007) 317–322
Optical measurement of flow field and concentration field inside a moving nanoliter droplet Cheng Wang, Nam-Trung Nguyen, Teck Neng Wong ∗ School of Mechanical and Aerospace Engineering, Nanyang Technological University, Nanyang Avenue 50, Singapore 639798, Singapore Received 8 July 2005; received in revised form 21 March 2006; accepted 15 May 2006 Available online 25 July 2006
Abstract Droplets-based method has been employed to enhance mixing in microfluidic systems. This paper presents experimental studies of the recirculating flow field inside a moving droplet and the characterization of the mixing of two aqueous droplets. In the first part, the velocity field inside the moving water droplet was measured using the micro-particle image velocimetry (micro-PIV) technique. The PIV measurements showed that recirculation flow exists inside the droplet. However, the findings suggested that the outer layer of droplets move at a faster velocity than the central part. The result is different from what is reported by other researchers. In the second part, two water droplets, a de-ionized (DI) water droplet and another DI water droplet with fluorescent dye, were brought together by the carrier fluid to form a bigger droplet. The mixing between the two aqueous droplets was characterized by the fluorescent dye concentration distribution. © 2006 Elsevier B.V. All rights reserved. Keywords: Droplet mixing; Micro-PIV; Fluorescence imaging
1. Introduction In microfluidic systems for biochemical and chemical applications, mixing is a very important process. Due to small Reynolds numbers, mixing of fluids in microscale is a very challenging task. Many studies have been carried out to promote mixing performance in microfluidics, methods such as using hydrodynamic focusing and time-interleaved segmentation [1], electrokinetic effect [2,3], and patterning the microchannel surface [4,5]. Different types of micromixers can be found in the review article by Nguyen and Wu [6]. Droplet mixing is one of the effective methods to enhance mixing behavior [7,8]. The basic idea is to form aqueous droplets in an immiscible carrier fluid, such as oil. At a high flow rate, the oil flow breaks the aqueous stream into single discrete droplets. Being moved away by the carrier fluid, the droplets experience a self-recirculation flow due to the shear interaction with the carrier fluid. Also at the corner turn, the droplet can stretch. Stretching and recirculation inside the droplets promote advection and consequently improve the mixing inside the droplet. ∗
Corresponding author. Tel.: +65 67905877; fax: +65 67911895. E-mail address: [email protected] (T.N. Wong).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.06.026
The mixing between two liquids can be characterized by using dye method to record the color change due to mixing, such as one liquid with dye and another colorless liquid [9], or two liquids with different color dye [10]. Another method is to use to liquid solutions that can react and produce another color [11,12]. Fluorescent dye method relating the light intensity to concentration of dye is often used as well [13,14]. Some temperature sensitive fluorescent dye can be used to characterize the temperature field [15,16]. This paper presents an experimental study of the recirculating flow field inside a moving nanoliter droplet and the characterization of mixing performance of two merging aqueous droplets. In the first part of the experiment, the velocity field inside a moving water droplet was measured using the micro-particle micro-PIV technique. A microfluidic device has been designed to study the mixing process during the coalescence of two aqueous droplets by using fluorescent dye method [14]. 2. Concept A T-shaped microfluidic device with bending corners was fabricated to study the recirculation flow field inside a moving droplet. The schematic is shown in Fig. 1. Oil with a viscosity of
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C. Wang et al. / Sensors and Actuators A 133 (2007) 317–322
Fig. 1. Schematic of a T-shaped microfluidic device. Fig. 3. The fabricated device used in our experiment.
Fig. 2. Schematic of the microfluidic device for studying the coalescence and mixing of two droplets.
6.52 × 10−2 Pa s and DI water with a viscosity of approximately 10−3 Pa s are introduced from the two inlets to form droplets. Tracing particles are diluted in DI water for the purpose of microPIV measurement. To study the mixing effect during the process of coalesce of two water droplet, a second microfluidic device was designed. The schematic of this device is shown in Fig. 2. Two aqueous droplets, one being pure water while the other being water with dye, form and meet at the downstream junction. Once the two droplets merge to a big droplet, the dye concentration field is measured at different time instances to study the mixing pattern. 3. Experiment 3.1. Experimental setup The microfluidic devices used in our experiment were fabricated using the polymeric lamination technique, shown in Fig. 3. This technique provides a fast way of prototyping. Polymer sheets commercially available as lamination porches are
cut using a CO2 laser machining system. The channels and other structures were designed on a CAD program and transferred on the polymer sheets by a laser cutting machine. We used the Universal M-300 Laser Platform (Universal Laser Systems Inc.) with a maximum CO2 -laser power of 25 W and a maximum beam speed of about 640 mm/s. The lamination process was carried out using a commercial hot laminator (Aurora LM-450HC, laminating temperature of 120 ◦ C, laminating speed of 0.3 m/min, and maximum laminating thickness of 600 m). Tubing connections are made to provide interface to a syringe pump (Cole-Parmer 74900-05, 0.2–500 mL/h, accuracy of 0.5%). A schematic of the experimental measurement setup is shown in Fig. 4. The setup can be used for both fluorescence imaging and micro-PIV measurement. It consists of four main components: an illumination system, an optical system, a coupled charger device (CCD) camera, and a control system. The control system consisting of a peripheral component interface (PCI) card, and its corresponding software, is implemented in a personal computer (PC). The PC can control and synchronize all actions related to illumination and image recording. Two different light sources were used for the measurements. For the fluorescence imaging measurement, a single mercury lamp was used for illumination. Because of the ability of precise timing and intensity control, a laser beam was used for the micro-PIV measurement. In our system, a double pulsed Qswitched (quality switched) Nd:YAG laser was used. By including a Q-switch inside the cavity the laser can work in a triggered mode. The microchannel in flow field measurement has a cross section of 100 m × 1000 m. The flow field inside the water droplet was measured by micro-PIV technique. In our experiments, Duke red particles (3 m, Duke scientific Co.) were used as the seeding particles. The particles have a maximum excitation wavelength of 540 nm (green, very close to the characteristic wavelength of Nd:YAG) and a maximum emission wavelength of 610 nm (red). The PIV-measurement uses an epifluorescent attachment of type Nikon G-2E/C (excitation filter for 540 nm, dichroic mirror for 565 nm and an emission filter for 605 nm). Both filters in the attachment have a bandwidth of
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Fig. 4. Schematic of the experimental setup for fluorescence imaging measurements.
25 nm. The measurement reported in this experiment was carried out with a 4× objective lens. With a CCD sensor size of 6.3 mm × 4.8 mm, the size of an image pixel is 2.475 m and the size of the measured area is 1584 m × 1188 m. After the images were recorded by the CCD camera, commercial software PIVview (PIVTEC GmbH, Germany) was used to evaluate the velocity field. 3.2. Measurement of recirculation in a single droplet A typical result of the velocity field inside the droplet in a straight channel is shown in Fig. 5. First, two images of the
droplet with particles are recorded (Fig. 5a). Next, the displacement of the particles and consequently the velocity field seen by an external observer can be evaluated (Fig. 5b). For the case where total flow rate is 1100 L/h, the average velocity of the carrier liquid is 2.78 mm/s and the average velocity of the droplet is 1.98 mm/s. The velocity of the out layer of the droplet is 2.8 mm/s, larger than the velocity at the center, 1.65 mm/s. Finally, the mean velocity of the droplet is subtracted (Fig. 5c). After subtracting the mean velocity, recirculation flow field can be clearly seen in Fig. 5c. However, the recirculation field is opposite from what is reported by other researchers [7]. In their study, they believed
Fig. 5. Micro-PIV measurement of the velocity field inside an aqueous moving droplet (channel size = 100 m × 1000 m, total flow rate = 1100 L/h, flow rate ratio = 10:1. (a) Original image, (b) velocity field and (c) velocity field after subtracting velocity of the droplet.
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Fig. 6. Maximum velocity difference inside the droplet for different flow rates.
that the center part of the droplet flows forward while the edge flows backwards. The difference might be due to the size of the droplet. In our experiment, the droplet size is less than the channel width. The droplet represents an obstacle or a big particle suspended in the carrier fluid. Thin film of carrier fluid flows between the outer layer of the droplet and the channel wall at higher velocity. This higher velocity will drag the outer layer of the droplet to move forward through the shear stress interaction. For a given mixing time, the stronger the recirculation is, the better the mixing can achieve. Inside the droplet, the recirculation can be characterized by the maximum velocity difference. The maximum velocity difference inside the droplet at different total flow rate is shown in Fig. 6. As the total flow rate increases, the maximum velocity difference increases. That means the recirculation is stronger. At the 90◦ bending corner, the deformation of the droplet’s shape can be clearly observed (Fig. 7). The droplet was stretched by the shear stress of the carrier fluid. The stretching of the droplet and recirculation flow field inside the droplet enhance the mixing process. From this experiment, the droplet size can also be determined. At a constant flow ratio, the size of the droplet depends on the flow rate. The flow ratio is defined as the flow ratio between oil and water phase. Fig. 8 shows that the droplet size decreases as the flow rate increases. Furthermore, the formation of droplets was found to be more rapid at a higher flow rate. When the droplet size decreases, the time required for mixing decreases, because the diffusion length is shorter.
Fig. 8. Droplet size for different flow rate and flow rate ratio.
3.3. Characterization of droplet mixing A microchannel with a cross section of 100 m × 600 m was used for mixing characterization. Fluorescent dye (fluorescein disodium salt C20 H10 Na2 O5 , also called Acid Yellow 73) was added into one of the water streams to form droplets with fluorescent dye. The pure water droplet and the droplet with dye travel along the microfluidic network. The coalescence of the droplets was observed and recorded as a video file for later analysis. When the fluoresein was illuminated by a mercury lamp, CCD camera (Nikon B-2A) was used to take the image frames. The system is able to take images at 30 frames per second. The exposure time of each frame was 1.2 ms. A program written in MATLAB was used to evaluate the light intensity distribution [14]. A proportional relationship is assumed between the light intensity and the concentration of the fluorescent dye. The coalescence and the concentration distribution across the droplets are shown in Fig. 9. Once the two droplets merge, the front droplet is slowed down. Complete mixing can be achieved in 1/6 s, (5 frames’ time, video frequency 30 Hz). The time required for complete mixing at total flow rate 220 L/h is 8 frames’ time, 4/15 s. The reason could be explained as below. The time of complete mixing depends on the droplet size and recirculation field inside the droplet. The smaller the droplet size is, the shorter the diffusion path is. Therefore, less time is taken to reach complete mixing. The stronger the recirculation field is, the less time is required to achieve complete mixing.
Fig. 7. The stretching of the droplet at the bending corner.
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Fig. 9. Concentration field of the fluorescent dye during the coalescence of two droplets (channel size = 100 m × 600 m), total flow rate = 330 L/h, flow rate ratio = 20:1:1.
4. Conclusions
References
In this paper, we presented the optical measurement of the flow field in a moving droplet and concentration field during the coalescence of two droplets. The experimental results showed that the recirculation inside the droplet is different from what is reported in the literature [8]. The underlying reason could be the size of the droplet. The recirculation field depends on the droplet size relative to the microchannel width. Stretching of droplets at the bending corner was observed. The stretching and recirculation make the mixing more efficient. The mixing behavior between two droplets was measured quantitatively. It is found that the droplet mixing depends on the droplet size and recirculation inside the droplet. The droplet size and recirculation depend on the flow rate and flow rate ratio between the liquids. The present study allows a better understanding of mixing mechanism of droplet based mixing and helps us to design droplet mixer for “lab-on-a-chip” applications.
[1] N.T. Nguyen, X.Y. Huang, Mixing in microchannels based on hydrodynamic focusing and time-interleaved segmentation: modelling and experiment, Lab on a Chip 5 (2) (2005) 1320–1326. [2] E. Biddiss, D. Erickson, D.Q. Li, Heterogeneous surface charge enhanced micromixing for electrokinetic flows, Anal. Chem. 76 (11) (2004) 3208–3213. [3] C. Chang, R. Yang, Computational analysis of electrokinetically driven flow mixing in microchannels with patterned blocks, J. Micromech. Microeng. 14 (4) (2004) 550–558. [4] S. Hu, X. Ren, M. Bachman, C. Sims, G. Li, N. Allbritton, Surface-directed, graft polymerization within microfluidic channels, Anal. Chem. 76 (7) (2004) 1865–1870. [5] A. Stroock, G. Whitesides, Controlling flows in microchannels with patterned surface charge and topography, Acc. Chem. Res. 36 (8) (2003) 597–604. [6] N.T. Nguyen, Z. Wu, Micromixers—a review, J. Micromech. Microeng. 15 (2) (2005) R1–R16. [7] H. Song, M.R. Bringer, J.D. Tice, C.J. Gerdts, R.F. Ismagilov, Experimental test of scaling of mixing by chaotic advection in droplets moving through microfluidic channels, Appl. Phys. Lett. 83 (22) (2003) 4664– 4666. [8] M.R. Bringer, C.J. Gerdts, H. Song, J.D. Tice, R.F. Ismagilov, Microfluidic systems for chemical kinetics that rely on chaotic mixing in droplets, Phil. Trans. R. Soc. Lond. A 362 (2004) 1087–1104. [9] S.H. Wong, M.C.L. Ward, C.W. Wharton, Micro T-mixer as a rapid mixing micromixer, Sens. Actuators B Chem. 100 (3) (2004) 365–385. [10] S.Y.M. Xia, H.M. Wan, C. Shu, Y.T. Chew, Chaotic micromixers using twolayer crossing channels to exhibit fast mixing at low Reynolds numbers, Lab on a Chip 5 (2005) 748–755. [11] R.F. Ismagilov, A.D. Stroock, P.J.A. Kenis, G. Whitesides, H.A. Stone, Experimental and theoretical scaling laws for transverse diffusive broad-
Acknowledgments This work was partially supported by the academic research fund of the Ministry of Education Singapore, contract number RG11/02. Cheng Wang gratefully acknowledges the Master of Engineering scholarship from the Nanyang Technological University.
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Biographies Cheng Wang received his BE (first class honors) degree in Mechanical Engineering from the Nanyang Technological University, Singapore in 2004. He studied for the Master of Engineering degree in the School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore from 2004 to 2006, in the area of the two-fluid flow in microfluidics for diffusionbased biomedical applications. He is now working in the Micromachines Center,
School of Mechanical and Aerospace Engineering of Nanyang Technological University as a research staff, with focus on the fabrication of photonic crystals. Nam-Trung Nguyen was born in Hanoi, Vietnam, in 1970. He received his Dip -Ing, Dr Ing and Dr Ing Habil degrees from Chemnitz University of Technology, Germany, in 1993, 1997, and 2004, respectively. In 1998, he worked as a postdoctoral research engineer in the Berkeley Sensor and Actuator center (UC Berkeley, USA). Currently, he is an associate professor with the School of Mechanical and Aerospace Engineering of the Nanyang Technological University in Singapore. His research is focused on microfluidics and instrumentation for biomedical applications. He published a number of research papers on microfluidics. His recent book “Fundamentals and Applications of Microfluidics” co-authored with S. Wereley was published in October 2002. The second edition of this book with updated contents was published in 2006. Teck Neng Wong received his PhD in Mechanical Engineering from University of Strathclyde, UK in 1990. He is an associate professor in School of Mechanical and Aerospace Engineering, Nanyang Technological University. His research interests include two phase flow and heat transfer, flow boiling phenomena, two-phase flow in evaporator, condenser and capillary tube expansion devices, heat driven pump and pulsating heat pipe for electronic cooling; microscale heat transfer and fluid flow, two-fluid electroosmotic pump for non-conducting fluid, and electroosmotic control of the interface between two fluids in microchannels and flow switching in microfluidic devices.
Optical measurement of flow field and concentration field inside a moving nanoliter droplet Cheng Wang, Nam-Trung Nguyen, Teck Neng Wong ∗ School of Mechanical and Aerospace Engineering, Nanyang Technological University, Nanyang Avenue 50, Singapore 639798, Singapore Received 8 July 2005; received in revised form 21 March 2006; accepted 15 May 2006 Available online 25 July 2006
Abstract Droplets-based method has been employed to enhance mixing in microfluidic systems. This paper presents experimental studies of the recirculating flow field inside a moving droplet and the characterization of the mixing of two aqueous droplets. In the first part, the velocity field inside the moving water droplet was measured using the micro-particle image velocimetry (micro-PIV) technique. The PIV measurements showed that recirculation flow exists inside the droplet. However, the findings suggested that the outer layer of droplets move at a faster velocity than the central part. The result is different from what is reported by other researchers. In the second part, two water droplets, a de-ionized (DI) water droplet and another DI water droplet with fluorescent dye, were brought together by the carrier fluid to form a bigger droplet. The mixing between the two aqueous droplets was characterized by the fluorescent dye concentration distribution. © 2006 Elsevier B.V. All rights reserved. Keywords: Droplet mixing; Micro-PIV; Fluorescence imaging
1. Introduction In microfluidic systems for biochemical and chemical applications, mixing is a very important process. Due to small Reynolds numbers, mixing of fluids in microscale is a very challenging task. Many studies have been carried out to promote mixing performance in microfluidics, methods such as using hydrodynamic focusing and time-interleaved segmentation [1], electrokinetic effect [2,3], and patterning the microchannel surface [4,5]. Different types of micromixers can be found in the review article by Nguyen and Wu [6]. Droplet mixing is one of the effective methods to enhance mixing behavior [7,8]. The basic idea is to form aqueous droplets in an immiscible carrier fluid, such as oil. At a high flow rate, the oil flow breaks the aqueous stream into single discrete droplets. Being moved away by the carrier fluid, the droplets experience a self-recirculation flow due to the shear interaction with the carrier fluid. Also at the corner turn, the droplet can stretch. Stretching and recirculation inside the droplets promote advection and consequently improve the mixing inside the droplet. ∗
Corresponding author. Tel.: +65 67905877; fax: +65 67911895. E-mail address: [email protected] (T.N. Wong).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.06.026
The mixing between two liquids can be characterized by using dye method to record the color change due to mixing, such as one liquid with dye and another colorless liquid [9], or two liquids with different color dye [10]. Another method is to use to liquid solutions that can react and produce another color [11,12]. Fluorescent dye method relating the light intensity to concentration of dye is often used as well [13,14]. Some temperature sensitive fluorescent dye can be used to characterize the temperature field [15,16]. This paper presents an experimental study of the recirculating flow field inside a moving nanoliter droplet and the characterization of mixing performance of two merging aqueous droplets. In the first part of the experiment, the velocity field inside a moving water droplet was measured using the micro-particle micro-PIV technique. A microfluidic device has been designed to study the mixing process during the coalescence of two aqueous droplets by using fluorescent dye method [14]. 2. Concept A T-shaped microfluidic device with bending corners was fabricated to study the recirculation flow field inside a moving droplet. The schematic is shown in Fig. 1. Oil with a viscosity of
318
C. Wang et al. / Sensors and Actuators A 133 (2007) 317–322
Fig. 1. Schematic of a T-shaped microfluidic device. Fig. 3. The fabricated device used in our experiment.
Fig. 2. Schematic of the microfluidic device for studying the coalescence and mixing of two droplets.
6.52 × 10−2 Pa s and DI water with a viscosity of approximately 10−3 Pa s are introduced from the two inlets to form droplets. Tracing particles are diluted in DI water for the purpose of microPIV measurement. To study the mixing effect during the process of coalesce of two water droplet, a second microfluidic device was designed. The schematic of this device is shown in Fig. 2. Two aqueous droplets, one being pure water while the other being water with dye, form and meet at the downstream junction. Once the two droplets merge to a big droplet, the dye concentration field is measured at different time instances to study the mixing pattern. 3. Experiment 3.1. Experimental setup The microfluidic devices used in our experiment were fabricated using the polymeric lamination technique, shown in Fig. 3. This technique provides a fast way of prototyping. Polymer sheets commercially available as lamination porches are
cut using a CO2 laser machining system. The channels and other structures were designed on a CAD program and transferred on the polymer sheets by a laser cutting machine. We used the Universal M-300 Laser Platform (Universal Laser Systems Inc.) with a maximum CO2 -laser power of 25 W and a maximum beam speed of about 640 mm/s. The lamination process was carried out using a commercial hot laminator (Aurora LM-450HC, laminating temperature of 120 ◦ C, laminating speed of 0.3 m/min, and maximum laminating thickness of 600 m). Tubing connections are made to provide interface to a syringe pump (Cole-Parmer 74900-05, 0.2–500 mL/h, accuracy of 0.5%). A schematic of the experimental measurement setup is shown in Fig. 4. The setup can be used for both fluorescence imaging and micro-PIV measurement. It consists of four main components: an illumination system, an optical system, a coupled charger device (CCD) camera, and a control system. The control system consisting of a peripheral component interface (PCI) card, and its corresponding software, is implemented in a personal computer (PC). The PC can control and synchronize all actions related to illumination and image recording. Two different light sources were used for the measurements. For the fluorescence imaging measurement, a single mercury lamp was used for illumination. Because of the ability of precise timing and intensity control, a laser beam was used for the micro-PIV measurement. In our system, a double pulsed Qswitched (quality switched) Nd:YAG laser was used. By including a Q-switch inside the cavity the laser can work in a triggered mode. The microchannel in flow field measurement has a cross section of 100 m × 1000 m. The flow field inside the water droplet was measured by micro-PIV technique. In our experiments, Duke red particles (3 m, Duke scientific Co.) were used as the seeding particles. The particles have a maximum excitation wavelength of 540 nm (green, very close to the characteristic wavelength of Nd:YAG) and a maximum emission wavelength of 610 nm (red). The PIV-measurement uses an epifluorescent attachment of type Nikon G-2E/C (excitation filter for 540 nm, dichroic mirror for 565 nm and an emission filter for 605 nm). Both filters in the attachment have a bandwidth of
C. Wang et al. / Sensors and Actuators A 133 (2007) 317–322
319
Fig. 4. Schematic of the experimental setup for fluorescence imaging measurements.
25 nm. The measurement reported in this experiment was carried out with a 4× objective lens. With a CCD sensor size of 6.3 mm × 4.8 mm, the size of an image pixel is 2.475 m and the size of the measured area is 1584 m × 1188 m. After the images were recorded by the CCD camera, commercial software PIVview (PIVTEC GmbH, Germany) was used to evaluate the velocity field. 3.2. Measurement of recirculation in a single droplet A typical result of the velocity field inside the droplet in a straight channel is shown in Fig. 5. First, two images of the
droplet with particles are recorded (Fig. 5a). Next, the displacement of the particles and consequently the velocity field seen by an external observer can be evaluated (Fig. 5b). For the case where total flow rate is 1100 L/h, the average velocity of the carrier liquid is 2.78 mm/s and the average velocity of the droplet is 1.98 mm/s. The velocity of the out layer of the droplet is 2.8 mm/s, larger than the velocity at the center, 1.65 mm/s. Finally, the mean velocity of the droplet is subtracted (Fig. 5c). After subtracting the mean velocity, recirculation flow field can be clearly seen in Fig. 5c. However, the recirculation field is opposite from what is reported by other researchers [7]. In their study, they believed
Fig. 5. Micro-PIV measurement of the velocity field inside an aqueous moving droplet (channel size = 100 m × 1000 m, total flow rate = 1100 L/h, flow rate ratio = 10:1. (a) Original image, (b) velocity field and (c) velocity field after subtracting velocity of the droplet.
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Fig. 6. Maximum velocity difference inside the droplet for different flow rates.
that the center part of the droplet flows forward while the edge flows backwards. The difference might be due to the size of the droplet. In our experiment, the droplet size is less than the channel width. The droplet represents an obstacle or a big particle suspended in the carrier fluid. Thin film of carrier fluid flows between the outer layer of the droplet and the channel wall at higher velocity. This higher velocity will drag the outer layer of the droplet to move forward through the shear stress interaction. For a given mixing time, the stronger the recirculation is, the better the mixing can achieve. Inside the droplet, the recirculation can be characterized by the maximum velocity difference. The maximum velocity difference inside the droplet at different total flow rate is shown in Fig. 6. As the total flow rate increases, the maximum velocity difference increases. That means the recirculation is stronger. At the 90◦ bending corner, the deformation of the droplet’s shape can be clearly observed (Fig. 7). The droplet was stretched by the shear stress of the carrier fluid. The stretching of the droplet and recirculation flow field inside the droplet enhance the mixing process. From this experiment, the droplet size can also be determined. At a constant flow ratio, the size of the droplet depends on the flow rate. The flow ratio is defined as the flow ratio between oil and water phase. Fig. 8 shows that the droplet size decreases as the flow rate increases. Furthermore, the formation of droplets was found to be more rapid at a higher flow rate. When the droplet size decreases, the time required for mixing decreases, because the diffusion length is shorter.
Fig. 8. Droplet size for different flow rate and flow rate ratio.
3.3. Characterization of droplet mixing A microchannel with a cross section of 100 m × 600 m was used for mixing characterization. Fluorescent dye (fluorescein disodium salt C20 H10 Na2 O5 , also called Acid Yellow 73) was added into one of the water streams to form droplets with fluorescent dye. The pure water droplet and the droplet with dye travel along the microfluidic network. The coalescence of the droplets was observed and recorded as a video file for later analysis. When the fluoresein was illuminated by a mercury lamp, CCD camera (Nikon B-2A) was used to take the image frames. The system is able to take images at 30 frames per second. The exposure time of each frame was 1.2 ms. A program written in MATLAB was used to evaluate the light intensity distribution [14]. A proportional relationship is assumed between the light intensity and the concentration of the fluorescent dye. The coalescence and the concentration distribution across the droplets are shown in Fig. 9. Once the two droplets merge, the front droplet is slowed down. Complete mixing can be achieved in 1/6 s, (5 frames’ time, video frequency 30 Hz). The time required for complete mixing at total flow rate 220 L/h is 8 frames’ time, 4/15 s. The reason could be explained as below. The time of complete mixing depends on the droplet size and recirculation field inside the droplet. The smaller the droplet size is, the shorter the diffusion path is. Therefore, less time is taken to reach complete mixing. The stronger the recirculation field is, the less time is required to achieve complete mixing.
Fig. 7. The stretching of the droplet at the bending corner.
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Fig. 9. Concentration field of the fluorescent dye during the coalescence of two droplets (channel size = 100 m × 600 m), total flow rate = 330 L/h, flow rate ratio = 20:1:1.
4. Conclusions
References
In this paper, we presented the optical measurement of the flow field in a moving droplet and concentration field during the coalescence of two droplets. The experimental results showed that the recirculation inside the droplet is different from what is reported in the literature [8]. The underlying reason could be the size of the droplet. The recirculation field depends on the droplet size relative to the microchannel width. Stretching of droplets at the bending corner was observed. The stretching and recirculation make the mixing more efficient. The mixing behavior between two droplets was measured quantitatively. It is found that the droplet mixing depends on the droplet size and recirculation inside the droplet. The droplet size and recirculation depend on the flow rate and flow rate ratio between the liquids. The present study allows a better understanding of mixing mechanism of droplet based mixing and helps us to design droplet mixer for “lab-on-a-chip” applications.
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Acknowledgments This work was partially supported by the academic research fund of the Ministry of Education Singapore, contract number RG11/02. Cheng Wang gratefully acknowledges the Master of Engineering scholarship from the Nanyang Technological University.
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Biographies Cheng Wang received his BE (first class honors) degree in Mechanical Engineering from the Nanyang Technological University, Singapore in 2004. He studied for the Master of Engineering degree in the School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore from 2004 to 2006, in the area of the two-fluid flow in microfluidics for diffusionbased biomedical applications. He is now working in the Micromachines Center,
School of Mechanical and Aerospace Engineering of Nanyang Technological University as a research staff, with focus on the fabrication of photonic crystals. Nam-Trung Nguyen was born in Hanoi, Vietnam, in 1970. He received his Dip -Ing, Dr Ing and Dr Ing Habil degrees from Chemnitz University of Technology, Germany, in 1993, 1997, and 2004, respectively. In 1998, he worked as a postdoctoral research engineer in the Berkeley Sensor and Actuator center (UC Berkeley, USA). Currently, he is an associate professor with the School of Mechanical and Aerospace Engineering of the Nanyang Technological University in Singapore. His research is focused on microfluidics and instrumentation for biomedical applications. He published a number of research papers on microfluidics. His recent book “Fundamentals and Applications of Microfluidics” co-authored with S. Wereley was published in October 2002. The second edition of this book with updated contents was published in 2006. Teck Neng Wong received his PhD in Mechanical Engineering from University of Strathclyde, UK in 1990. He is an associate professor in School of Mechanical and Aerospace Engineering, Nanyang Technological University. His research interests include two phase flow and heat transfer, flow boiling phenomena, two-phase flow in evaporator, condenser and capillary tube expansion devices, heat driven pump and pulsating heat pipe for electronic cooling; microscale heat transfer and fluid flow, two-fluid electroosmotic pump for non-conducting fluid, and electroosmotic control of the interface between two fluids in microchannels and flow switching in microfluidic devices.