Engineering and Technology Degree Level: Ph.D. Abstract ID
Graduate Category: Engineering and Technology Degree Level: Ph.D. Abstract ID# 1343
Multiscale Thermal Fluids Laboratory
Pooyan Tirandazi – Carlos H. Hidrovo Abstract Over the last few years, microfluidic systems known as Lab-on-a-Chip (LOC) and micro total analysis systems (μTAS) have been increasingly developed as essential components for numerous biochemical applications. Droplet microfluidics, however, provides a distinctive attribute for delivering and processing discrete as well as ultrasmall volumes of fluid, which make droplet-based systems more beneficial over their continuous-phase counterparts. Droplet generation in its conventional scheme usually incorporates the injection of a liquid (water) into a continuous immiscible liquid (oil) medium. In this study we demonstrate a novel scheme for controlled droplet generation in confined gas-liquid microflows. We experimentally investigate the manipulation of water droplets in flow-focusing configurations using a high inertial air stream. Different flow regimes are observed by varying the gas and liquid flow rates, among which, the “dripping regime” where monodisperse droplets are generated is of great importance. The controlled size and generation rate of droplets in this region provide the capability for precise and contaminant-free delivery of microliter to nanoliter volumes of fluid. Furthermore, the high speed droplets generated in this method represent the basis for a new approach based on droplet pair collisions for fast efficient micromixing which provides a significant development in modern LOC and μTAS devices.
Introduction Conventional droplet microfluidics usually rely on contact of two immiscible liquids (e.g. water in oil) Liquid-in-gas droplet microfluidics present a lot of new perspectives for aerosol applications and aerobiology. A novel method is presented which enables the generation of monodisperse droplets in a high-speed gaseous microflow and subsequent collection of the droplets in a second liquid medium after being generated in the gas phase. The high speed droplets generated in this method, in contrast to conventional liquid-in-liquid generation, represent the basis for a new micromixing approach based on droplet pair collisions for implementation in modern LOC and μTAS devices.
Experimental Setup
Fig. 2 Schematic of the experimental setup used for droplet generation and subsequent collection of the drops. Air is controlled and regulated in multiple steps previous to entering the microfluidic chip inlets. Outlet of the chip is immersed in an oil bath composed of Hexadecane mixed with 2.5wt% of a nonionic surfactant (span 80).
Fig. 6 Images of the collected droplets are captured and a set of image processing techniques are performed in order to obtain the droplet sizes present in each image.
B
A
Results & Discussion Fig. 3 Microchannel configuration used for formation of droplets in air. A flowfocusing junction is utilized with liquid stream in the middle and two gas streams on the sides. The height of all the channels are 40 µm.
Fig. 7 The two generation mode which occur within the dripping regime. Liquid flow rate for both cases is 1µL/min A) Monodisperse generation mode. In this mode by having a constant liquid and gas flow rate the size of the generated droplets are almost the same. Gas Reynolds number this mode is 150. B) Satellite generation mode. In this mode small daughter droplets are generated beside the main droplet which results in polydisperse distribution of the droplets. Gas Reynolds number in this mode is 300.
Conclusion & Future Works Fig. 4 Representation of the flow regimes observed in the experiments. The above images were taken with the speed of 10000 fps for the constant water flow rate of 1 µl/min. As the air flow rate increases three different patterns are distinguished which are (a) co-flow regime (Re~10), (b) jetting regime (Re~30), and (c) dripping regime (Re~100)
A novel method has been presented for the first time for microfluidic generation of monodisperse droplets in air down to 50 µm in diameter. We are actively working on synchronized generation of airborne particles and subsequent processing of the droplets all in an integrated microfluidic device. Chip 1 Chip 2
LW
LG
LO
H
40 µm 40 µm
100 µm 80 µm
180 µm 120 µm
40 µm 40 µm
Fabrication Process
Fig. 1 Process flow for fabrication of microfluidic chips. A photomask containing the microchannels features is designed using a CAD software. This mask is used in Photolithography process with SU-8 to fabricate a Silicon master mold. The fabricated mold is then used in standard soft lithography process with PDMS. After casting the PDMS solution on the mold, each microfluidic chip is peeled off the mold. Required holes are punched and the chip is bonded to a pre-cleaned glass slide using a plasma cleaner.
Fig. 5 (Top) Actual images of droplets moving inside the microchannel and their size difference for a range of gas Reynolds numbers corresponding to the dripping regime. For the same rate of liquid input, increasing gas Reynolds number result in smaller droplets. (Bottom) Experimental data of detached droplet size for different liquid flow rates and gas Reynolds numbers for a channel with aspect ratio of 1.25. Data has been nondimensionalized by the channel hydraulic diameter.
Fig. 7 Actual images and size distributions of collected droplets for two microfluidic chips with the dimension shown in Table. Liquid flow rate and gas flow rate for both cases are 2µL/min and 15mL/min respectively. In both cases more than %60 of the drops have the same average diameter which are 86µm and 54µm for each case.
References 1. S.-Y. Teh, R. Lin, L.-H. Hung, and A. P. Lee, “Droplet microfluidics.,” Lab Chip, vol. 8, pp. 198–220, 2008. 2. B. Carroll and C. Hidrovo, “Droplet collision mixing diagnostics using single fluorophore LIF,” Exp. Fluids, vol. 53, no. 5, pp. 1301–1316, Aug. 2012. 3. J. H. Xu, S. W. Li, J. Tan, Y. J. Wang, and G. S. Luo, “Preparation of highly monodisperse droplet in a T-junction microfluidic device,” AIChE J., vol. 52, no. 9, pp. 3005–3010, Sep. 2006. 4. P. Garstecki, M. J. Fuerstman, H. a Stone, and G. M. Whitesides, “Formation of droplets and bubbles in a microfluidic Tjunction-scaling and mechanism of break-up.,” Lab Chip, vol. 6, pp. 437–446, 2006.
This project is currently being supported by an NSF CAREER Award grant CBET- 1151091.
Multiscale Thermal Fluids Laboratory
Pooyan Tirandazi – Carlos H. Hidrovo Abstract Over the last few years, microfluidic systems known as Lab-on-a-Chip (LOC) and micro total analysis systems (μTAS) have been increasingly developed as essential components for numerous biochemical applications. Droplet microfluidics, however, provides a distinctive attribute for delivering and processing discrete as well as ultrasmall volumes of fluid, which make droplet-based systems more beneficial over their continuous-phase counterparts. Droplet generation in its conventional scheme usually incorporates the injection of a liquid (water) into a continuous immiscible liquid (oil) medium. In this study we demonstrate a novel scheme for controlled droplet generation in confined gas-liquid microflows. We experimentally investigate the manipulation of water droplets in flow-focusing configurations using a high inertial air stream. Different flow regimes are observed by varying the gas and liquid flow rates, among which, the “dripping regime” where monodisperse droplets are generated is of great importance. The controlled size and generation rate of droplets in this region provide the capability for precise and contaminant-free delivery of microliter to nanoliter volumes of fluid. Furthermore, the high speed droplets generated in this method represent the basis for a new approach based on droplet pair collisions for fast efficient micromixing which provides a significant development in modern LOC and μTAS devices.
Introduction Conventional droplet microfluidics usually rely on contact of two immiscible liquids (e.g. water in oil) Liquid-in-gas droplet microfluidics present a lot of new perspectives for aerosol applications and aerobiology. A novel method is presented which enables the generation of monodisperse droplets in a high-speed gaseous microflow and subsequent collection of the droplets in a second liquid medium after being generated in the gas phase. The high speed droplets generated in this method, in contrast to conventional liquid-in-liquid generation, represent the basis for a new micromixing approach based on droplet pair collisions for implementation in modern LOC and μTAS devices.
Experimental Setup
Fig. 2 Schematic of the experimental setup used for droplet generation and subsequent collection of the drops. Air is controlled and regulated in multiple steps previous to entering the microfluidic chip inlets. Outlet of the chip is immersed in an oil bath composed of Hexadecane mixed with 2.5wt% of a nonionic surfactant (span 80).
Fig. 6 Images of the collected droplets are captured and a set of image processing techniques are performed in order to obtain the droplet sizes present in each image.
B
A
Results & Discussion Fig. 3 Microchannel configuration used for formation of droplets in air. A flowfocusing junction is utilized with liquid stream in the middle and two gas streams on the sides. The height of all the channels are 40 µm.
Fig. 7 The two generation mode which occur within the dripping regime. Liquid flow rate for both cases is 1µL/min A) Monodisperse generation mode. In this mode by having a constant liquid and gas flow rate the size of the generated droplets are almost the same. Gas Reynolds number this mode is 150. B) Satellite generation mode. In this mode small daughter droplets are generated beside the main droplet which results in polydisperse distribution of the droplets. Gas Reynolds number in this mode is 300.
Conclusion & Future Works Fig. 4 Representation of the flow regimes observed in the experiments. The above images were taken with the speed of 10000 fps for the constant water flow rate of 1 µl/min. As the air flow rate increases three different patterns are distinguished which are (a) co-flow regime (Re~10), (b) jetting regime (Re~30), and (c) dripping regime (Re~100)
A novel method has been presented for the first time for microfluidic generation of monodisperse droplets in air down to 50 µm in diameter. We are actively working on synchronized generation of airborne particles and subsequent processing of the droplets all in an integrated microfluidic device. Chip 1 Chip 2
LW
LG
LO
H
40 µm 40 µm
100 µm 80 µm
180 µm 120 µm
40 µm 40 µm
Fabrication Process
Fig. 1 Process flow for fabrication of microfluidic chips. A photomask containing the microchannels features is designed using a CAD software. This mask is used in Photolithography process with SU-8 to fabricate a Silicon master mold. The fabricated mold is then used in standard soft lithography process with PDMS. After casting the PDMS solution on the mold, each microfluidic chip is peeled off the mold. Required holes are punched and the chip is bonded to a pre-cleaned glass slide using a plasma cleaner.
Fig. 5 (Top) Actual images of droplets moving inside the microchannel and their size difference for a range of gas Reynolds numbers corresponding to the dripping regime. For the same rate of liquid input, increasing gas Reynolds number result in smaller droplets. (Bottom) Experimental data of detached droplet size for different liquid flow rates and gas Reynolds numbers for a channel with aspect ratio of 1.25. Data has been nondimensionalized by the channel hydraulic diameter.
Fig. 7 Actual images and size distributions of collected droplets for two microfluidic chips with the dimension shown in Table. Liquid flow rate and gas flow rate for both cases are 2µL/min and 15mL/min respectively. In both cases more than %60 of the drops have the same average diameter which are 86µm and 54µm for each case.
References 1. S.-Y. Teh, R. Lin, L.-H. Hung, and A. P. Lee, “Droplet microfluidics.,” Lab Chip, vol. 8, pp. 198–220, 2008. 2. B. Carroll and C. Hidrovo, “Droplet collision mixing diagnostics using single fluorophore LIF,” Exp. Fluids, vol. 53, no. 5, pp. 1301–1316, Aug. 2012. 3. J. H. Xu, S. W. Li, J. Tan, Y. J. Wang, and G. S. Luo, “Preparation of highly monodisperse droplet in a T-junction microfluidic device,” AIChE J., vol. 52, no. 9, pp. 3005–3010, Sep. 2006. 4. P. Garstecki, M. J. Fuerstman, H. a Stone, and G. M. Whitesides, “Formation of droplets and bubbles in a microfluidic Tjunction-scaling and mechanism of break-up.,” Lab Chip, vol. 6, pp. 437–446, 2006.
This project is currently being supported by an NSF CAREER Award grant CBET- 1151091.
