Dual-nozzle microfluidic droplet generator
- Ji Wook Choi†1,
- Jong Min Lee†1,
- Tae Hyun Kim1,
- Jang Ho Ha1,
- Christian D. Ahrberg2 and
- Bong Geun Chung1Email authorView ORCID ID profile
© The Author(s) 2018
Received: 9 March 2018
Accepted: 21 April 2018
Published: 8 May 2018
The droplet-generating microfluidics has become an important technique for a variety of applications ranging from single cell analysis to nanoparticle synthesis. Although there are a large number of methods for generating and experimenting with droplets on microfluidic devices, the dispensing of droplets from these microfluidic devices is a challenge due to aggregation and merging of droplets at the interface of microfluidic devices. Here, we present a microfluidic dual-nozzle device for the generation and dispensing of uniform-sized droplets. The first nozzle of the microfluidic device is used for the generation of the droplets, while the second nozzle can accelerate the droplets and increase the spacing between them, allowing for facile dispensing of droplets. Computational fluid dynamic simulations were conducted to optimize the design parameters of the microfluidic device.
The development of microfluidics and micro total analysis systems (µTAS)  has led to a paradigm shift in many research areas. Microfluidics allows the precise handling of small volumes of liquids, while maintaining a high control over mass and thermal transport, as well as fast response times at low cost and automation . Two options exist for the operation of a microfluidic device, either continuous or segmented flow. While in continuous flow only one phase is used [3, 4], segmented flow breaks up the flow using two or more different phases . Despite the higher complexity, the segmented flows possess a number of advantages over continuous flows. Typically, droplets provide faster mass and thermal transfer, while preventing boundary effects, such as axial dispersion. Furthermore, they provide small, reproducible volumes, can be manipulated independently, and serve as individual units for reactions . Due to their high homogeneity and fast mass transfer, they are commonly used for the controlled synthesis of nanoparticles [7, 8]. Other applications can be found in the creation of artificial cells , the analysis of single cells , or in digital polymerase chain reaction . For all of these applications, the generation of stable and monodispersed droplets is necessary.
In microfluidics, droplets can be made either following an active or a passive method. In active methods, droplets are generated by applying an external force. This can be done either by applying a direct or alternating current. In systems consisting of one conducting and one insulating phase, charges accumulate on the interface due to electrochemical reactions. The resulting electrical field force results in the formation of droplets . Alternatively, a force can be created through thermal expansion of one of the two phases, as can be done by localized laser irradiation [13, 14]. Lastly, droplets can be generated by active methods utilizing active valves or pneumatically actuated membranes [15, 16]. In passive method, pressure-driven flows of the dispersed and continuous phase meet at a microchannel junction. The characteristics of the junction determine the interface deformation and the formation of droplets. One, infrequently used, option is to arrange both streams in coaxial microchannels. The dispersed phase is introduced in the central channel, while the continuous phase flows through outer channels [17, 18]. Similarly, flow-focusing geometries use a central flow of the dispersed phase and outer flows of segmented phase. In contrast to coaxial microchannels, the flows pass a contraction region after which the central flow breaks up into droplets [19, 20]. The most popular method for passive droplet generation is the cross-flow method. Here, the flow of the continuous phase is partially blocked by a flow of the dispersed phase coming from a secondary channel. Through this, a shear gradient develops, the dispersed phase elongates and eventually breaks into droplets [21–23]. Some applications, such as filling of nanowells  or production of micro-lenses  require the dispensing of the generated droplets. Previously, this has been obtained by generating droplets using a pinched flow channel, followed by injecting the droplets into a stream of a carrier gas for analysis in ion coupled mass spectroscopy (ICPMS) . Other groups have been able to achieve dispensing by either precise timing control of the dispensing process , or by the use of an active piezo-electric droplet generator . Through the use of an active droplet generation method, the issue of droplet aggregation and merging can be prevented. However, this reduces the throughput of the microfluidic device and adds complexity to the system.
Here, we show a novel method of droplet dispensing using a dual-nozzle microfluidic setup. While the first nozzle is used for the generation of droplets, the second nozzle is used for the acceleration of the generated droplets and to increase the spacing between them. Through this droplets can be dispensed at a high frequency without the issue of aggregation and merging at the device outlet. A computational fluid dynamic (CFD) model was created before experiments to optimize the design of microfluidic devices.
2.1 Computational model of the microfluidic device
For the simulations, a value of ρ1 = 800 kg/m3 and dynamic viscosity of μ1 = 0.01 Pa s was used. For water, the values were ρ2 = 1000 kg/m3 and μ2 = 0.001 Pa s, respectively. Furthermore, all fluids were assumed to be incompressible, homogenous Newtonian fluids. A model of the microfluidic droplet dispensing device was constructed based on the AutoCAD drawing used for device fabrication. The walls were defined as wetted boundaries with a contact angle of 120° for the water phase and no pressure was set at the outlet of the microfluidic device.
2.2 Fabrication of the dual-nozzle microfluidic device
A microfluidic dual-nozzle device consisting of two inlets for each nozzle was designed using AutoCAD (Autodesk, USA) and printed onto photomasks. All inlet channels were designed with a width of 70 µm with the exception of the water inlet in the first nozzle which had a width of 100 µm. The design from the masks was transferred to silicon wafers (Wangxing Silicon-Peak Electronics, China) using a standard soft-lithography process as shown previously . Briefly, silicon wafers are cleaned using a wafer washing system and afterwards dried for 5 min at 200 °C on a hotplate. 5 mL of SU-8 50 photoresist (Microchem Corp., USA) was spin-coated onto the silicon wafers at 3000 rpm for 60 s, resulting in a 40 µm photoresist layer. The spin-coated wafer was soft-baked at 65 °C for 5 min and afterwards further heat treated at 95 °C for 15 min on a hotplate to evaporate the solvent. After UV-exposure for 10 s at an intensity of 20 mW/cm2, the wafers were baked at 65 °C for 1 min, followed by heat treatment at 95 °C for 4 min on a hotplate. The silicon masters were developed using SU-8 developer (Microchem Corp., USA) and dried with air. Poly(dimethylsiloxane) (PDMS, Dow Corning, USA) was poured onto the silicon wafers. After curing in an oven at 80 °C, the PDMS was peeled off from the silicon wafer and was subsequently bonded into glass slides using oxygen plasma.
2.3 Droplet dispensing experiments
Syringe pumps (PHD 2000, Harvard Apparatus, USA) were connected to the four inlets of the microfluidic device using tygon tubing (Sigma Aldrich, USA) to conduct droplet dispensing experiments. For experiments, de-ionized water (DI water) was used as the continuous phase and mineral oil (M5904, Sigma Aldrich, USA) as the dispersed phase. For experiments, all flow rates were systematically varied between 10 and 50 µL/min in increments of 10 µL/min, in accordance with the values previously used for numerical simulations. Images of the resulting droplets were captured using an inverted microscope (Olympus IX73, Japan) and were also analyzed using Image J (National Institute of Health, USA) regarding their droplet diameter and the distance between droplets.
3 Results and discussion
3.1 Fabrication of dual-nozzle microfluidic device
3.2 Computational model
3.3 Droplet generation in a dual-nozzle microfluidic device
Here, we have shown a microfluidic device for the generation and dispensing of droplets. The microfluidic device consists of two separate nozzles. While the first nozzle is used for the generation of droplets, the distance between the individual droplets can be adjusted using the second nozzle. Using this method, the agglomeration and merging of droplets at the microfluidic device outlet can be prevented and dispensing of homogenous droplets can be achieved. The microfluidic device could be a valuable tool for a wide range of applications. Through the small size of the device, it might be particularly interesting for point-of-care applications.
JWC and JML fabricated and analyzed the dual-nozzle microfluidic droplet generator. THK and JHH performed CFD simulation and analyzed droplets. CDA and BGC discussed the experimental data and wrote the paper. All authors wrote the final manuscript. All authors read and approved the final mansucript.
The authors declare that they have no competing interests.
Availability of data and materials
The authors have no data to share since all data are shown in the submitted manuscript.
Ethics approval and consent to participate
This work was supported by the National Research Foundation (NRF) of Korea grant funded by the Ministry of Science and ICT (MSIT) (Grant Numbers 2016M3A7B4910652, 2016R1A6A1A03012845, 2017H1D3A1A02013996).
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