Volume 7 Issue 7 - February 20, 2009
Droplet formation utilizing controllable moving-wall structures for double emulsion applications
Yen-Heng Lin, Chun-Hong Lee and Gwo-Bin Lee*

Department of Engineering Science, College of Engineering, National Cheng Kung University

IEEE/ASME Journal of Microelectromechanical Systems, accepted for publishing, 2008 (SCI; Impact factor=2.659)

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The developed chip is capable of generating single and double emulsion microdroplets and has the potential to be used for high-quality emulsification processes, including the analysis of pico-liter biochemical reactions, drug delivery systems, and in the cosmetics industry. The formation of microdroplets in liquids using an emulsification process plays an essential role in a wide variety of applications including the pharmaceutical, biomedical, agricultural, food, printing and cosmetics industries. A uniform size distribution of emulsion droplets is crucial in determining the quality of the final products. In this study, we report a new microfluidic device capable of double emulsions by using two types of moving-wall structures. By using a simple and reliable PDMS fabrication process, the first controllable moving-wall structure near the T-junction microchannels can be activated under specific applied air pressure to fine-tune the size of the internal droplets. After the first emulsion stream passes downstream of the flow-focusing zone, the double emulsion microdroplets with a uniform diameter can be generated by another pair of moving-wall structures. The size of the external droplets can be also fine-tuned by the applied compressed air pressure or the width of the pre-focusing flow.

Figure 1 shows a schematic illustration of the emulsion chip which has a controllable moving-wall structure at the T-junction channels and another pair of moving-wall structure as “pneumatic choppers” downstream. These two moving-wall structures can control the diameters of the internal and external droplets. The operating procedure of the emulsion chip can be described as follows. First, two immiscible liquids, including a continuous phase (sample A) with a volume flow rate of R1 and a dispersed phase (sample B) with a volume flow rate of R2 are injected into the T-junction channels to generate the internal emulsion droplets. Then the internal emulsion droplets are hydrodynamically focused into a narrow stream by the neighboring sheath flows (sample C) with a volume flow rate of R3. Finally, the pneumatic choppers are used to cut the pre-focused emulsion flow into double emulsion microdroplets with well-controlled sizes. The double emulsion droplets are then collected at the outlet reservoir.

Figure 2(a) shows a schematic illustration of the size-tunable microdroplet generation by utilizing the controllable moving-wall structure, which can control the width of the T-junction microchannel to squeeze the continuous phase flow. Figure 2(b) shows a schematic illustration of the working principle behind the formation of double emulsion microdroplets by utilizing the pneumatic choppers. The pneumatic choppers are membrane structures allowing horizontal movement to squeeze the pre-focused stream. When the pneumatic side-chambers are filled with compressed air, the PDMS membranes are deflected and the upstream liquid is then “chopped” to generate emulsion microdroplets. The diameter of the external droplets can be controlled by the applied compressed air pressure or the width of the pre-focusing flow.

A photograph of the assembled microfluidic chip is shown in Fig. 3. The dimensions of the chip are 4 cm and 3 cm in width and length, respectively. As shown in Fig. 4, the size of the o/w emulsion droplets could be fine-tuned using different applied air pressures. The average diameters for these four cases are 50.07, 43.09, 35.93, and 21.80 μm for corresponding applied air pressures of 10, 15, 20, 25 psi, respectively, under a fixed continuous/dispersed flow rate ratio of 1600. The coefficients of variation are 1.28, 2.78, 1.61, and 3.53%, respectively. It can be seen that the larger the applied pressure is, the smaller the droplet size is under the same continuous/dispersed flow rate ratio. The deformation of the controllable moving-wall structure squeezes the continuous-phase flow locally and increases the flow velocity near the intersection of the T-junction channels. It therefore increases the shear force to form droplets with smaller diameters.

Figure 5 shows a series of photographs for w/o/w double emulsions with different external and internal droplet sizes at a fixed volume flow rate ratio (R1:R2:R3 = 0.5:30:1000 μl/hr). Double emulsion droplets with ratios of external/internal droplet sizes ranging from 1.69 to 2.75 can be successfully produced. The experimental results clearly show that the size of the external droplet can be decreased as the applied pressure of the pneumatic chopper is increased while keeping the same internal droplet size. Therefore, double emulsion droplets with the same internal droplet size and different external droplet sizes can be formed. Similarly, double emulsion droplets with the same external droplet size and different internal droplet sizes can also be produced.
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