1. MEMS design and Micro-fabrication Lab MML Glass PDMS #2 Choppers PDMS #1 Inlet hole Outlet hole Air inlet A NEW MICROFLUIDIC CHIP FOR FORMATION OF MICRO-DROPLETS IN LIQUIDS UTILIZING ACTIVE PNEUMATIC CHOPPERS Cheng-Tso Chen and Gwo-Bin Lee Department of Engineering Science, National Cheng Kung University, Tainan, Taiwan 701 Abstract Mono-disperse emulsion is of great significance for both chemical and industrial applications. The current study reports a new microfluidic system capable of formation of micro-droplets in liquids for emulsification applications. The new emulsion chip can precisely generate uniform droplets using a novel combination of hydrodynamic-focusing and liquid-chopping techniques. Experimental data show that micro-droplets with a diameter ranging from several to tens of micrometers could be precisely generated. The emulsification behavior at different ratios of sample flow rate (v 1 ) and sheath flow rate (v 2 ) and chopping frequency is systematically investigated. The micro-droplets have a uniform size while compared to previous studies. This novel device can be promising for emulsification and other related applications. Design and Principle Fig. 1 (a) Schematic illustration of the working principle of the microfluidic chips. (b) Sample flow is focused into a narrow stream. (c) An active “chopper” is used to cut liquids into droplets with well-controlled sizes. Experimental Fig. 4 SEM images of (a) SU-8 mold of the chopper structure, and (b) PDMS replication from the SU- 8 mold. Conclusions This chip is capable of generating uniform microdroplets and has the potential to be used for high-quality emulsification processes. Experimental data show that the developed chip is capable of creating emulsions by combining hydrodynamic flow focusing and liquid- chopping techniques. The droplet size is dependent on the ratio of sheath and sample flow rates and the chopping frequency. Emulsions with droplet sizes ranging from 6-100  m are successfully observed. The standard variation in size of the droplets is within 5 %. Acknowledgement The authors gratefully acknowledge the financial support provided to this study by the National Science Council of Taiwan under Grant No. NSC 94-3112-B-006-002. (a) (b) Continuous phase Dispersed phase (b) Focusing (c) Choppers Sample collection reservoir v2v2 v2v2 v1v1 （a）（a） Micro-droplets Air chamber PDMS #2 Compressed air PDMS #1 Focused flow Glass Fig. 2 Schematic illustration of the working principle of the active pneumatic choppers. Fig. 3 Explosive view of the microfluidic chip. (a) (b) ◇ No chopping △ f EMV = 3.42 Hz Diameter (μm) v 2 / v 1 Droplet Diameter (μm) ◇ = 3.42 Hz □ = 4.89 Hz ○ = 6.90 Hz △ = 13.84 Hz v 2 / v 1 Fig. 6 The relationship between the average droplet diameter and the relative sheath/sample flow rate ratio for (a) no chopping frequency and 3.42Hz chopping frequency. (b) at various driving frequencies. (a)(b)(c) 150μm Fig. 5 Formation of microdroplets using active pneumatic choppers at a constant chopping frequency. The size of the emulsion droplets can be well controlled using different sheath flow rates. Average droplet diameters are (a) 100 μm, (b) 40 μm, and (c) 25 μm, respectively. 2006