Droplet fission and fusion in cross-overlapping microchannels

• Main Authors: Chang, Wei-Chun, 張偉軍
• Other Authors: Yang, Jing-Tang
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/62684157459711117394

碩士 === 國立清華大學 === 奈米工程與微系統研究所 === 99 === This research developed an innovative droplet microfluidic platform, which is a simple-designed 3-D cross-overlapping microchannel capable of manipulating the fission and fusion of droplets. Numerical simulation and experiments both verified that the platform is able to flexibly control the size of daughter droplets by modulating flow rates, and also fuse, mix, and split droplets in high efficiency. Hence, it is expected to be successfully applied to biomedical technology and chemical materials. Based on our proposed design concepts, we first examined mechanisms of splitting by theory, analyzed the system stability of fusion and fission through numerical simulation, and then finally observed the effects of distinct flow rates on the behaviors of droplet fission and fusion. In the part of droplet fission, the size of daughter droplets split from single droplet flow can be controlled by adjusting the flow rate from a lateral inlet. To develop this function, we first inferred from theories that two mechanisms were related to this study— squeezing mechanism and dripping mechanism. In the experiment, we switched the two mechanisms by altering the ratio of droplet size and outlet width and investigated how they affected the fission. It was found that only when the platform was operated in the dripping mechanism, daughter droplets, which can be at smallest size of femtoliter, were controllable by adjusting the flow rate. In the part of droplet fusion and mixing, two droplet flows can merge and mix fast and stably in the cross-overlapped region, and then split into two daughter droplets with the same concentration and size at two outlets. This function was developed by first inducing from numerical simulation that reducing the outlet width would enhance the shear stress and then indirectly increased the system stability. Therefore, in the experiment, we designed two outlets with reduced width (30, 60 micrometer). The results showed that droplets favored to be merged at low flow rate in both of the two designs. Besides, due to distinct flow fields produced by the two designs, the merging patterns of them differed: shifted coalescence in the narrow outlet (30 micrometer), and side-by-side coalescence in the wild one (60 micrometer). The narrow outlet performed better than the wild one since its shifted coalescence diminished the influence of phase difference on the fusion, thus enabling two daughter droplets to have the same concentration. In addition, the narrowness of the outlet resulted in stronger shear stress, which helped splitting a mother droplet into two same size daughter droplets. In summary, this research extended the application of droplet microfluidics from the level of 2-D structure to 3-D structure and developed two functions of it: first, to control the size of daughter droplets by modulating flow rate; second, to integrate the operations of fusion, mixing and fission in a platform. The two aforementioned functions are versatile. For example, the splitting function can be used for reducing the waste of reagents in drug dosing. The function of fusion and mixing can be used to dynamically examine the concentration of specific products in two daughter droplets at the same time.