Dynamic control of water-in-oil droplets in a continuous flow by active and passive methods
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The objective of this thesis is studying the dynamic control of the droplet in microfluidic systems. We developed new microfluidic designs and demonstrated its functionality based on two different approaches: active and passive method. The active method is based on application of electric field, and the passive method is based on the microfluidic channel modification. The first microfluidic device investigated in this thesis is two-phased droplet sorter using single electrode pair. This device was fabricated using soft lithography and individual droplet sorting was also demonstrated. The two-phased droplets are generated and charged by electrostatic actuation and sorted into designated outlet simultaneously. The concurrent charging and sorting of up to 600 droplets per second was achieved. And, the optimized voltages for stable device operation were also investigated. Beyond operational voltage (∼160 V), the droplet generation is unstable and results in unsuccessful droplet charging and sorting mechanism. Our next study in this thesis is also related with two-phase droplet sorting device based on electric field, but the different configuration. This device combines two independent modules: hydrodynamic flow focusing structure and the two paired electrodes for droplet charging and sorting. Unlike the previous study, two sets of electrodes were employed to enable the preformed and chemically or biologically treated droplets. The droplet sorting of individual and a train of droplets were demonstrated depending on the polarity of the charged surface. And device working range was also studied such as the minimum and maximum operational region in terms of two-phased droplet size and applied actuation voltage. The previous studies focus on the droplet control using active method, specifically electric field. In following study, we focus on the passive method (e.g., geometry modification). Firstly, we investigated droplet synchronization method using railroad-like channel network. The droplet synchronization efficiency was achieved up to 95%. The pressure difference between the top and bottom channels results in the cross flow of carrier oil through the interconnected channels and there are no necessary of any active source to droplet synchronization. The physical parameters (e.g., droplet length and droplet generation frequency) were studied to improve the droplet synchronization efficiency. In addition, one-to-multiple droplet synchronization was demonstrated by matching the product of droplet length and the droplet generation frequency for both the top and bottom channels. Lastly, the straightforward method of guiding, distribution, and storing of a train of shape-dependent droplets by using side flows and guiding tracks was investigated. The squeezing flow makes a train of flattened droplets to align to one side wall and the pushing flow guides it to one of the designated guiding tracks. The guided droplets are then flowing along the cavity guiding track due to the lowered surface energy when they flow along the track. In addition, simultaneous droplet guiding and storing process has been demonstrated by the proposed method.
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