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UBC Theses and Dissertations

Study of droplet splitting in an electrowetting based digital microfluidic system Bhattacharjee, Biddut


This thesis focuses on the symmetric and asymmetric splitting of droplets, which is a prominent fluidic operation in a digital microfluidic system (DMFS). The prerequisite part of the investigation of droplet splitting is to understand the electrowetting-on-dielectric (EWOD) based droplet actuation. This thesis demonstrates that not only the EWOD actuation is a self-feedback system - implying that the actuation force depends on the position of the droplet, but also the size of the droplet affects the magnitude of actuation force. However, a sensing mechanism is essential for complex operations, e.g. dispensing and splitting. One contribution of this thesis is a novel method of sensing the droplet position that requires connections to the two adjacent electrodes in the lower plate only. For the fabrication of prototype DMFS, a new polymeric material, cyanoethyl pullulan (CEP), is proposed as the dielectric layer resulting in a simple and low-cost fabrication of DMFS. The required voltage for droplet manipulation is drastically reduced owing to high relative permittivity of CEP. Droplet splitting is investigated both numerically and experimentally. Numerical investigation of droplet splitting in FLOW-3D®, a commercial computational fluid dynamics software, revealed that the strength of viscous forces relative to the surface tension force determines the success of splitting. For successful asymmetric splitting, performed by applying voltages of unequal magnitude to left-hand and right-hand sides of the droplet, there exists a minimum voltage for the low-voltage side that guarantees splitting. This minimum voltage increases if the aspect ratio (i.e., diameter to height) of the droplet is reduced while keeping the diameter of the droplet constant. Investigation of the asymmetric splitting with different ratios of applied voltage revealed that the ratio between the volumes of accumulated liquid on either sides increases with voltage ratio. The feasibility of asymmetric splitting as well as the effects of different ratios of applied voltages were studied in prototype DMFS. The results verify the existence of a minimum voltage for successful splitting. The ratio between the volumes of the sister droplets increases with that of the applied voltages. Moreover, the general characteristics of flow-rates and liquid accumulation were found to be similar to those in simulations.

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