Modelling and Development of Droplet Generation Method for Magnetically Actuated Digital Microfluidics
Digital microfluidics is useful in miniaturizing different types of biological tests due to an ability to perform multiple operations on a single chip with a single common geometry. However, one of the largest problems in using digital microfluidics has been biofouling during these processes, a phenomenon which is exacerbated when using electric fields for fluid manipulation. Magnetic microfluidics is an alternative actuation mechanism, which does not have this same limitation. However, until recently it has been impossible to create drops, which can also be subsequently moved to perform multiple step analyses. In this thesis, a method for creating drops on superhydrophobic surfaces using magnetic force as the primary actuation mechanism is outlined. Using numerical simulations in Comsol Multiphysics, the method was explored to determine several of the limitations of the system, including the minimum actuatable volume (5 μL). Additionally, the flow within the drops was explored to obtain a better understanding of the experimental results. Experimental validation of the results followed, and it was found that the system showed a 1-to-1 linear change in drop volume relative to expected dispensed volume, verifying the theoretical model. Experimental validation found that the error of the system is constant over any dispensed volume at ±0.5 μL. The method was applied to miniaturize a standard test for hard water determination, a complexometric EDTA titration. The results from a specially designed chip are compared to a titration of a 10 mL volume of the same sample and two were found to be equal within error (100-170 ppm for the chip, 124±2 ppm for the standard method). The water hardness estimate from the chip was further improved upon by image analysis, and the range narrowed to 120±15 ppm, again within the same error as the standard method. Together with the theoretical analysis, this result verifies the practicality of the method for analytical operations. Finally, preliminary work into making the magnetic droplet manipulation method more general was conducted, and it was found to be possible to create chips which utilize any combination of droplet mixing, merging, moving, and splitting.
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