Systematic analysis of geometrical based unequal droplet splitting in digital microfluidics

This paper presents the thorough analysis of a new operator developed for unequal droplet splitting by geometrical modification of a conventional electrode in digital microfluidic (DMF) platforms. This operator functions in an area as small as the size of a conventional electrode divided into several smaller sub-electrodes addressable independently. Using this operator, droplets can be precisely split unequally with a high volume ratio, as well as being dispensed with a wide range of sizes from a reservoir. This operator functions without complications in the fabrication process and the need for additional external modules such as feedback control systems. To characterize the range of the applicability of this operator, the effects of the applied voltage, the gap height between the two plates, and the sub-electrode geometry on the splitting performance are studied. For high applied voltages, splitting is performed with an error close to 5% (similar to the splitting precision obtained in conventional DMF devices); whereas, for voltages close to the threshold value, the error is less than 1%. In this way the droplet size becomes fairly independent of the geometry and the gap height. The threshold splitting voltage versus gap height has also been studied and shows a linear behavior, facilitating the selection of proper voltages for high precision splitting. For the three sub-electrode patterns studied here (i.e. square, horizontal stripe, and vertical stripe), the results show that the square sub-electrodes provide higher reliability and a wider range of sizes for the split droplet as compared to the other patterns. The final part of the study illustrates that there is a linear relation between the area of the split droplets and the number of actuated sub-electrodes. This linear behavior allows for the selection of an appropriate number of the sub-electrodes to be actuated based on the desired volume of the droplet.