KEYWORDS: Electrodes, Dielectrics, Microfluidics, Capacitors, Liquids, Motion models, Capacitance, Systems modeling, Chemical analysis, Biological research
Digital microfluidic systems (DMFS) manipulate liquid droplets with volumes in submicroliter range in two dimensional
arrays of cells. Among possible droplet actuation mechanisms, Electrowetting-on-dielectric (EWOD) actuation has been
found to be most feasible and advantageous because of low power consumption, ease of signal generation and basic
device fabrication. In EWOD based DMFS, droplets are actuated by applying an electric field and thus increasing the
wettability on one side of the droplet. In this paper, we show that the EWOD actuation of a droplet can be modeled as a
closed loop system having unity feedback of position. Electrode, dielectric and droplet are modeled as a capacitor with
variable area as the droplet, considered as a conductor, moves over the dielectric layer. The EWOD force depends on the
rate of change of droplet area over the actuated electrode, which in turn depends on the direction of motion and the
position of the droplet between the actuated and previous electrode. Thus, EWOD actuation intrinsically utilizes the
droplet position to generate sufficient force to accelerate the droplet. When the droplet approaches the final position, the
magnitude of force reduces automatically so the droplet decelerates. In case the droplet has sufficient momentum to
exceed the final position, the EWOD force, according to the model, will act on the opposite side of the droplet in order to
bring it back to the desired position. The dynamic response has been characterized using the proposed model for
different droplet sizes, actuation voltages, dielectric thicknesses and electrode sizes.
KEYWORDS: Microfluidics, Electrodes, Detection and tracking algorithms, Computer simulations, Very large scale integration, Visualization, Silicon, Sensing systems, Medical research, Inspection
Digital microfluidic systems (DMFS) are poised to provide fully automated, high-throughput, dynamically
reconfigurable sensing devices superior to those available today. Efficient droplet routing algorithms for these systems
have not yet been established, though several solutions have been proposed. Such algorithms are ultimately required to
generate droplet movement schedules and must be robust enough to handle the inevitable increases in problem
complexity that will come as this technology matures. We have proposed a new solution based on a classic VLSI lineprobe
algorithm to meet these demands for the detailed routing of droplets within a multi-stage algorithm. The most
significant addition includes a sub-algorithm that calculates the routing complexity for any DMFS configuration based
on the size, shape, number, type, and distribution of rectilinear obstacles throughout a DMFS biochip surface. By
determining the complexity of the routing of each droplet, routing schedules may be prioritized, minimizing the number
of fluidic and time constraint violations that affect high priority droplet routes. The complexity characterizations
generated by our algorithm may also be used to create consistent, standardized benchmarks for the evaluation of existing
droplet routing solutions. The efficiency of the proposed algorithm has been verified using the simulation presented in
this paper.
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