Research On Manipulation And Control Of Droplets Based On Electrowetting On Dielectric

Electrowetting on dielectric (EWOD) based digital microfluidics(DMF) has shown enormous advantages and potential in biology, medicine, chemistry and so on, and it has been used extensively in all these subjects, such as cell manipulation, clinical tests, DNA analysis, food and environmental monitoring and so on. It can be seen that, with the development of MEMS, digital microfluidics will be an aspect of the science and technology on the leading edge in the twenty-first century.Aim at the poor functions of DMF developed nowadays and low flexibility in manipulation of droplets, in this paper, a mixed structure DMF was designed based on the research on the theory of EWOD and on the mechanism of droplets manipulation. Combine numerical simulation and MEMS technics, the DMF was successfully developed. Besides,we also designed a control circuit and carried out a series of experimental research on droplets manipulation and control.Firstly, the forces acting on droplets in micro-scale was analysed, several methods for changing surface tension was compared. Because of its unique advantages, EWOD was selected as driver for droplets manipulation. The model of EWOD was built and the internal mechanism of EWOD was studied. We also improved the mathematical equation of EWOD based on energy minimization principle. After the model of droplets manipulation was founded, we proposed preliminarily methods for the creating, transporting, cutting and merging droplets.Combine the advantages and disadvantages of single-plate and double-plate DMF, a mixed structure DMF was proposed for the first time, and the mathematical relations for dimensions of electrodes between single region and double region in the designed DMF was deduced. After the governing equations and their boundary conditions were built, we solved for the electric field and calculated the electrodynamic actuation forces acting on the whole surface of droplet by using Maxwell-stress tensor in different kinds of structures of digital microfluidics with the Comsol software package, and the optimal distribution and dimensions for different kinds of electrodes were maked. Based on the electrodynamic simulation model, the internal mechanism of EWOD was also studied by comparing three kinds of simulatation structures, which have the same contact domain but different contact line. Besides, the electrical field, fluidic field and temperature field coupled in the designed DMF were numerically simulated, and based on the coupling simulation results, we generalized the move of droplet to two stage and analysed the influence of dielectric layer thickness on temperature and velocity in the droplet.Based on the theoretical calculation and numerical simulation results, a process flow for fabricating digital microfluidics was designed by employing MEMS technics. Then a DMF with lamellar composite-structure dielectric layer was successfully developed by combining Si₃N₄ and SiO₂. In addition, a switching hardware circuit and relative program for controlling the manipulation of droplets was successfully designed and debugged based on MSP430F149 microcontroller.Lastly, an experimental platform including microscopic vision system, computer, dc electrical source, control circuit board and so on was set up for carrying out experimental research on manipulation and control of droplets. It was testified by measuring a series of dynamic contact angle and contact angle saturation under different voltages that SiO₂-Si₃N₄-SiO₂ and Si₃N₄-SiO₂ lamellar composite structure for dielectric layer of DMF was superior to Si₃N₄-SiO₂-Si₃N₄ and SiO₂-Si₃N₄ dielectric layers. The influence of applied voltage, the frequence of switch circuit, the volume of droplets and the space between upper and bottom plate of DMF on manipulation of droplets was compared and analysed by experiments.Transporting, merging aqueous droplets and two aqueous droplets transporting side-by-side were successfully realized, and the feasibility of manipulation and control of droplets based on EWOD was testified.This paper involves multi-disciplinary intercross investigation and application, the theory, methods and conclusions in this paper can be useful and valuable to a certain extent for further development of DMF and research on manipulation of droplets.