Driving Mechanism And Manipulation Of Droplets Based On Electrowetting On Dielectric

Electrowetting-on-dielectric (EWOD) based digital microfluidics is a new technologythat permits manipulation of liquid droplets on an array of electrodes. Using thistechnology, nanoliter to microliter size droplets of different samples and reagents can bedispensed from reservoirs, moved, split, and merged together. This technology has a greatdeal of advantages, such as less regent consumption, less time consuming and device ofsmall size. Thereby, EWOD based digital microfluidics has a very wide range ofapplications in fields of biology, chemistry, optical lens and so on. This paper focuses onthe impending demand on micro-droplet manipulation. Firstly, we go about the mechanismof droplet’s motion from the condition of rest to moving. Whereafter, we study the dropletforce analysis and implementations to reduce the driving voltage by means of modelingand numerical simulation. Finally, experimental results validate the efficiency of reducingdriving voltage and the performance of droplet position detecting system for successivedroplet motion.Firstly, the principles of electrocapillary, the electrowetting and electrowetting on thedielectric are presented, then the mechanism of bound charge effect on the solid-liquidcontact surface is studied; and afterwards the electric field, the pressure field and the flowfield of EWOD device–droplet are numerically simulated in software COMSOLMultiphysics4.3, simulated results demonstrate that static force will arise when positiveand negative ionsmolecules attract each other in electric field, and the static force willmake the surface force of droplet change, then fluid statics pressure will arise due to theasymmetrical change of surface force, then droplet will move under the enough fluidstatics pressure;―three free body diagrams‖is used to depict droplet’s motion from thecondition of rest to moving.Secondly, the dynamic equation is set up baesd on the forces exerted on the droplet；whereafter deducing the force equations exerted on the droplet during different timequantum in one period of motion based on energy minimization principle, and the forces are numerically simulated；the simulated results demonstrate that the longer width ofelectrode can be able to obtain greater driving force which will contribute to the optimalstructure design.Thirdly, optimization design is exerted on the factors reducing the driving voltage, asuspended ground electrode is designed, and the numerical simulated results demonstratethat the suspended ground electrode structure can be able to reduce the tree contact lineforce and plate shear force. Meanwhile, a crescent electrode was designed in comparisonwith conventional square and jagged driving electrodes. The numerical simulated resultsdemonstrate that among three different electrodes (four different layouts), the dropletlocating on the crescent electrode can reach the maximum stress value occupyingminimum duration, with larger partial motion at the same voltage before it is successfullydriven. Therefore, the crescent electrode can not only drive droplet at low voltage, but alsomake the droplet obtain the maximum velocity at the same voltage. Afterwards we analyzethe influence of different electrode layouts to the droplet splitting on the basis of dropletsplitting principle, and the results demonstrate that the opposite crescent electrode ispropitious to droplet splitting.The experimental results demonstrate that the suspended ground electrode EWODdevice can obtain higher droplet velocity and reduce the driving voltage relatively.Experiments in comparison with conventional square and jagged driving electrodesdemonstrate that the crescent electrode EWOD device possess the ability that reducing thedriving voltage. At the same time, the opposite crescent electrode is the most efficient todroplet splitting at lowest driven voltage among different electrode layout. We sense thedroplet’s position through detecting the equivalent capacitance of―EWOD device-droplet‖and prove that the location of the droplet can get feedback in real time to achievesuccessive droplet motion while the droplet’s movement is broken down.This paper involves multi-disciplinary intercross investigation and applications, thetheory, methods and conclusions in this paper can be useful andvaluable to a certain extent for further development of EWOD based DMF andresearch on manipulation of droplets.