Research On The Droplet Movement In Electrowetting On Dielectric Based Ditigal Microfluidic System

As an emerging technology, digital microfluidics has a good prospect in the field of biology,medicine, and chemical with the advantage of less reagent consumption, configurable, parallelprocessing and easy to automate. The basic performance and dynamic characteristics of closedelectrowetting on dielectric(EWOD) digital microfluidic system was studied through theoreticalanalysis, modeling and simulation, experimental validation and model improvement. The majorcontent includes:1) Research on the basic theory and stress analysis of the droplet motion.2) The hydrodynamic model was established and basic performances such as distributionof the electric field, effect of the dielectric layer thickness and the contact angle change withvariation of driving voltage were discussed based on the two-dimensional model.3) Dynamic characteristics including the changes of contact angle, the pressure and speedof the droplets, the stretch of droplet, the impact of electrode shape, electrode size and gasbetween plate were studied based on three-dimensional model.4) The experimental system was set up and the characteristics including the contact anglechanges with variation of applied voltage, saturation of the contact angle, the effect of dielectriclayer thickness, the influence of the hydrophobic layer, contact angle hysteresis, the thresholdvoltage, velocity under different voltages, repeatability of movement and the breakdownphenomenon of dielectric layer were validated.5) With experimental data, the coefficient of the contact line friction and shear resistancewith plates was calculated. Then the predictive value based on improved model was comparedwith experimental data. The main innovatin points including:1) Derivation of the optimal initial volume for droplet movement based on force analysis.In ideal situation, when the radius r of droplet and the length L of square electrode satisfied therelationship of r≤2L, driving force increased with r, while r>2L, the driving force isconstant. With increasing of radius, the droplets might interfere each other and couldn’t becontrolled accurately. In a word, the driving efficient was the best when r=2L.2) In most simulation reported about digital microfluidic system based on EWOD, VODwas adopted to tracking the interface of droplet. Some researchers used level set method to studythe open system and some others discussed the contact angle change with2D model. To theknowledge of author, it hadn’t been reported about simulating the moving and splitting processof droplet with level set method.3) Influence of electrode shape, electrode size and gas between plates were discussed with3D model, and some conclusions were drawn, including: while the threshold voltage ofinterdigital electrodes was a little lower than the square electrodes’, the shape of electrode didn’taffect the average velocity obviously. The splitting velocity grew faster with the decreasing ofdroplet volume. When the volume was fixed and the ratio of droplet the diameter R to gapbetween plates was in the range of6-17, the smaller of ratio, the easier to split. Out this range,splitting would fail if the ratio was smaller and broken daughter droplets were produced if theratio was bigger.4) The contact line friction and shear resistance with plates were taken into account inimproved model, which increased the accuracy of simulation result greatly.5) Laser engraving PCB board was adopted in chip fabrication and the smallest distancebetween lines was about50μm. ITO glass was used as top plate, and a thin film of paryleneabout0.5μm was coating as dielectric layer. Laser engraving PCB method reduced the costgreatly comparing with the clean room processing. Also, the fabrication precision was improvedsignificantly comparing with conventional PCB. It might lead a new way to relative low-cost andhigh-precision experiment method.