Research On Digital Microfluidic Devices For High-performance Droplet Separation Based On Electrowetting On Dielectric

Electrowetting-on-dielectric(EWOD)based digital microfluidics has been widely used due to its unique advantages such as simple structure,easy operation and strong driving capability.Droplet splitting is an important part of this technology.So far,traditional chip still has problems such as low separation efficiency,high voltage,and poor accuracy,which greatly limit its application.Therefore,it has important practical significance and research value for promoting the development of digital micro fluidics to research the droplet splitting process and propose optimization solutions for the problems existing in the separation process to meet the requirements of different fields.This paper first introduced the origin and principle of the electrowetting-on-dielectric,and explained some special phenomena(contact angle hysteresis,contact angle saturation,etc.).Then,the model of droplet splitting was established,and the N-S equation and level set method were introduced.Based on that,the droplet splitting in the closed system was numerically simulated,the model was verified to be correct.Finally,the static model of droplet splitting was analyzed,and the dynamic behaviors of droplet(electrowetting force and resistance on droplet,droplet velocity,etc.)were studied,which provide theoretics support for the optimal design of the chip.Secondly,the factors affecting the performance of droplet splitting(voltage,separation success rate and separation accuracy)were analyzed.The channel height and electrode size were related to the voltage.The voltage must increase with the increase of d/L with other conditions unchanged.The material of dielectric layer was also related to the voltage.And,the materials with high dielectric constant can be selected to reduce the voltage.It can be seen that the effective Triple Contact Line(TCL)also affects the voltage through the analysis of driving force on the droplet.The larger the effective TCL,the lower the voltage applied.The influences of channel height on the success rate of droplet splitting were analyzed.The droplets cannot be split on the single plate.In the closed system,the success rates of droplet splitting were limited by the channel height.The way of voltage application also affected the success rate.And,the success rate can be incresed by adjusting droplet position before separation.The initial position of the droplet affected the separation accuracy.The less the error of the droplet position at the beginning,the higher the separation accuracy.The stability of the separation process also affects the accuracy.The more stable the separation process,the higher the accuracy.Then,the chips were optimized according to the problems in the separation processes.Several digital microfluidic chips with the opposite layout of crescent electrodes were designed according to the theory of effective TCL.Numerical results show that the droplet on the crescent electrodes subjected the larger force and had the shorter separation time,therefore,it is expected to achieve the purpose of reducing the voltage.In order to avoid the limitation of channel height on the success rate,the EWOD chips using spacers for droplet splitting were proposed,and the effect of different spacer configurations(length,thickness and shape)on droplet splitting were discussed.The fan-shaped electrodes were designed according to the analysis of the reasons of poor separation accuracy on the square electrode and the reference of some previous studies(Y-shaped electrode,bow electrode and round dumbbell electrode).This chip can adjust the initial position of the droplets and ensure the stability of separation process,thereby greatly improving the accuracy and success rate of droplet splitting.Finally,the droplet splitting processes were researched experimentally.The results of droplet splitting on the traditional square electrodes showed that the voltage and the channel height had the great effects on the success rate.In addition,the success rates of second method(applied voltage on three electrodes firstly,and switched off the middle circuits after the droplet was elongated)were higher under the same conditions.Compared with the traditional electrode,the results show that the droplet splitting can be completed on opposite layout of crescent electrode at a lower voltage.The results of droplet splitting on the fan-shaped electrodes show that this chip can adjust the initial position of the droplet before separation.The separation process was more stable and the errors in the volume of the daughter droplets were smaller comparing with the square electrodes.Moreover,the success rates using this chip were higher.