Numerical Investigation Of Droplet Splitting Process In Digital Microfluidic Systems

Digital microfluidic(DMF)chips based on electrowetting of dielectric(EWOD)have been rapidly developed in past decades due to its distinguished advantages such as energy saving,low sample consumption.Therefore,EWOD-based DMF platform has become one of the most famous hotspots in some fields such as bio-chemical,medical,energy systems and so on.Due to different characteristics of macro and micro fluid flow,the inner mechanism of droplet motion based-on DMF chips at the microscale is still unclear.The accuracy of generated droplet volume is one of the most important focuses and difficulties in DMF technology at home and abroad.What's more,the droplet moves extremely fast with complicated topological geometry deformation,and the experiment figures can't be captured absolutely due to many influenced factors.This paper concentrates on numerical investigation of droplet splitting and generation process,simultaneously based-on theoretical analysis and experimental investigations in references.Firstly,we select EWOD method to actuate droplet.The EWOD method is based on electrowetting of dielectric effect,which imposes voltage on droplet to change the surface tension,enventually causing a certain pressure difference on surface to actuate droplet to move toward specified direction.Secondly,based on numerical formulation,a complete numerical model of the micro droplet moving and splitting process is established in this paper,which includes the influences of some key EWOD chips parameters.After that,A Finite-Volume formulation with a two-step projection method is used to solve droplet splitting on a fixed computational domain besed-on EWOD chips.When droplet is moving,the liquid-gas interface is tracked by a coupled level set and volume of fluid(CLSVOF)method,and then the surface tension force regarded as body force is computed by continuum surface force(CSF)method.While based on the Hele-Shaw cell model,the 3D model in this paper is transformed into 2D,while a simplified viscous force scheme derived from the Couette flow model is adopted to evaluate the viscous force exerted by the parallel plates when droplet is moving,and theeffect of contact angle hysteresis is embeded as an significant component of EWOD numerical scheme.Thirdly,based on EWOD chips,the generation and splitting process have been simulated with three distinguished electrode arrangements,namely square pattern,uneque square pattern and strip pattern.In square electrode pattern,we summarize three typical splitting modes: satellite droplet,normal splitting and splitting cessation,and the parameters study has been discussed.Significantly,a dimensionless parameter has been proposed todetermining the droplet splitting mode and volume of satellite droplet.we simulate droplet generation process on three distinguished electrodes from published reference.Finanlly,we propose a uneque strip electrode pattern for droplet splitting with controllable volume,parameter study which influences droplet splitting has been discussed.This paper concentrates on numerical formulation to simulate droplet splitting process,which also combines theoretical and experimental research.A comprehensive and accurate numerical model for EWOD-induced droplet splitting and sub-drop generation has been proposed,which provides mature and practical significance for droplet actuation,and the achievements in this paper have made great contribution to structural optimization and parameters setting for DMF platforms.