In this paper, the splitting of microdroplet in a closed electrowetting-based digital microfluidic system has been studied via a numerical model. The governing equations for the fluid flow are solved by a finite volume formulation with a two-step projection method on a fixed computational domain. The free surface of the liquid is tracked by a coupled level-set and volume-of-fluid method, with the surface tension at the free surface computed by continuum surface force scheme. Contact angle hysteresis is implemented as an essential component of electrowetting modeling, and a simplified viscous force model is adopted to evaluate the viscous stress based on the Hele-Shaw model. Excellent agreement has been achieved between the numerical and published experimental results. A parametric study has been performed in which the effects of viscous stress, channel height, static contact angles, contact angle hysteresis, and electrode size on the splitting process have been analyzed. Three distinct splitting modes, which are “splitting with satellite droplet,” “normal splitting,” and “splitting cessation,” have been discussed. Based on the competition between the curvature in the z-direction (κz) and that on the x-y plane (κxy), the physical mechanism that separates the splitting into these three modes has been revealed. More importantly, a dimensionless parameter κ̃ has been proposed, which can be used for (a) determining the splitting mode and (b) estimating satellite droplet volume for electrowetting-induced droplet splitting process.
Microdrop generation with excellent controllability and volume precision is of paramount significance for a large variety of microfluidic applications. In this work, we propose a new configuration comprising only stripped electrodes of rectangular shape for the closed electrowetting-on-dielectric digital microfluidic (EWOD DMF) system and investigate its parallel microdrop generation outcomes via a numerical approach. The microfluidic droplet motion is solved by a finite-volume scheme on a fixed computational domain. The numerical model is verified by an experimental study of microdrop production from an EWOD DMF device with three different electrode designs. After model verification, we examine the influences of the equilibrium contact angle and the spacing of the microchannel on stripped electrode based microdrop generation outcomes and discover five different regimes including the phenomena of satellite droplet formation and separation cessation. Despite the various generation outcomes, the daughter droplet size is found to vary linearly with a dimensionless EWOD parameter κ*. More importantly, for all successful generations, the deviation of the daughter droplet size from that of the stripped electrode is smaller than 3.5%, which even reaches zero in proper conditions. This new configuration can be utilized as a convenient alternative for electrowetting-induced parallel microdrop production with excellent precision and controllability.
Microwater droplet splitting and merging in a parallel-plate electrowetting-on-dielectric (EWOD) device have been studied numerically. The transient governing equations for the microfluidic flow are solved by a finite volume scheme with a two-step projection method on a fixed computational domain. The interface between liquid and gas is tracked by a coupled level set (LS) and volume-of-fluid (CLSVOF) method. A continuum surface force (CSF) model is employed to model the surface tension at the interface. Contact angle hysteresis which is an essential component in EWOD modeling is implemented together with a simplified model for the viscous stresses exerted by the two plates at the solid–liquid interface. The results of the numerical model have been validated with published experimental data and the physics of droplet motion within the EWOD device has been examined. A parametric study has been performed in which the effects of channel height and several other parameters on the fluid motion have been studied.
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