A numerical study of microfluidic droplet transport in a parallel-plate electrowetting-on-dielectric (EWOD) device

The fluid dynamics of microwater droplet transport in a parallel-plate electrowetting-on-dielectric (EWOD) device have been investigated via a numerical model. The transient governing equations 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 and volume-of-fluid method. A continuum surface force model is employed to evaluate surface tension at the interface. A simplified model is adopted for the viscous stresses exerted by the parallel plates at the gas-liquid interface in conjunction with contact angle hysteresis implemented as a crucial element in EWOD modeling. Excellent agreement has been achieved between the numerical and published experimental results. Special attention has been focused on some localized areas near the ON/OFF electrode border where the transport process is primarily influenced. A dimensionless curvature has been introduced and a critical value has been identified beyond which the droplet would split during the transport. A parametric study has been performed in which the effects of several crucial parameters including initial droplet shape, static contact angles, contact angle hysteresis, viscous stress, channel height and electrode size on the transport process have been revealed.