Experimental and theoretical investigation of microfluid mechanics of liquid actuation between parallel plates

Liquid transport between parallel electrodes by electrowetting on dielectric (EWOD) is an effective method for microfluid manipulation in many applications. However, the mechanics of liquid motion are complex since many parameters and forces (i.e., electric force, surface tension force, contact line friction force, and viscous force) influence the liquid motion. In this study, we present a new electromechanical model to analyse the transient dnamics of liquid motion. Three common liquids (NaCl and KCl solutions: 3*10− 4 S/m and DI water: 1.5*10− 3 S/m) are used to verify our model at three frequencies (i.e., 1 kHz, 10 kHz and 100 kHz). Compared to previous studies, there are three novel features of this study: (1) the absolute value of complex permittivity was added into the model to clarify conductivity-dependent and frequency-dependent liquid motion; (2) the conductivity-dependent and frequency-dependent transient dynamics of liquid motion were investigated; and (3) four conductivity-dependent and frequency-dependent forces were investigated. In the model, the electric force and surface tension force are constant during the full process of liquid motion. The liquid moves slowly with time since the viscous force increases with time. In addition, NaCl and KCl solutions also showed similar behaviour since their conductivities and other experimental conditions are the same. In this work, the experimental data showed good agreement with theoretical predictions. Our model can successfully clarify the conductivity-dependent and frequency-dependent transient behaviour of liquid motion. The developed model can be used to predict the dynamics of fluids in parallel-plate microfluidic devices for many applications.