Experimental and theoretical investigations of the threshold actuation voltages of aqueous solutions, glycerol–water solutions, and ethanol–water solutions on a coplanar electrode device

In digital microfluidics, actuation voltage is critical in droplet manipulation using electrowetting-on-dielectric. Compared to parallel electrode devices, the droplet manipulation on coplanar electrodes requires a higher actuation voltage due to its structure. This study derives an equation related to the threshold actuation voltage and many critical parameters (applied frequency, surface tension, fluid conductivity, droplet volume, dielectric thickness, etc.). Our developed model has three innovations: (1) liquid and device parameters related to the threshold actuation voltage are integrated into the model, (2) the absolute value of the complex permittivity of the fluid is added into our model to clarify the conductivity-dependent and frequency-dependent threshold actuation voltage, and (3) the extrand model is used to derive the effects of the surface tension, contact angles, and droplet radius on the threshold actuation voltage. Additionally, we used many solutions for biochemical and biomedical assays—DI (Deionized) water, DMEM (Dulbecco's Modified Eagle Medium), PBS (phosphate buffered saline), glycerol–water solutions, ethanol–water solutions, potassium chloride solutions, and sodium chloride solutions—to verify the developed Vth in our model. By using the model and conducting an experiment, we investigated the Vth of different liquids on coplanar electrodes. Our model can successfully explain the difference of Vth between these solutions on coplanar electrode devices. The model and experimental data can be utilized to predict the actuation voltage for a broad range of buffers and organic solvents on coplanar electrodes.