Dynamic Contact Angles and Hysteresis under Electrowetting-on-Dielectric.

By designing and implementing a new experimental method, we have measured the dynamic advancing and receding contact angles and the resulting hysteresis of droplets under electrowetting-on-dielectric (EWOD). Measurements were obtained over wide ranges of applied EWOD voltages, or electrowetting numbers (0 ≤ Ew≤ 0.9), and droplet sliding speeds, or capillary numbers (1.4 × 10–5≤ Ca≤ 6.9 × 10–3). If Ewor Cais low, dynamic contact angle hysteresis is not affected much by the EWOD voltage or the sliding speed; that is, the hysteresis increases by less than 50% with a 2 order-of-magnitude increase in sliding speed when Ca 0.4. Data are interpreted with simplified hydrodynamic (Cox–Voinov) and molecular-kinetic theory (MKT) models; the Cox–Voinov model captures the trend of the data, but it yields unreasonable fitting parameters. MKT fitting parameters associated with the advancing contact line are reasonable, but a lack of symmetry indicates that a more intricate model is required. [ABSTRACT FROM AUTHOR]