Dynamic Contact Angles and Hysteresis under Electrowetting-on-Dielectric

Abstract

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 Ew or Ca is low, dynamic contact angle hysteresis is not affected much by the EWOD voltage or the sliding speed, i.e., the hysteresis increases by less than 50% with a two order-of-magnitude increase in sliding speed when Ca < 10-3. If both Ew and Ca are high, however, the hysteresis increases with either the EWOD voltage or the sliding speed. Stick-slip oscillations were observed at Ew>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 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.

Contact angle hysteresis at the nanoscale: a molecular dynamics simulation study

“…CAH is also sensitive to the volume fraction of the microbead suspension [14]. Experiments on CAH were carried out under various situations such as electrowetting-on-dielectric [15] and asymmetric vibration [16]. Recently, Bormashenko et al presented an ingenious experimental technique to measure CAH based on the slow deformation of the droplet [3].…”

Evaluation of the threshold trimming method for micro inertial fluidic switch based on electrowetting technology

“…Wyatt C. Nelson et al have studied the dynamic contact angle and hysteresis under electrowetting-on-dielectric. 7 Measurements show that if E w or Ca (capillary number) is low, dynamic contact angle hysteresis is not affected 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 < 10 −3 . If both E w and Ca are high, the hysteresis increases with either the EWOD voltage or the sliding speed.…”

A Langevin model for fluctuating contact angle behaviour parametrised using molecular dynamics

“…Strong elastic bonds are applied at the surface with sixteen atom molecules used to hold the liquid region together and prevent evaporation with Tanner's law and MKT also reproduced by the presented results. Experimental studies of liquid bridges also provide some insight into the general behaviour where the study of Wang and McCarthy 20 observed pinch off while Nelson et al 21 studied the angle sliding rate dependence in the presence of electrowetting.…”