Electric Field Enhanced Spreading of Partially Wetting Thin Liquid Films

Abstract

Equilibrium and dynamic electrowetting behavior of ultrathin liquid films of surfactant (SDS) laden water over silicon substrate (with native oxide) is investigated. A nonobtrusive optical method, namely, image analyzing interferometry, is used to measure the meniscus profile, adsorbed film thickness, and the curvature of the capillary meniscus. Significant advancement of the contact line of the liquid meniscus, as a result of the application of electric field, is observed even at relatively lower values of applied voltages. The results clearly demonstrate the balance of intermolecular and surface forces with additional contribution from Maxwell stress at the interline. The singular nature of Maxwell stress is exploited in this analysis to model the equilibrium meniscus profile using the augmented Young-Laplace equation, leading to the in situ evaluation of the dispersion constant. The electrowetting dynamics has been explored by measuring the velocity of the advancing interline. The interplay of different forces at the interface is modeled using a control volume approach, leading to an expression for the interline velocity. The model-predicted interline velocities are successfully compared with the experimentally measured velocities. Beyond a critical voltage, contact line instability resulting in emission of droplets from the curved meniscus has been observed.

Magnetowetting of Ferrofluidic Thin Liquid Films

“…It could be inferred from the augmented Young Laplace equation35455060, that the rise in the curvature of the capillary region applies a net capillary suction causing the film to retract. The suction is predominantly felt by the thinner part of the film (first few fringes) wherein, the relative magnitude of the forward acceleration is lower in comparison to those in the vicinity of the capillary region.…”

“…. Using the lubrication approximation35456061, the governing equation for liquid flow inside a thin film could be modified so as to include the body force (magnetic force) term and is given as…”

“…Subjecting a thin liquid film to external perturbations resulting in the actuation of liquid motion is used to probe and manipulate the transport phenomena near the contact line region. The analysis of the dynamics of partially wetting thin liquid films, experiencing an external perturbation, is a complex process353637. Evaporating thin liquid films may provide very high heat transfer coefficients across the liquid-vapour interface3839.…”

“…. Since then, the response of thin liquid films subjected to various modes of perturbation such as thermal3841 and electrical35 have been investigated and reported. Extensive reviews on intermolecular forces and interactions present in a thin liquid-solid-vapour system have been presented by Derjaguin et al 42…”

“…probed the dynamics of evaporating and condensing thin liquid films subjected to thermal perturbations. Bhaumik et al 35. studied the contact line dynamics of thin liquid films and reported the contact line instabilities associated with satellite drop formation, subjected to electrical actuation.…”