Poster session and reception
Tuesday, March 13, 2018 - 3:15pm - 4:45pm
- Ion injection mechanism of contact angle saturation in electrowetting
Tetsuya Yamamoto (Nagoya University)
Electrowetting is a process with which the contact angle θ(sub)w of a droplet decreases with increasing applied voltage. In a typical experiment, an aqueous droplet, immersed in an oil medium, is prepared on a dielectric film and voltage is applied between the droplet and an electrode at the opposite side of the dielectric film. With small applied voltage, cosθ(sub)w is a quadratic function of applied voltage, in agreement with the Young-Lippmann theory. For larger applied voltage, cosθ(sub)w deviates from the prediction of the Young-Lippmann theory and eventually saturates. We here use the Poisson-Boltzmann theory to predict that ions are injected from the aqueous droplet to the oil due to the large applied voltage and this deviates the contact angle from the Young-Lippmann theory. This theory predicts that the contact angle saturation is not a real saturation, but a broad maximum of cosθ(sub)w.
- Electrostatic Assist of Liquid Transfer between Flat Surfaces
Chung-Hsuan Huang (University of Minnesota, Twin Cities)
Transfer of liquid from one surface to another plays a vital role in printing processes. During liquid transfer, a liquid bridge is formed and subjected to substantial extension, but incomplete liquid transfer can produce defects that are detrimental to the operation of printed electronic devices. One strategy for minimizing these defects is to apply an electric field, a technique known as electrostatic assist (ESA). However, the physical mechanisms underlying ESA remain a mystery. To better understand these mechanisms, slender-jet models are developed for both perfect dielectric and leaky dielectric axisymmetric Newtonian liquid bridges with moving contact lines. Nonlinear partial differential equations describing the evolution of the bridge radius and interfacial charge are derived, and then solved using finite-element methods. For perfect dielectrics, application of an electric field enhances liquid transfer to the more wettable surface over a wide range of capillary numbers. The electric field modifies the pressure differences inside the liquid bridge, and as a consequence, drives liquid toward the more wettable surface. For leaky dielectrics, charge can accumulate at the liquid-air interface. Application of an electric field can augment or oppose the influence of wettability differences, depending on the direction of the electric field and the sign of the surface charge. Flow visualization experiments reveal that when an electric field is applied, more liquid is transferred to the more wettable surface due to a modified bridge shape that causes depinning of the contact line. The measured values of the amount of liquid transferred are in good agreement with predictions of the perfect dielectric model.