Electrostatic Assist of Liquid Transfer in Printing Processes

Abstract

Printing processes are being explored for the large-scale manufacture of electronic de- vices. Transfer of liquid from one surface to another plays a key role in most printing processes. During liquid transfer, a liquid bridge is formed and then undergoes sig- nificant extensional motion. Incomplete liquid transfer can produce defects that can be detrimental to device operation. One important printing process is gravure, which involves transfer of liquid from micron-scale cavities at high speeds. Electric fields are sometimes used to enhance liquid transfer, a technique known as electrostatic assist (ESA). However, its underlying physical mechanisms remain a mystery. This thesis uses a combination of theory and experiment to understand the fundamental mechanisms by which electrostatic forces influence liquid transfer. Liquid transfer without electric fields and cavities must be understood before study- ing the mechanism of ESA. We develop one-dimensional (1D) slender-jet and two- dimensional (2D) axisymmetric models of this phenomenon and compare the resulting predictions with previously published experimental data. At relatively low stretching speeds, predictions from both models of the amount of liquid transferred agree well with the experimental data. When the stretching speed is high enough, the models predict that each surface receives half the liquid, in agreement with experimental observations. For intermediate values of the stretching speed, predictions from each model can deviate substantially from the experimental data, which we speculate is due to the influence of surface defects that are not included in the models. The 1D and 2D model are modified to include electrostatic effects. The liquid be- haves like a perfect (non-conducting), or leaky dielectric (poorly conducting) material. The liquid is confined between two plates, with the top plate having a constant electro- static potential while the bottom plate is grounded. For perfect dielectrics, application of an electric field enhances liquid transfer to the more wettable surface because it slows the surface-tension-driven breakup of the bridge, thereby allowing more time for the con- tact line to retract on the less-wettable surface. For leaky dielectrics, 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 interfacial charge. Experimental results confirm the enhancement of the amount of liquid transferred when the electric field is present, and the measured values are in good agreement with the predictions of the 1D perfect dielectric model. When one of the plate is replaced by a cavity, the presence of the cavity causes the contact line on the cavity wall to effectively pin and inhibits the liquid transfer. For perfect dielectrics, application of an electric field unpins the contact line on the cavity and leads to improvement of cavity emptying. For the leaky dielectrics, the presence of the surface charge does not further improve liquid transfer because of nearly zero electric tangential stress near the contact line on each surface.