Formation and Modeling of Droplet Ejection for Electrohydrodynamic Inkjet Printing

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EHD(Electrohydrodynamic) inkjet printing based on electrospray and EHD discharging for tiny droplet/jet is known to be able to generate higher resolution and higher aspect ratio of patterns than the conventional inkjet printing(Lee, 2014). EHD inkjet printing is a suitable ejection mode for industrial inkjet printing technology in micro-dripping mode, pulsed cone jet mode and continuous cone jet mode among ejection modes at the classification of electrospray phenomenon. The meniscus shape of continuous cone jet mode and pulsed cone jet mode are most similar to the conical Taylor cone(Taylor, 1964). The analysis of meniscus of conical shape(Taylor cone) which are pulsed cone jet mode and continuous cone jet mode has been conducted long time study on the principle of the meniscus in electric field(Collins 2007 and Gennari 2015). However, the study on micro-dripping mode, which is similar to the hemisphere or elliptical shape and discharges one droplet, has not been well established. For the modeling of ejection at micro-dripping mode, the meniscus is divided into layers (see Fig. 1), and use the method of calculating the force (surface tension, pressure and electric force) acting on each layer based on the coordinates on the nozzle and analyzed how each force generates and changes at the boundary of meniscus. To obtain the exact electric field intensity and surface tension acting on the meniscus in micro-dripping mode, the coordinate values of the meniscus were obtained through image processing, and the radius of curvature of each layer divided by the curve fitting was obtained(see Fig. 2 and 3). Using the obtained values, the sum of electric force and pressure of the divided layers according to the respective meniscus shape during ejection at micro-dripping mode and the surface tension were compared. Fig. 4 is a graph comparing the sum of electric force and pressure of the divided layers and the surface tension at the meniscus shape with the nozzle radius being x axis when ejection at micro-dripping mode. As the meniscus gets closer to the ejection, the equilibrium point (color circle of each graph) gradually goes to the center of the nozzle. This is because the meniscus interface at the center of the nozzle is initially hemispherical or elliptical shape, so that the distance from the substrate is the closest, the electric force is pulled in the +z direction by the electric force. Since the surface tension of the interface is larger in right part of the equilibrium point, the meniscus is pulled in the -z direction. For this reason, the shape of the meniscus gradually changes, so that the size of the portion to be torn off (red circle on image) is gradually reduced, which is a condition for forming droplet. In the final stage of ejection (ocher graph), left part of the equilibrium point is formed as droplet because the electric force is much larger than the surface tension, and the right part of the equilibrium point is the stable part of the equilibrium state for preparing next ejection.

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Journal: TechConnect Briefs
Volume: 4, Informatics, Electronics and Microsystems: TechConnect Briefs 2018
Published: May 13, 2018
Pages: 181 - 184
Industry sectors: Advanced Materials & Manufacturing | Sensors, MEMS, Electronics
Topic: Inkjet Design, Materials & Fabrication
ISBN: 978-0-9988782-1-8