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Electrohydrodynamic (EHD) inkjet or E-jet printing is a high-resolution, direct write, non-contact patterning technology for micro fabrication. High registration accuracy, vacuum free operation along with the ability to handle flexible stretchable substrate makes this new fabrication methodology a suitable candidate for flexible electronics application (Onses et al., 2015). The capability of dispensing a wide range of materials both liquid as well as colloidal solutions of solid nanoparticles (Kim et al., 2011), makes this emerging technology diverse. It finds its application in printed electronics (Kamyshny et al., 2014), biotechnology (Jayasinghe et al., 2005), human wearable flexible sensors (Nothnagle et al., 2015), display devices (Park et al., 2013), combinatorial chemistry (Kim et al., 2010) etc. Being a simple process with no complicated pre or post processing operation, it could be a suitable alternative to lithography technology. Layered manufacturing or additive printing is also an added feature of this technology (Qin et al., 2017). In conventional inkjet, there are mainly two kinds of actuation mechanisms used for material ejection namely thermal actuation and piezo-based actuation. But limited resolution operation is one of the inherent shortcomings of these technologies. It has been reported that the maximum achieved resolution in the range of 10-20 µm (Derby et al., 2010). To get higher resolution deposition small size nozzles are required, as the printing dimension is about two times of the nozzle dimension (Back et al., 2012). But incorporating smaller size nozzle creates nozzle clogging problem with high viscous ink material. Moreover, with small size nozzles, the ejection pressure becomes very high. The fabrication of these small size nozzles itself is a costly affair. The aforementioned limitations can be successfully mitigated by E-jet printing. The functional material gets ejected at the tip of the Taylor cone (Taylor, 1964) instead of the nozzle tip. And as the tip of cone dimension is smaller than the nozzle dimension, the high-resolution operation can be achieved with conventional nozzle configuration. Both continuous as well discrete features can be produced with this direct write technology.
In EHD inkjet printing a potential difference is applied between the nozzle and the substrate. The applied electrical field causes the charges in the ink to move towards the tip of the nozzle. Due to mutual columbic repulsion in the pendant liquid meniscus, it produces an electric stress which deforms the liquid meniscus into a conical form towards the substrate. This deformed shape is known as Taylor’s cone. When the applied electrostatic force overcomes the restraining surface tension force of the liquid material, a droplet or continuous jet of functional material is produced (Yudistira et al., 2011; Nguyen & Byun, 2009). The main components of EHD printing system consist of: a pressure chamber to supply the ink at the tip of the nozzle, a voltage source to apply the potential difference, a conducting nozzle to dispense the material, a substrate on which printing is done, a CNC stage for mounting the substrate and a vision system to observe the droplet or jet formation process. Depending on the geometry of the product, CNC stage moves. The following schematic diagram shows different components of EHD inkjet system (see Figure 1).
Figure 1. Schematic Diagram of EHD Jet Printing