A study on the optimization of viscoelastic ink and processing conditions for the formation of Taylor cone jet in electrohydrodynamic jet printing

Abstract

Electrohydrodynamic(EHD) jet printing is considered to be a promising tool for making nano-scale patterns as the demand for miniaturization of electronic devices increases. The formation of Taylor cone jet is a key process in EHD jet printing that is used to produce high resolution patterns. It is well known that the formation of Taylor cone jet mainly depends on the processing conditions, geometry conditions, and the liquid properties: elasticity, viscosity, conductivity, surface tension, and dielectric constant. Especially, even though the viscoelastic inks are complex fluids composed of particles, binder, and solvent, most of previous researches have assumed the ink as a Newtonian fluid. To obtain uniform and high resolution printing products, controlling the physical properties of viscoelastic ink and predicting the processing ranges for Taylor cone jet are very important. In this study, therefore, we design the desired properties of viscoelastic ink and optimize the processing conditions for the formation of Taylor cone jet by investigating the effect of rheological and electrical properties of low viscous elastic fluids, which are suitable materials for EHD jet printing. Firstly, we organize the seven dimensionless number based on the all parameters involved in the EHD jet printing to systemize the variables affecting the Taylor cone jet. Dimensionless flow rate (α) and dimensionless voltage (β) are used for dimensionless processing condition. Two dimensionless numbers related to physical properties of ink, elastic parameter (ξ) and viscosity parameter (χ), are key parameter that determines whether elasticity and viscosity has an effect on the Taylor cone jet formation, respectively. We controlled the ξ and χ using two model systems designed to control the elasticity and viscosity of the ink independently. The results can be summarized in terms of two parameters: elasticity parameter,ξ, and viscosity parameter,χ. The increase in elasticity widens the range of voltage for Taylor cone jet zone, while the range of flow rate remains independent of elasticity. Especially, an increase of elasticity increased the initial voltage, the voltage at which the Taylor cone jet first appears. The results demonstrated that elasticity enhanced the inward direction of normal stress on the surface of conical shape. The effect of elasticity is dominant for ξ>1 while it is nearly negligible for ξ<1. When the viscosity is increased, the Taylor cone jet zone widened mainly by the flow rate when χ<1, while the voltage stabilizes the Taylor cone jet for χ>1 . As a result, the viscoelasticity improves the stability by expanding the processing range for Taylor cone jet. Secondly, we investigated the effect of the interaction between the electrical and rheological properties of the ink by designing model systems that control both the electrical conductivity and the viscoelasticity of the ink to observe how they affect the Taylor cone jet formation. The ink with high electrical conductivity produced the conical shape with large surface area. It induces a higher initial voltage. The results demonstrated that the large surface area is needed to accumulate sufficient charge for the formation of Taylor cone jet. After forming the Taylor cone jet, the ink with high electrical conductivity increased the final voltage, the voltage at which the jet becomes unstable by stabilizing the conical shape. When the electrical conductivity increased by the same rate, the voltage ranges for Taylor cone jet decreased as the inks viscoelasticity increased. The results implied that the viscoelasticity obstructs the charge transport to the surface of conical shape. Consequently, increased viscosity and elasticity also lead to the similar result with increase of electrical conductivity: increase of the initial and final voltages, which can be attributed to the slower charge transport that minimizes the stabilizing effect of the inks electrical conductivity. This study suggests guidelines for optimization of the process parameter and design of desired properties of the viscoelastic ink to produce Taylor cone jet. All variables affecting the EHD jet process are systemized by dimensional analysis. The effect of rheological and electrical properties of viscoelastic ink on Taylor cone jet was investigated by designing the model systems. The relationship between the properties of ink and the processing conditions for Taylor cone jet is shown through the processing window maps by using the dimensionless processing conditions, dimensionless flow rate (α), and dimensionless voltage (β). This study will contribute to the optimal design of viscoelastic ink and processing conditions on the formation of Taylor cone jet in EHD jet printing.