PhD Thesis Abstract

Copy of thesis is now available on D-Space.

Title

Fluid characterisation and drop impact in inkjet printing for organic semiconductor devices

Authors

Hutchings, Ian and Jung, SungJune

Abstract

An inkjet printer can deposit a very small volume of liquid with high positional accuracy, high speed and low cost. As a maskless, non-contact additive patterning method, inkjet printing technology is increasingly being explored as an alternative to lithography, etching and vapour deposition processes to pattern electrical conductors and thin films with applications in printed electronic devices. The functional inks used in many of the applications involve non-linear viscoelasticity and their behaviours in the context of inkjet printing have not been fully understood. This thesis aims to characterise Newtonian and non-Newtonian properties of inkjet fluids and identify the key parameters affecting drop impact and spreading processes.  Various fluid characterisation techniques such as the filament stretching rheometer and piezoelectric axial vibrator are explored. We propose an experimental method to assess the jettability of non-Newtonian inkjet fluids, without using an inkjet print head. The oblique collision of two continuous liquid jets leads to the formation of a thin oval liquid sheet bounded by a thicker rim which disintegrates into ligaments and droplets. Under certain conditions the flow structure exhibits a remarkably symmetrical “fishbone” pattern composed of a regular succession of longitudinal ligaments and droplets. Good correlation was found between the maximum included angle of the fishbone pattern and the maximum ligament length in the jetting experiments, which suggests that a test based on oblique impinging jets may be useful in the development of fluids for ink jet printing. High-speed imaging is used to analyse the impact and spreading of sub-30 μm drops of diethyl phthalate or polystyrene solutions in diethyl phthalate on to smooth glass surfaces with controlled wettability at speeds from 3 to 8 m s-1, under conditions representative of drop-on-demand inkjet printing. Data on drop height and spreading diameter are generated with high time and spatial resolution, over eight orders of magnitude in timescale. The effects of fluid viscosity and elasticity, which significantly affect jetting performance, are negligible throughout the whole deposition process, with no significant difference between spreading curves. The values of the fluid surface tension and the substrate wettability also have no effect on the kinematic, spreading or relaxation phases, but a marked influence on the wetting phase, in terms of the speed of expansion of the contact diameter and the final spreading factor.

Research Interests Sung June Jung, PhD Student

1. Imaging and characterisation of inkjet drop impact for OLED production process Ink-jet printing has been used for the production of high quality plastic electronic devices, including both organic LED displays and organic TFT backplanes, because of its ability to accurately control the placement of the organic materials on the substrate. This project aims to achieve a complete understanding of the mechanics of the liquid drop impact process and of fluid/substrate interaction in the context of industrial inkjet printing of Organic LEDs. (Supported by Cambridge Display Technology)
2. Atomisation produced by the oblique collision of two liquid jets The oblique collision of two identical jets leads to the liquid broadening radially from the impact point, creating a thin oval sheet bounded by a thicker rim. We focus on the regime where the impinging jets form a liquid sheet which then breaks up into a regular succession of ligaments and droplets, the so-called 'fishbone' pattern. This research offers a detailed insight into the effects of different degrees of viscoelasticity on the formation, destabilisation and the atomisation of a liquid sheet by oblique non-Newtonian jet impact. It is suggested that a test based on oblique impinging jets may be useful in the development of fluids for inkjet printing.