Computational modelling for inkjet-printed electronics.

PEDOT is an electrically-conductive polymer which shows potential for use in small-scale electronic components. Experimentally, thin films of PEDOT been shown to exhibit a conductivity of up to 6300 S/cm [Gueye et al. 2020], only a factor of 10 smaller than most conductive metals. This conductivity, however, is highly dependent on the method of preparation. In an experimental setting, PEDOT is first introduced in the form of the PEDOT:PSS complex, which is dissolved in a water-DMSO co-solvent mixture and then jetted onto a substrate. As the co-solvents evaporate, some of the PEDOT:PSS dissociates, leaving a residue containing both PEDOT-rich and PSS-rich regions. For optimal performance, this residue should exhibit good interconnectivity between the PEDOT-rich sites and should have a thickness profile which is close to uniform. It is therefore important that the dynamics of each constituent during solvent evaporation are well-understood.

The fluid dynamics of drop evaporation has a strong influence in the morphology of the PEDOT deposit. Thus, we will examine the evaporation-induced capillary flows in an evaporating sessile droplet and how they lead to contact-line aggregation of solute particles. In the literature, this is a ubiquitous phenomenon known as the 'coffee ring effect' (CRE) and is undesirable in most industrial settings (including the one outlined here). In addition to understanding the formation of coffee rings, we hope to explore which experimentally accessible physical parameters (ambient temperature, humidity, particle size/shape etc.) can be exploited to suppress them. This will include some novel work in surface assembly, which has been demonstrated experimentally at high evaporation rates [Li 2016] but lacks a mathematical framework that gives results consistent with experimental data.

Many existing CRE models are limited to axisymmetric geometries due to complications in tracking fronts of jammed particles. In addition, these models cannot capture the full evaporation process due to topological changes in the drop surface that arise in the late stages of drying. Using the finite element library Oomph-lib [Heil, Hazel 2006], we will develop a novel computational framework for CRE that remedies both these issues. This framework will allow us to study the influence of contact line curvature on the local CRE intensity and apply our model to printed lines which are built up dropwise.

The analytical and computational work will be carried out alongside partners at the Centre for Additive Manufacturing (CfAM) in Nottingham, where tailored experiments will allow us to benchmark our results and give insight into how to capture the relevant physics in our modelling.