Computational modelling for 3D 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.

This project will be broken down into two stages. In the first stage, we will study the induced capillary flows in an evaporating 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', 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. More exotic phenomena such as Marangoni flow and Rayleigh-Benard convection, often exhibited in fluids with binary/tenary composition, may also be included in our modelling. Current studies are limited to an axisymmetric drop. However, in future work we hope to mimic the experimental procedure by considering a liquid line which is built up dropwise. In the second stage, the interaction between the individual constituents of the mixture will be studied in finer detail, perhaps through the use of a reaction-diffusion type model.

To study an effect in isolation, asymptotic and semi-analytical methods may be used. However, a comprehensive study of all these effects will require a primarily numerical treatment, and will be conducted using the finite element library Oomph-lib [Heil, Hazel 2006].

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.