Complex inkjets: particles, polymers and non-linear driving

Can inkjet technology revolutionise manufacturing processes as we know them? By extending the existing benefits of inkjet methods to attain the speed, coverage and material diversity of conventional printing, we can transform inkjet from its present status as a niche technology into a mainstream process, with the UK as a major player. However, we require a better understanding of the science underlying the formation of small droplets and the effect of complex additives. First, we highlight key inkjetting methods and discuss well-known effects that particles and polymers have on jet evolution. We describe how jetting and filament-thinning experiments can be used to measure key characterisation parameters and how these techniques can be modelled via an established simulation method. Second, we review the literature exploring jet stability and break-up, including the Rayleigh stability analysis and universal self-similar thinning laws. In Chapter 3, we develop a simple one-dimensional model. First, we model particulate effects on the decay of a liquid bridge and identify three thinning regimes. In particular, we describe a mechanism for acceleration, which agrees quantitatively with experiments. In contrast, the addition of viscoelasticity retards thinning processes and delays break-up. Our viscoelastic jetting model demonstrates the theoretical exponential thinning law, `beads-on-string' structures and is in quantitative agreement with axisymmetric simulations. In Chapter 4, we develop a simplified drop-on-demand jetting model to predict the printability of polymer solutions. We demonstrate three known jetting regimes and the predicted `jettable' concentration threshold is in quantitative agreement with experimental data. Using axisymmetric simulations, we identify a `pre-stretch' mechanism that is able to fully extend polymers within the nozzle. Consequently, we show that molecules can undergo central scission due to high strain rates at the nozzle exit. In Chapter 5, we simulate a one-dimensional continuous inkjet using an adaptive mesh technique. We explore non-linear behaviour caused by finite-amplitude modulations in the driving velocity profile, where jet stability deviates from Rayleigh behaviour. We identify a modulation range where pinching becomes `inverted', occurring upstream of the filament connecting the main drops, rather than downstream. This behaviour can be controlled by the addition of a second harmonic to the initial driving signal. Our results are compared to full axisymmetric simulations in order to incorporate the effects of nozzle geometry.