Abstract
We have developed a simplified jetting model that predicts the printability of dilute, monodisperse polymer solutions in drop-on-demand (DoD) inkjet printing. Polymer molecules are modelled as finitely extensible non-linear elastic (FENE) dumbbells with fluid parameters chosen to fit the Zimm model. Three distinct jetting regimes are predicted, defined by the Weissenberg number Wi and the extensibility L of the molecules. The behaviour of the jet depends upon a critical factor that limits jet speed; regime 1 is restricted by fluid viscosity, regime 2 by elasticity and regime 3 by high strain extensional viscosity. We study two polymer solutions of disparate viscosity under different jetting conditions (i.e. print speed and nozzle geometry) and compare our results with experimental data and axisymmetric simulations. The maximum polymer concentration that can be jetted at a desired speed is found to scale with molecular weight Mw and is dependent on the solvent quality factor ν. We find that polymers can be stretched out in the print head for particular nozzle geometries, which has a considerable effect on the maximum polymer concentration that can be ejected. Furthermore, this ‘pre-stretch’ mechanism can fully extend molecules in the nozzle and consequently, molecules can undergo central scission due to high strain rates at the nozzle exit.
Highlights
The break-up of liquid jets is a classical problem in fluid mechanics [16] with a wide range of applications including spray painting, agricultural irrigation, pharmaceuticals and DNA sampling
Evidence of polymer scission occurring in inkjet printing was recently reported by A-Alamry et al [1] in jetting experiments, in which approximately monodisperse polystyrene solutions where jetted through two different print heads
In this paper we have tested the predictions of the simple model of jetting given by Hoath et al against both experimental observations and full numerical simulations
Summary
The break-up of liquid jets is a classical problem in fluid mechanics [16] with a wide range of applications including spray painting, agricultural irrigation, pharmaceuticals and DNA sampling. Hoath et al [24] have recently analysed experimental results on the jetting behaviour for mono-disperse, linear polystyrene dissolved in two solvents of disparate viscosity, jetted through different nozzle diameters at different print speeds They introduce a simple model (based on one originally proposed by Bazilevskii et al [4]), in which the fluid is modelled as a solution of finitely extensible dumbbells (FENE model) and the parameters are chosen to fit the Zimm model [11]. Central scission is observed for polystyrene in a number of good solvents under certain jetting conditions for a bounded range of molecular weights Since only those molecules that are fully extended can be fractured at the centre of the polymer chain [29], in this paper we investigate whether flow-induced central scission is possible under the conditions of DoD jetting
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