Modelling the jetting of dilute polymer solutions in drop-on-demand inkjet printing

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 M 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.