Ink jet rheology

Ink jet processing is very sensitive to the formulation of the ink jet fluid and in particular  the rheology. Normally ink jet fluids have a  viscosity  below 5 mPas, close to that of normal solvents and water. Ink jet processing usually involves high jet speeds, typically 6 m/s and this results in high strain rate deformation with dominant extensional behaviour during drop formation outside the jet nozzle. Strain rates in excess of 1000 s-1 are often achieved. Ink jet fluids whilst generally being Newtonian in steady shear can be viscoelastic where the relaxation times involved can be very short, typically a millisecond or less. This takes rheology into an area of “extreme rheology” needing to characterise the viscoelasticity of “watery” fluids with short relaxation times. In addition nonlinear extensional behaviour of these low viscosity, viscoelastic fluids can be important.

Our own work on ink jet fluids started with measurements of high shear rate apparent viscosity and we soon discovered that ink jet formulations were essentially Newtonian even at shear rates in excess of 1000 s-1. Tuladhar, T.R. and Mackley, M.R. Filament stretching rheometry and break-up of low viscosity polymer solutions and inkjet fluids Journal of Non-Newtonian Fluid Mechanics, 148 97-108. (2008). In this paper we also described the filament stretching behaviour of ink jet formulations using the Multipass Rheometer (MPR) as a non linear filament stretching device. We now call this apparatus the Mk1. Cambridge Trimaster. Subsequently we built a Mk2 Cambridge Trimaster filament stretch apparatus that used a simple stepper motor drive, rather than the more complex MPR servo hydraulics. D.C.Vadillo, T.Tuladhar, A.C. Mulji, S. Jung, S.D. Hoath,and M.R. Mackley Evaluation of the inkjet fluid’s performance using the “Cambridge Trimaster” filament stretch and break-up device  Journal of Rheology 54, 2 .261-282 (2010)

In order to characterise the high frequency linear viscoelastic response of ink jet fluids  a Piazo Axial Vibrator (PAV) that had been pioneered by the late Prof Pechold from the University of Ulm was used. This is a brilliant and delicate apparatus that is capable of exploring linear viscoelasticity at high frquencies.  D.C. Vadillo, T.R. Tuladhar, A.C. Mulji and  M.R. Mackley The rheological characterization of linear viscoelasticity for ink jet fluids using piezo axial vibrator and torsion resonator rheometers. Journal of Rheology, 54, 4. 781-795 (2010 )

The link between ink jet performance and ink jet rheology is given in,    S. Hoath,  I. M.Hutchings, G.D. Martin, T.R.Tuladhar, M.R.Mackley and D. Vadillo    Links Between Ink Rheology, Drop-on-Demand Jet Formation, and Printability  Journal of Imaging Science and Technology 53, 4, pp. 041208-(1-8) (2009).

The way in which Trimaster filament stretch, thinning and breakup can be modelled is told in two papers.       M. Tembely, D. Vadillo, M. R. Mackley and  A. Soucemarianadin The matching of a “one-dimensional” numerical simulation and experiment results for low viscosity Newtonian and non-Newtonian fluids during fast filament stretching and subsequent break-up  J. Rheology. 56, 159- 184   (2012).

D. C. Vadillo, M. Tembely,  N.F. Morrison, O. G. Harlen, and M. R. Mackley The matching of polymer solution fast filament stretching, relaxation, and break up experimental results with 1D and 2D numerical viscoelastic simulation. J. Rheology . 56, 1491-1516 (2012)

A summary of the ink jet rheology story is given below from a 2009 Seminar presentation at Monash University, Australia (Unfortunately some of the photographs and graphs have not come out in the reproduction).

Papers relevant to Ink Jet Rheology and Processing