Effects of fluid viscosity on drop-on-demand ink-jet break-off

Abstract

Simulations erf the jetting of Newtonian fluids from drop-on-demand print heads show that the radial jet pinch-off region, which may lie inside the nozzle, is strongly affected by the fluid viscosity over the range of values that are commonly used Jet profiles beyond the nozzle exit predicted in these simulations match previously published high resolution images very well and validate the code used The simulations show that the radial velocity at the minimum radius in the pinch-off region falls exponentially soon after neck formation but then approaches a speed near that predicted theoretically for filament rupture. The overall jet length is primarily controlled by the slow speed of radial pinch-off. Towards the final break-off time, competition between the original radial minimum and a developing second radial minimum can alter the flow conditions towards symmetry. The simulations also explain why visible jets are shaped like truncated cones. Pinch-off occurs typically within one nozzle radius of the nozzle exit, and while it may be located within the nozzle region, another radial minimum also forms outside the nozzle, close to the exit for low viscosity fluids but well beyond it for higher viscosity fluid The radial collapse follows a power law with time, with the power-law index n varying between the value of n=2/3 expected for an inviscid fluid and n=l law expected for a viscous fluid The transition in behavior occurs at a viscosity of ∼20 mPa s, which is within the range of ∼10-40 mPa s typical of most DoD inks formulations.