Fundamental Aspects of Ink Jet Printing

Abstract

Ink jet printing is a fabrication technique with uses in graphical applications, printed electronics, and solid freeform fabrication in which a droplet of 1-1000 pL volume is generated at a nozzle and then placed at an arbitrary position on a substrate. A series of droplets can be deposited in a pre-defined pattern, and by the droplets spreading, coalescing and changing phase a solid deposit is formed upon the substrate. Models are presented for high-vapour pressure inks, together with experimental observation, which show that deposit size is primarily controlled by the droplet size and the surface energy interactions. This is due to the timescale of phase change being significantly longer than spreading because of a saturated vapour environment in the vicinity of each droplet. The saturated vapour environment of adjacent droplets can interact, as shown by an elongation of the deposit due to a change in the wetting kinetics caused by an adsorbed vapour film on the substrate. Models are presented for the unstable deposition of a droplet with a random rough surface and a step obstacle that explain the threshold behaviour of splashing in terms of a Rayleigh-Taylor instability and an energy balance respectively. These models show good agreement with experimental results for a range of fluids of varying viscosity, contact angle and surface tension, and varying roughness or obstacle height. The coalescence of droplets into beads is modelled, with three different morphologies identified. The boundaries between the three regions (periodic curvature, stable and bulging) are modelled using volume conservation and a balance between pressure-driven and capillary flow within the bead. The models are found to show good agreement with experimental observation for a variety of different ink and substrate combinations. The width of the bead within the stable region is modelled by assuming volume conservation and a cylindrical segment geometry with good experimental agreement. A stability map that enables graphical inspection of expected morphology as a function of fluid and operating parameters is derived, showing good agreement with experimental data.