Drying Inkjet Droplets - Internal Flows and Deposit Structure

Abstract

Inkjet printing delivers droplets with picolitre volumes onto a substrate. This thesis focuses on improving the quality of inkjet prints by controlling the particle distribution in the deposit. To this end, the internal flows in evaporating droplets were compared on purpose-built imaging rigs. Formulations were developed for deposit control based on the results of this analysis.

Initial studies of pure solvents determined the influence of the substrate wettability and thermal conductivity on evaporation. It was demonstrated that an isothermal model accurately predicted the drying times of picolitre droplets, provided that evaporative cooling was insignificant.

Evaporation of simplified model inks (two solvents + latex particles) was then considered. Marangoni flows transported particles along streamlines, with a circulating region that switched from the droplet centre to the edge on reversing the flow direction. Particles also migrated across streamlines towards the centre of the droplet, independent of the Marangoni flow direction. Large particles migrated faster than smaller particles, forming a tighter central group. Migration mechanisms were considered: Thermophoresis was ruled out due to particle migration in droplets with negligible thermal effects. Chemophoresis was not consistent with all observations of particle migration, though chemophoretic velocities are large enough to contribute. Shear-induced migration to regions of low shear rate is a promising potential migration mechanism.

The deposit macro/microstructures were investigated for pure and binary mixtures. In pure solvents, evaporation-driven radial flow built up a ring stain at the contact line. This stain was inhibited for some binary solvent mixtures. However, in most cases, a ring stain developed after the Marangoni flow period ended. Hence, alternative routes of deposit control were investigated.

Two strategies were developed to control the deposit structure: i) a sol-gel transition in a suspension of a nano-particulate clay (laponite), or ii) depletion flocculation induced by a free polymer (PSS). The latter strategy was the most successful for ethanol/water mixtures, producing a printed dot smaller than the droplet contact area. In water droplets, the sol-gel transition proved a successful method for obtaining a uniform particle distribution in the deposit.