A simple experimental setup to study the impact and coalescence of deposited droplets is described. Droplet impact and coalescence have been investigated by high-speed particle image velocimetry. Velocity fields near the liquid-substrate interface have been observed for the impact and coalescence of 2.4 mm diameter droplets of glycerol∕water striking a flat transparent substrate in air. The experimental arrangement images the internal flow in the droplets from below the substrate with a high-speed camera and continuous laser illumination. Experimental results are in the form of digital images that are processed by particle image velocimetry and image processing algorithms to obtain velocity fields, droplet geometries, and contact line positions. Experimental results are compared with numerical simulations by the lattice Boltzmann method.

This article describes a series of modern developments carried out by the inkjet community in its quest to improve material compatibility, printing quality, and reliability. Recent progresses in rheology have advanced our understanding of liquids at the time scales that are characteristic of inkjet printing processes. As a result, microsecond rheology now permits the formulation of inks with tailored viscosities that vary according to the time-scale of their dynamics, i.e. low effective viscosity during jetting but high at break up and landing. These advances have permitted the community to assess, and often predict, the ink jetting behaviour, at a given printing frequency, based on the linear or non-linear viscoelasticity and other fluid characteristics. Advances in fluidic systems and in waveform design have now enabled the printing of high viscous inks that were previously impossible to jet on demand. This capability is opening up new markets and opportunities for inkjet, from the printing of glues to the use of heavily loaded ceramic inks. Advances in printhead design, and the assessment of printing patterns using common standards, now allow the verifiable and reliable operation of industrial-scale digital inkjet printing in a wide range of environments. Recent improvements on printhead cleaning protocols, have contributed to an increase in printing speed and operating time by reducing the production of mist and satellite droplets neighbouring the printhead region. Thanks to these improvements, inkjet is displacing traditional technologies, such as offset and screen printing, in large markets including graphics, packaging and labelling.

No abstract

The effects of corona discharge treatment (CDT) on ink drop impact and spreading on a coated polypropylene film substrate were investigated. Substrate surface energies were determined from static contact angles with water and ethylene glycol. The polar component increased with increasing CDT. Drops 39 lm in diameter of an acrylate-based UV-curable ink were printed on to the substrate, and the spreading process studied by high-speed photography. No changes occurred during the initial stages, but the wetting phase was shorter for higher doses of CDT. Drops spread further on substrates with low doses of CDT than with higher doses. White light interferometry was used to determine the final heights of drops after UV-curing. The height was significantly affected by CDT, with minimum height at low doses. The relationship was investigated between the static contact angle for large sessile drops and the equilibrium contact angle for printed drops after spreading. Contact angle measurements with millimeter-sized sessile drops of ink provide a reliable method to determine the effects of corona treatment on wetting by ink jet printed drops.

No abstract