Ultra-fine bubbles control inkjet ink drying without additives

Researchers at Tokyo Metropolitan University have developed a novel method for controlling the drying behavior of inkjet-printed ink droplets by incorporating ultra-fine bubbles. This technique allows for the tuning of the shape of the particle-laden film left after drying, without relying on chemical additives. The study, slated for publication in Precision Engineering, demonstrates that adjusting the number of bubbles can significantly alter the resulting deposit.

Inkjet printing is a vital technology for applications ranging from printed media to microelectronics and MEMS. A key challenge in these areas is controlling how ink droplets dry and how the suspended submicron particles arrange themselves. The common "coffee ring" effect, where particles accumulate at the droplet's edge, is often undesirable. While chemical additives are typically used to modify surface tension and achieve more uniform coatings, these additives can remain in the final film and negatively impact the properties of the deposited particles.

To circumvent this issue, Professor Arata Kaneko and his team at Tokyo Metropolitan University employed nanoscale ultra-fine bubbles dispersed within the ink. They generated suspensions of silica nanoparticles in water, passed them through an ultra-fine bubble generator, and then deposited 1-nanoliter droplets onto a silicon substrate using an inkjet nozzle. The drying process of these bubble-laden droplets was then observed.

The team observed that suspensions without bubbles exhibited a pronounced coffee ring effect. However, the addition of bubbles led to gradual changes in the deposit morphology. A small number of bubbles resulted in a more uniform coating, while an increased bubble concentration caused particles to concentrate at the center of the droplet. Crucially, these bubbles do not leave any residue after the droplet dries, which is essential for preserving the intrinsic properties of the nanoparticles.