Inkjet printing is no longer used only for documents and images. It has become an essential manufacturing technique for advanced technologies such as microelectronics and MEMS (microelectromechanical systems), where microscopic coatings and intricate circuit patterns must be deposited with exceptional accuracy.

A major challenge is controlling what happens after each droplet lands on a surface. As the liquid dries, the suspended particles inside can gather into uneven patterns. One of the best known examples is the "coffee ring" effect, where most of the solid material collects around the outer edge of the droplet, much like the stain left behind by a dried drop of coffee.

Manufacturers often add chemicals that alter the liquid's surface tension to create smoother, more uniform coatings. However, those additives remain behind after drying and can change the behavior of the printed material, which is undesirable for many advanced applications.

Controlling Drying With Ultra Fine Bubbles

To avoid that problem, a research team led by Professor Arata Kaneko took a different approach. Rather than modifying the ink with surfactants or chemically altering the particles, they dispersed nanoscale ultra fine bubbles throughout the liquid.

For their experiments, the researchers suspended silica nanoparticles in water and passed the mixture through an ultra fine bubble generator. They then deposited 1 nanoliter droplets onto a silicon substrate using an inkjet nozzle and allowed them to dry.

The results showed that the bubbles gave researchers remarkable control over the final particle pattern. Droplets without bubbles produced a pronounced coffee ring effect. Introducing a modest number of bubbles created a much more even coating, while increasing the bubble concentration further caused particles to collect near the center of the droplet instead. The bubbles did not alter the nanoparticles themselves, including their electrical charge. Instead, they changed the liquid's surface tension and how it spread across the surface.

Cleaner Printing for Advanced Electronics

One of the biggest advantages of the technique is that the bubbles disappear completely as the droplet dries, leaving behind no residue. That makes the method especially useful when the original properties of nanoparticles must remain unchanged.

For example, graphene and molybdenum dioxide nanoparticles are commonly used in gas sensors because their electrical conductivity changes when they absorb gases. The sensitivity of these sensors depends heavily on the shape of the printed deposit. Conductive nanoparticles used in electronic circuits also perform best when their surfaces remain as clean and pristine as possible.

Because the ultra fine bubbles vanish after drying, the researchers believe their method could provide a cleaner, more precise way to produce next generation microdevices without the drawbacks of conventional ink additives.

The research was supported by JSPS KAKENHI Grant Numbers JP22H01377 and JP25K01136, along with a JKA Promotion Fund under Grant Number 2024M-394.