When the ink hits the page

The physics of the most printable inkjet printer ink

Hitting “print” is easy. Getting a perfect printout is not. All too often, inkjet printers spew out smudged or smeared pages. Now, a new study of the physics of ink concludes that the culprit is the gloopiest ink.

THE GLOP FACTOR INK PHYSICS: The best ink forms one well-defined droplet before hitting the paper. IMAGE CREDIT: Daehwan Jang and Jooho Moon

Inkjet printers have a tiny nozzle, or inkjet, that squirts droplets of ink onto the paper as it passes through. Ideally, the ink forms into a perfect round droplet as it launches from the inkjet, hitting the paper right on target. But droplet formation is affected by ink properties including density, surface tension and viscosity, which is the measure of resistance to flow or “gloopiness.” And if the droplets aren’t just right, a splotch appears instead of a crisp line of text, the researchers say.

Scientists use a ratio called the Z number to describe the surface tension and viscosity of a particular ink. Inks with lower Z are more viscous, while inks with higher Z have more surface tension, explains material scientist and study coauthor Jooho Moon of Yonsei University in Seoul, Korea.

Recent theoretical work found that the most printable inks would have Z values between one and 10. But the new research suggests the best droplets form from inks with Z values between four and 14, Moon and his colleagues report in the March 3 Langmuir. That means, of the inks now in common use, the more free-flowing ones print better, Moon says.

“A kind of balance is what is needed for the most printable inks,” explains material scientist Damien Vadillo of the University of Cambridge in England.

When ink is launched from the inkjet, a teardrop-shaped ball hurtles toward the paper. The teardrop elongates until a tail of ink, called a filament, stretches away like the tail of a comet.

In the study, researchers captured images of the droplets of different types of lab-made inks, up to a Z score of 17. Inks with Z values above 14 had filaments that easily broke away from the ink drop, forming a secondary ink drop that created a smear. Droplets of inks with Z values below 14 held together better, and the viscosity of the ink pulled the filament back into the droplet. But droplets of very viscous inks with Z values below four stuck to the inkjet instead of launching properly, the scientists note.

The researchers acknowledge in the paper that the work only tested ink on one printer, and the best Z values could vary slightly depending on how far a printer’s inkjet is from the paper’s surface.

Despite the new range of Z values, it’s not necessary to rush out to purchase new ink cartridges. The study’s conclusion “is not really a big surprise, just refining what we already knew,” Vadillo says.

But understanding the properties of the most printable inks will help in future printer and ink design. And inkjet printers are useful for more than desktop publishing. “Droplets of chemicals could be printed onto a substrate, then we could test thousands of different reactions at once,” Vadillo speculates. “Understanding how to make the chemicals into good ‘inks’ will help us to do that.”

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