The tendency of ink to form coffee ring-like stains seeds an idea for a new kind of touch-screen device.
It's a touch screen, a solar panel, a computer circuit, and soon, it could be used at home.
Taking advantage of ink's natural tendency to create "coffee rings," a group of Israeli scientists has developed a type of ink jet dye that could one day create a range of power-hungry, and power-producing, devices at home.
"Usually these 'coffee stains' are a major problem in ink jet printing," said Shlomo Magdassi, a professor at the Hebrew University of Jerusalem in Israel and co-author of a new paper in the journal ACS Nano. "I got the idea that we could turn this big problem into a big advantage."
"Coffee ring" stains appear as a drop of water or solvent dries. The edge of the drop evaporates more quickly than the center, creating tiny currents inside the drop. These currents continuously move dye particles from the drop's interior to its outer edge, where it's then permanently deposited.
The end result is a thin ring around a large, relatively colorless area, reminiscent of the stain a coffee cup can leave on a surface.
To eliminate the stains, scientists have used fast-drying solvents and additional dye particles. But Magdassi and his colleagues realized that coffee stains could solve a problem with electrically-conductive touch screens.
Magdassi wanted to create such a touch screen using silver nanoparticles. But there was a hitch. Spreading the silver nanoparticles evenly over the plastic ensures transparency, but spread the particles too far apart and they can't conduct electricity.
Adding enough particles to make the screen electrically conductive makes the screen opaque.Encouraging coffee rings solves both problems.
The rings squish the metallic nanoparticles close enough for an electrical current to pass through them. The empty holes allow 95 percent of light to pass through the screen, making it almost completely see-through. The scientists did this slightly above room temperature using water, ensuring that the technique could be easily done with commonly-available printing materials.
Absorbing some light is not necessarily a bad thing, however. Left in a beam of sunlight, the silver nanoparticles can convert a small amount of energy to charge a device. Solar charging with the touch screen wouldn't be nearly as efficient as plugging a device into a wall socket, but it could give a user a few extra minutes of talking time when a plug isn't available.
Silver isn't the only metal Magdassi used. Since submitting the ACS Nano paper, Magdassi has used copper nanorings with similar results. The next step, said Madgassi, is creating concentric rings of copper and silver nanoparticles to get the best of both properties.
Magdassi's results come weeks after Xerox announced a similar breakthrough using silver nanoparticles and ink jet printers to create printable electronic circuits on paper or other substrates.
Ana Arias of the Palo Alto Research Center in California has also been experimenting with silver nanoparticles for touch-screen devices.
"They were able to turn a big problem into a good feature," said Arias.
Instead of allowing the silver rings to form naturally, Arias formed the silver nanoparticles into long lines, packing the particles against each other while remaining transparent. A patent on her method is pending.
Whether any of the research translates into commercial products remains to be seen, say Arias and Madgassi. The current industry go-to material for touch screens is indium tin oxide, although scientists are searching for alternatives.
"Everyone complains about the indium tin oxide's problems, but at least we know what those problems are and can work around them," said Arias.
Years of research is needed to find any potential problems, and their solutions, for newer materials like silver nanoparticles before they appear in consumer devices.