Inkjet printing of two-dimensional crystals will be crucial for ushering in the next generation of printed electronics. While the technology has made a lot of progress in recent years, a major challenge to industrial-scale printed electronic components is achieving uniform distribution of the crystals; uneven distribution can result in faulty devices. The culprit is a phenomenon known as the “coffee ring effect.” Now scientists have created a new family of inks that can suppress the effect, according to a new paper in the journal Science Advances.
Coffee rings are the pattern you get when a liquid evaporates and leaves behind a ring of previously dissolved solids—coffee grounds in the case of your morning cup of joe, 2D crystals in the case of inkjet printing of electrical components. (You can also see the effect with single-malt scotch. A related phenomena is wine tears.) The coffee ring effect occurs when a single liquid evaporates and the solids that had been dissolved in the liquid (like coffee grounds or 2D crystals) form a telltale ring. It happens because the evaporation occurs faster at the edge than at the center. Any remaining liquid flows outward to the edge to fill in the gaps, dragging those solids with it. Mixing in solvents (water or alcohol) reduces the effect, as long as the drops are very small. Large drops produce more uniform stains.
Similarly, when a drop of watercolor paint dries, the pigment particles of color break outward, toward the rim of the drop. So artists who work with watercolors also have to deal with the coffee ring effect if they don’t want that accumulation of pigment at the edges to happen. As we reported in 2018, adding alcohol to the watercolor paint can prevent it. Alternatively, an artist may wet the paper before applying the paint. Instead of the drop remaining pinned to the paper, the ink runs off. This allows the artist to play with various effects, such as generating unusual color gradients.