Adding a dash of alcohol suppresses coffee ring effect in 2D print jets

Inkjet printing of two-dimensional crystals will be essential for the introduction of the next generation of print electronics. While technology has made great strides in recent years, a major challenge for industrial scale-printed electronic components is achieving uniform distribution of the crystals; uneven distribution can result in defective 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 with joe, 2D crystals in the case of inkjet printing of electrical components. (You can also see the effect with single-malt scotch. A related phenomenon is wind tears.) The coffee-ring effect occurs when a single liquid evaporates and the solids that were dissolved in the liquid (such as coffee grounds or 2D crystals) form a narrative ring. This is because evaporation occurs more rapidly at the edge than at the center. Any remaining liquid flows outward to fill the holes, entraining these solids. Mixing in solvents (water or alcohol) reduces the effect, as long as the drops are very small. Large drops produce more uniform spots.

In the same way the pigment particles of color break outwards as a drip of watercolor paint dries outwards, towards the edge of the drip. That artists who work with watercolors should also deal with the coffee ring effect if they do not want the accumulation of pigment to happen on the sides. 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 remaining drip on the paper, the ink runs off. This allows the artists to play with various effects, such as generating unusual color gradients.

The shape of the drops is also a factor in the coffee ring effect. As we reported in 2011, researchers found that the effect can be ignored if the particles are ellipsoidal instead of spherical. In this way, they form loosely packed structures that can withstand the capillary flow when transported to the edge of the drip. When the drip has completely evaporated, these particles are more evenly distributed. The longer the particles are, the more uniform the deposition, and providing a way to control the distribution of material.

In the case of inks for 2D printing of electronic components, adding the right type of alcohol to the ink mixture can affect droplet shape and suppress the coffee effect, according to this latest study, led by researchers from the University of Cambridge, Durham University, and Beihang. University. The researchers used 2D crystal flakes of graphene, bismuth telluride, tungsten disulfide, bornitride, and black phosphorus, among other materials. The crystals were then dispersed in isopropanol (IPA) alone, plus isopropanol mixed with ethanol, t-butanol, and 2-butanol, in various samples.

They found that the coffee ring effect was still prevalent for the IPA, IPA / ethanol, and IPA /t-butanol measurements. But the effect was suppressed in the IPA / 2-butanol mixture, producing uniform thickness in the print forms. The authors suggest that thicker areas of the drip are richer in IPA and therefore have a lower surface tension. The addition of 2-butanol to the mix results in a Marangoni stream from thicker to thinner areas, creating a natural negative feedback mechanism to suppress the capillary currents leading to the coagulation effect and spreading the droplet out like a pancake .

“The natural shape of ink droplets is spherical – however, because of their composition, our ink droplets take on pancake forms,” ​​said co-author Tawfique Hasan of the University of Cambridge.

The team has already successfully printed gas sensors and photodetectors as proof of concept that overcomes the current requirements of the industry, and they are confident that the wide range of materials they can use in their ink, printing more efficient catalysts , solar cells, could turn on batteries, and functional coating. The ink mixtures can even print nanoparticles and organic molecules. And the process is scalable: scientists print 4,500 nearly identical sensors and photodetectors. This makes it a promising method for cheap, industrial scale processing of electronic devices.

“Understanding this fundamental behavior of ink droplets has allowed us to find this ideal solution for inkjet printing of all kinds of two-dimensional crystals,” said co-author Guohua Hu of the University of Cambridge and the Chinese University of Hong Kong. “Our formulation can be easily scaled up to print new electronic devices on silicon wafers, such as plastic, and even in spray painting and portable, even if it matches the manufacturing requirements for printed devices.”

DOI: Science Advances, 2020. 10.1126 / sciadv.aba5029 (About DOIs).