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Adding a little alcohol will prevent the coffee ring effect in 2D ink



The coffee ring effect occurs because evaporation occurs at the edge faster than in the center.
Enlarge / The coffee ring effect occurs because evaporation occurs at the edge faster than in the center.

The two-dimensional crystal inkjet printing will be vital to usher in the next generation of printing electronics. While the technology has made great progress in recent years, a major challenge for industrial-scale printed electronic components is to achieve an even distribution of the crystals; Uneven distribution can lead to defective devices. The culprit is a phenomenon known as the “coffee ring effect”

;. Scientists have now created a new ink line that can prevent this effect, according to a new article in the Science Advances journal.

The coffee ring is the type you get when the liquid evaporates and leaves a ring of previously dissolved solids – coffee grounds in the case of your morning joe cup, 2D crystal in case of inkjet printing of ingredients electricity. (You can also see the effect with single malt scotch. A related phenomenon is wine tears.) The coffee ring effect occurs when a liquid evaporates and the solid has been dissolved in the liquid ( like coffee grounds or 2D crystals) forming a storytelling ring. It happens because the evaporation occurs at the edge faster than at the center. Any remaining liquid flows outwards to fill the gaps, dragging those solids. Mixing in a solvent (water or alcohol) reduces the effect, provided the droplets are very small. Larger droplets create more even stains.

Likewise, when a drop of watercolor paint dries, pigment particles of the color break out, toward the rim of the paint droplet. So artists working with watercolor also have to deal with the coffee ring effect if they don’t want the buildup of pigmentation in the edges to occur. As we reported in 2018, adding alcohol to watercolor paints may prevent that. Also, an artist can wet the paper before painting. Instead of the remaining droplet pinned to the paper, the ink will flow. This allows the artist to play with various effects, such as creating irregular gradients.

The shape of the water droplets is also a factor in the coffee ring effect. As we reported in 2011, the researchers found that this effect could be negated if the particles were elliptical instead of spherical. In that way, they form loosely packed structures that can resist capillary outflow as they are transported to the edge of the droplet. When the droplet has completely evaporated, the particles are more evenly distributed. The longer the particles elongate, the more uniform the deposition, providing a way to control the distribution of the material.

In the case of 2D ink electronic components, simply adding the correct alcohol to the ink mixture could affect the droplet shape and prevent the coffee ring effect, according to this latest study, by researchers. Durham led the way. University and Beihang University. The researchers used 2D crystal fragments of graphene, bismuth telluride, tungsten disulfide, boron nitride and black phosphorus, among other materials. The crystals are then dispersed in isopropanol (IPA) alone, plus isopropanol mixed with ethanol, t-butanol and 2-butanol, in different samples.

They found that the coffee ring effect still occurs for IPA, IPA / ethanol and IPA /t-butanol mixture. But the effect is suppressed in the IPA / 2-butanol mixture, creating a uniform thickness in the prints. The authors suggest that the thicker areas of the water drop are richer in IPA and therefore have lower surface tension. Adding 2-butanol to the mixture results in a flow of Marangoni from thicker to thinner areas, creating a natural negative feedback mechanism to suppress capillary outflows resulting in a coffee and orchid ring effect. spills out like a pancake.

“The natural form of the ink droplets is spherical – however, due to their composition, our ink droplets are pancake-shaped,” said co-author Tawfique Hasan of Cambridge University.

The team has successfully printed the gas sensor and photodetector as proof of concept exceeding current industry requirements, and they are confident that the variety of materials they can use in ink can also permit. print catalysts, solar cells, batteries are more efficient. , and the functional overlays. Ink mixes can even print nanoparticles and organic molecules. And the process is scalable: scientists have printed 4,500 almost identical sensors and optical detectors. That makes it a promising method for making low-cost, industrial-scale electronics.

Co-author Guohua Hu of Cambridge University and Chinese University of Hong Kong said: “Understanding this basic behavior of the ink droplets allowed us to find the ideal solution for inkjet printing of all types of crystals. two-dimensional ”. “Our formulations can be easily extended to print new electronics on silicon or plastic sheets, and even in spray paints and wearables, have met or exceeded productability requirements. to print devices. “

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


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