Combating the Coffee-Ring Effect With New Printing Technique For Electronics

The coffee-ring effect is something we can observe every day. But what is just a slight annoyance for us, is a challenging issue for scientists. Now, a team of researchers has found a way to combat the hindering effect with new inks, thereby enabling large-scale printing of electronic devices with simple inkjet technology.

Everyone, who enjoys their morning coffee, has come to know them quite well: coffee stains. But what has been a nuisance for us turned out to be the catalyst for a new, optimal printing technique for electronics. The coffee-ring effect (CRE) has for a long time been one of the most puzzling phenomena of fluid mechanics, having hindered the industrial deployment of functional inks with graphene, 2D materials, and nanoparticles, making printed electronic devices behave irregularly. After studying this process for years, researchers were now able to find a way to modify the way ink droplets dry, using an alcohol mixture that redresses this problem and enables cheap industrial-scale printing of electronic devices, such as sensors, light detectors, batteries and solar cells, at unprecedented scales with this new family of inks.

Combating the Coffee-Ring Effect

Coffee rings form because the liquid evaporates quicker at the edges of the rings, causing an accumulation of solid particles that result in the characteristic dark rings all of us have created far too often in our everyday life before. Ink has proved to behave in the same way, accumulating around its edges, creating irregular shapes and uneven surfaces. This property turned out to be especially challenging when printing on hard surfaces like silicon wafer or plastics, as one would do when printing electronic devices.

After studying the physics of ink droplets by combining particle tracking in high-speed micro-photography, fluid mechanics and experimenting with different combinations of solvents, the team of researchers lead by Tawfique Hasan (University of Cambridge’s Cambridge Graphene Centre), Colin Bain (Durham University’s Department of Chemistry) and Meng Zhang (Beihang University’s School of Electronic and Information Engineering) came to the conclusion that using a mixture of isopropyl alcohol and 2-butanol can help ink particles distribute evenly across a droplet, thereby generating shapes with uniform thickness and properties. Their composition makes the natural, spherical form of ink droplets turn into more of a “pancake shape”. After adding this mixture, new ink droplets will deform smoothly across the surface, spreading particles consistently while drying.

Exceeding Manufacturability Requirements

With this universal formulation, manufacturers could now finally adopt inkjet printing as a cheap, easy access method of fabricating electronic devices and sensors. Before, commercial additives like polymers of surfactants were used to combat the coffee-ring effect. But those additives also interfered with the electronic properties of graphene and other 2D materials. With the new inks, such obstructions can now be avoided entirely.

But the new approach doesn’t only improve the quality of the results, it also enables a high level of reproducibility and scalability: The researchers managed to print 4500 nearly identical devices on a silicon wafer and plastic substrate. For their test, they went with printed gas sensors and photodetectors. In the past, printing just a few hundred devices such as these was considered a success, even if they still showed uneven behaviour, yet, the scientists managed to print devices in large numbers, displaying very little variations in performance.

First author of their research paper Guohua Hu said:

“Understanding this fundamental behaviour of ink droplets has allowed us to find this ideal solution for inkjet printing all kinds of two-dimensional crystals. Our formulation can be easily scaled up to print new electronic devices on silicon wafers, or plastics, and even in spray painting and wearables, already matching or exceeding the manufacturability requirements for printed devices.”

Setting A New Standard

The researching team went beyond just working with graphene: They optimized over a dozen ink formulations containing different materials, such as black phosphorus and boron nitride, and even more complex hetero-structures and nano-structured materials, claiming their ink formulations can print pure nanoparticles and orfanic molecules. This new variety of available materials creates a realm of new possibilities for innovation and manufacturing of electronic and photonic devices, as well as more efficient catalysts, solar cells, batteries and functional coatings.

With their first proofs of concept showing promising results in terms of sensitivity and consistency, setting a new standard for the future of industrial requirements, the team expects to see industrial applications of their technology very soon. But the team is looking beyond the established application abilities:

“Our technology could speed up the adoption of inexpensive, low-power, ultra-connected sensors for the internet of things. The dream of smart cities will come true.”

Their full research results have been published in Science Advances. Read their original paper here.

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