The “coffee ring effect” is one of the most puzzling phenomena in fluid mechanics. A quick Google search will unearth any number of scholarly articles about what it is, what it does and how to try to beat it.
One thing it has done, says a team of researchers led by the University of Cambridge in the UK, is to hinder the industrial deployment of functional inks with graphene, 2D materials, and nanoparticles because it makes printed electronic devices behave irregularly.
Now, after studying this process for years, Tawfique Hasan and colleagues say they have created a new family of inks that overcomes this problem, enabling the fabrication of new electronics such as sensors, light detectors, batteries and solar cells.
Coffee rings form because the liquid evaporates more quickly at the edges, causing an accumulation of solid particles – and inks behave like coffee, the researchers say. Particles accumulate around the edges, creating irregular shapes and uneven surfaces, especially when printing on hard surfaces like silicon wafers or plastics.
After studying the physics of ink droplets by combining particle tracking in high-speed micro-photography, fluid mechanics and different combinations of solvents, their solution, as explained in a paper in the journal Science Advances, is alcohol.
Specifically, it’s a mixture of isopropyl alcohol and 2-butanol. Using these, ink particles tend to distribute evenly across the droplet, generating shapes with uniform thickness and properties. While drying, the droplets deform smoothly across the surface, spreading particles consistently.
Using this universal formulation, the researchers suggest, manufacturers could adopt inkjet printing as a cheap, easy-to-access strategy for the fabrication of electronic devices and sensors.
The new inks also avoid the use of polymers or surfactants, commercial additives used to tackle the coffee ring effect that at the same time thwart the electronic properties of graphene and other 2D materials.
“Understanding this fundamental behaviour of ink droplets has allowed us to find this ideal solution for inkjet printing all kinds of two-dimensional crystals,” said first author Guohua Hu.
“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.”
The authors say that in proof-of-concept trials they printed 4500 nearly identical devices on a silicon wafer and plastic substrate.
The project was a collaboration with researchers from Durham University, UK, and China’s Beihang University.
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