how to overcome coffee ring effect

the coffee ring effect. This effect has hindered the industrial deployment of functional inks with graphene, 2-D materials, and nanoparticles because it makes printed electronic devices behave irregularly.

Now, after studying this process for years, a team of researchers 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 quicker at the edges, causing an accumulation of solid particles that results in the characteristic dark ring. Inks behave like coffee—particles in the ink accumulate around the edges creating irregular shapes and uneven surfaces, especially when printing on hard surfaces like silicon wafers or plastics.

Their solution: alcohol, specifically 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. 

"The natural form of ink droplets is spherical—however, because of their composition, our ink droplets adopt pancake shapes,"

The new inks also avoid the use of polymers or surfactants—commercial additives used to tackle the coffee ring effect, but at the same time thwart the electronic properties of graphene and other 2-D materials.

reference

"A general ink formulation of 2D crystals for wafer-scale inkjet printing" Science Advances (2020). DOI: 10.1126/sciadv.aba5029