An international team of researchers have been working together to examine the so-called “coffee ring effect” that has so far been an obstacle to developing functional inks using graphene and other nanoparticles.
These “coffee rings” form as particles from the ink gather around the edges to form uneven, irregular shapes and surfaces. Actual coffee rings form because liquid tends to evaporate more rapidly at edges, resulting in an accumulation of particles. In this way, graphene inks behave similarly to coffee.
In their research, the team studied ink droplets using a combination of methods, including high-speed micro-photography, fluid mechanics, and various solvents. They found by adding a mixture of isopropyl alcohol and 2-butanol, the ink particles distributed more evenly, resulting in more uniform shapes across surfaces as the ink tries.
Perhaps the biggest breakthrough here is in the repeatability. Using their method, the researchers were able to print 4,500 devices on a silicon wafer and plastic substrate that were nearly identical in performance. Previous efforts in which a few hundred such devices were printed were considered successful runs, even if their behavior varied.
“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,” said project first author Guohua Hu.