Avoiding the coffee ring effect in printed electronics

Researchers from Tokyo University of Agriculture and Technology have discovered a solution to a problem that occurs when printing words on electronics — the dreaded ‘coffee ring effect’.

Drying is an important part of printing words and electronics. Particles suspended in liquid are applied to a surface and the liquid evaporates leaving the particles behind. But sometimes, these particles dry unevenly.

Much like when spilled coffee dries up and leaves behind a hollow ring, particles tend to move to the outside of the liquid droplet. This is a problem, particularly for printed electronics, which require uniform application of a liquid for maximum performance.

Looking to rectify this, the researchers tested three different concentrations of cellulose nanofibres added to a solution with suspended particles. They also tried increasing the particle concentration in a solution with no added nanofibres. They photographed the drying process under a microscope over time and published the results in the journal Science and Technology of Advanced Materials.

The solutions with nanofibres dried much more evenly than those without. Instead of a hollow ring, the particles condensed into a solid dot, slightly shrinking in size as the liquid evaporated. Particles in the mixtures with nanofibres also moved at a consistent pace. There was no final rush to the periphery as was observed in the solutions without nanofibres.

Schematic diagram of the experimental set-up for time-lapse measurement by digital camera. The sample of a droplet on a cover glass was placed on a transparent styrol case where black paper was located beneath the sample. The brightness of the LED backlight was mediated by tracing paper. Image courtesy of the study authors under CC BY 4.0

The researchers concluded that cellulose nanofibres can improve the drying process and avoid problems stemming from uneven drying, such as degradation of paint coatings, clarity of printed characters on paper and conductivity of printed electronics. Once the solution dries, the nanofibres are left behind along with the desired particles.

The researchers do not yet know whether the nanofibres impede or benefit the material, claiming that is a topic for further research. Their study concluded, “The addition of cellulose nanofibres may alter the electrical resistivity of conductive wires in the printed electronics, but the fine-tuning of the concentration might be exploited for the control of electric resistivity itself.”

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