This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A drop of coffee on a tabletop often leaves a dark ring behind. This everyday mark is a reminder that a drying droplet is not passive: As water evaporates, it can transport the particles suspended inside it toward the edge.
Scientists call this the coffee-ring effect. For inkjet printing and surface coatings, however, a ring is often exactly what engineers do not want. They need the material to land in a controlled, predictable pattern.
In our new study published in Langmuir, we found that graphene oxide can produce two strikingly different outcomes. Depending on the size of its nanosheets and the amount of oxygen attached to them, a drying droplet can leave behind deposits that look like a spider's web or a mosaic. The result offers a simple way to tune how the nanoflakes are arranged on a surface—a useful capability for printed electronics, coatings and potentially supercapacitors.
Graphene is a one-atom-thick sheet of carbon known for its exceptional electrical, thermal and mechanical properties. Graphene oxide is a chemically modified form of graphene: Oxygen-containing groups decorate its surface, making the sheets easier to disperse in water. That combination of conductivity-related properties and water compatibility makes graphene oxide attractive for printing electronic components.
But graphene oxide does not simply float around as isolated flakes. In water, the sheets can organize themselves into layers. The suspension still flows like a liquid, yet the sheets retain a degree of order more commonly associated with crystals.
Such materials are called liquid crystals. Their internal organization affects not only how they flow, but also what they become after the water has evaporated. We therefore asked a straightforward question: Can we control the final deposit by changing two features of the sheets—their size and their degree of oxidation?
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