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.
From spider webs to mosaics: How nanosheets of graphene oxide control the patterns left by a drying droplet
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