Small beads scattered in liquids can pack into square arrays, an unusual arrangement for floating objects.

Particles at the boundary of a liquid — such as bubbles at the surface of a soft drink — usually clump together in space-saving hexagons to minimize disruption to the surface tension of a liquid. Jasper van der Gucht and his group at Wageningen University in the Netherlands created an experimental set up to explore what would happen if the boundary was curved. They deposited oil droplets on a glass slide, added a layer of water and placed micrometre-sized plastic beads on top. The spheres clustered at the interface between the oil and water. The team could control the curvature of the interface by changing how oil droplets were attached to the glass, and could generate forces to make the beads group in squares.

Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1222196110 (2013)