A liquid droplet coated with shiny solid particles floating on the surface of water

What looks like a floating disco ball is actually a ‘liquid marble’ formed from a water droplet coated in hexagonal plastic particles. Credit: F. Geyer et al./Adv. Funct. Mater.

Materials science

Liquid marbles self-assemble from a pile of powder

Armoured ‘footballs‘ can be coaxed into cylinder and dumbbell shapes.

Wrapping droplets of water in a common type of plastic creates liquid marbles, which can function as microscopic ball bearings, miniature vessels for chemical reactions, and various types of sensor.

Scientists have been engineering spherical liquid marbles for more than a decade by encasing droplets in ‘armour’ made of spherical particles. But Syuji Fujii of the Osaka Institute of Technology in Japan and his colleagues sought to cover the marbles with hexagonal plates, which could lead to a wide variety of useful shapes.

The researchers started with powdered poly(ethylene terephthalate) — the plastic used for soft-drink bottles — in the form of tiny, six-sided plates. When the scientists placed water droplets on a layer of the plastic, the plates surrounded them, creating marbles that look like lumpy miniature footballs.

Marbles coated with the main ingredient used in superglue retained their shapes instead of collapsing when the water inside them evaporated. The researchers also found that they could force the droplets into cylinders, dumbbells and even letters by applying external pressure with something as simple as a spatula.