If ever a science fiction movie director were seeking inspiration for how to depict tiny robotic alien life forms, they need look no further than those created by a team of scientists in Bulgaria, Poland, the UK and China.
Beginning as innocuous oily droplets about 20–40μm across floating in water, these structures take on faceted, crystal-like shapes when cooled to around 2-8°C – even though they aren’t frozen. Then things get really weird.
Some of the particles’ facets grow while other shrink, producing a variety of geometrical forms such as kites, isosceles triangles and spiked tetrahedra. Then, from some of the sharp corners emerge tentacle-like strands, as if being extruded from a nozzle. As they grow, the strands bend into undulating shapes – and the droplets start to swim, propelled through the fluid by the tentacles’ extension.
The researchers first reported polygonal and polyhedral droplets in 2015. They are made from simple alkanes such as pentadecane or tetradecane, emulsified in water by a surfactant coating. The facets appear on cooling because the droplet surfaces develop a thin layer of a loose crystalline phase, in which the alkane chains are aligned but fairly free to rotate. This surface layer is rather stiff, and a complex balance of forces causes it to straighten and buckle into facets.