If a science fiction film director ever looked for inspiration for depicting tiny robotic alien life forms, he had no further look than those created by a team of scientists in Bulgaria, Poland, Britain and China.
Starting out as harmless oily droplets around 20–40 μm in diameter that float in the water, these structures take on faceted, crystal-like shapes when they cool down to around 2–8 ° C – although they are not frozen. Then it gets really strange.
Some facets of the particles grow while others shrink, creating a variety of geometric shapes such as dragons, isosceles triangles, and spiky tetrahedra. Tentacle-like strands then emerge from some of the sharp corners, as if extruded from a nozzle. As they grow, the strands bend into undulating shapes – and the droplets begin to swim, propelled by the liquid through the elongation of the tentacles.
The researchers first reported polygonal and polyhedral droplets in 2015. They consist of simple alkanes such as pentadecane or tetradecane, which are emulsified in water by a surfactant coating. The facets appear on cooling because the droplet surfaces develop a thin layer of loose crystalline phase in which the alkane chains are aligned but are fairly freely rotatable. This surface layer is quite stiff, and a complex balance of forces causes it to straighten up and bend into facets.
Then the materials chemist Stoyan Smoukov of the Queen Mary University of London and colleagues discovered that tendrils could sprout from the corners of the droplets. A team led by Smoukov, Nikolai Denkov from the University of Sofia, Maciej Lisicki from the University of Warsaw and Eric Lauga from the University of Cambridge have now investigated how these strands drive the particles. When they first saw this strange movement, says Smoukov, “we had a lot of questions: where would they go, how and why are they doing it?”
The researchers suspect that the filaments, which are only a few hundred nanometers wide, also consist of the elastic surface phase. Their calculations predict that as the strands grow, viscous friction with the water bends them into undulating shapes like meandering rivers. As a result of the friction, a filament emerging in one direction pushes the droplets in the opposite direction.
By slowly raising and lowering the temperature by around 5 ° C, Smoukov and colleagues can repeatedly shrink and grow the filaments so that the droplets stop or pull back and then move again. In fact, the droplets store energy from the warmer environment, which they recharge for more swimming when they cool down afterwards.
“This work is amazing,” says physicist Tom McLeish of the University of York, UK. “The structures remind me of some of the strange filamentous forms of crystal growth for some simple inorganics in the presence of polymers.” He adds that swimming behavior doesn’t look like anything he’s seen before in natural or synthetic systems. “I really have no idea what’s going on,” he admits.
Daniela Kraft, physicist for soft matter at the University of Leiden in the Netherlands, calls the work “beautiful” and says: “This is a novel drive mechanism that uses the elasticity of the ejected fibers to generate a net movement”.
The movement of the droplets, driven by filaments, is reminiscent of agile bacteria swimming with whip-like appendages called flagella. Smoukov hopes that her droplets provide a simple model system to study such movements.