Self-propelled Balls the Size of a Human Hair

When it comes to technological breakthroughs in the nanoworld, it seems nature has still a lot to teach us. Squids, octopuses and jellyfish swim by creating jets of water which push the animal forward based on the principle that every action has an equal and opposite reaction.

This is pretty much what a group of researchers replicated when they created microscopic polystyrene balls that swim using jets of liquid. Each ball is as wide as a strand of human hair and proves that this form of navigation also works on a microscopic level.

Ramin Golestanian led the team of researchers of the University of Sheffield, which designed the polystyrene balls, which are coated with a thin layer of platinum on one side. When dropped into a solution of water and hydrogen peroxide, the platinum layer acts as a catalyst in a reaction that breaks hydrogen peroxide into water and oxygen.

This reaction produces three molecules for every two that it consumes, molecules that push the polystyrene ball from the platinum side, acting like a microscopic jet propulsion system. To remove the possibility of misleading results, caused by the Brownian motion - which manifests itself in vibrating atoms and molecules that naturally makes small objects wander about randomly - the scientists tested the platinum-coated balls against polystyrene balls with no coating.

The results clearly showed that the half-coated ones preferred a particular direction of movement, even though they tended to meander over long distances. This meandering, however, was clearly not the result of Brownian motion alone.

It seems that the trajectories looked like a random walk with step sizes that were conditioned by the concentration of the chemical that produces oxygen - hydrogen peroxide - which amplifies the step with its concentration.

Practical applications are still a thing of the not so near future, since there is still the problem of making the balls heading in a particular direction, but one day, this type of propulsion could move microscopic drug-delivering robots through the bloodstream or tiny maintenance robots inside self-healing devices.