Moving forward: self-propelling oil droplets

Ned Stafford/Hamburg, Germany

In the latest step toward creation of artificial living cells in a laboratory, a team of Japanese researchers has developed a new variety of oil droplets that propel themselves through an aqueous solution as the result of a chemical reaction at the interface between the droplet and the solution.¹

Previously, scientists had succeeded in creating two categories of self-propelled oil droplets whose movements were limited - essentially because they consumed themselves as they moved or because the fuel needed for movement was inside the droplet and eventually ran out.

Tadashi Sugawara of the University of Tokyo tells Chemistry World that his team has now developed a new third category of self-propelled oil droplets, which he describes as ’a primitive type of inanimate chemical machinery’. The new category is a step forward from the two other types of self-propelling droplet, he says, as ’the oil droplet consumes not itself but [surfactant fuel] dissolved in water. Therefore, the oil droplet can sustain its self-movement as long as fuel is added.’

The team, led by Taro Toyota, started by preparing a suspension of surfactant dissolved in water and added oil droplets. The oil droplets were made of 4-octylaniline, which Sugawara says is a derivative of aniline, often used in the synthesis of dyes. A fluorescent catalyst was added to the oil droplets and the suspension was placed in a glass chamber of two thin glass slides.

The team speculates that the droplets propel themselves ’by the convection flow caused by chemical reaction occurring on its surface,’ Sugawara says. ’Once the hydrolysis of the surfactant occurs at a certain site on the surface of the oil droplet, the products accumulate at that site. In order to level off the local concentration of the product, a convection flow emerges.’

Noting that ’our model is just a beginning of self-propelled matters,’ Sugawara says his group is not only interested in the mechanism of this new category of self-propelled droplets, but of the interaction they observed in the glass chamber between droplets. He describes the inter-droplet dynamics as a ’primitive communication model’ representing what might have been occurring between cells during prebiotic evolution.

’We observed a loosely synchronised motion [between droplets,]’ he said. ’This means that the two droplets have attractive forces between them. We are very much interested in the mechanism of such synchronisation.’

Potential applications stemming from additional research into self-propelled droplets could, for example, be autonomous chemical transporters or a pollutant disposers, says Sugawara.

Itay Budin, a member of the Jack Szostak Lab at Harvard University and Massachusetts General Hospital, said the work is conceptually similar to a previous paper by the team published in 2007.²

’However, what they did here is use a chemical system where they can introduce the surfactant externally,’ he says. ’This is interesting conceptually because it mimics a simple cellular metabolic function. The droplet takes in an externally supplied fuel, performs a catalysed reaction and uses that process to drive a cell-like function.’

In practical terms, it is significant because it allows for indefinite locomotion and control over the locomotion via addition of new fuel surfactant, Budin says. He adds that potential applications could include micro- and nano-scale devices capable of autonomous locomotion to perform complex tasks.

While praising the paper, he adds: ’I do think there are some mechanistic aspects, especially the product segregation within the droplet and the release of the lipophilic reaction product that could be further explored. But the paper does well to describe the system and its unique behaviors.’