Synthetic Microswimmers: Inanimate Microparticles Display Complex Behavior

Janus particles under the electron microscope. The titanium dioxide microswimmers are barely larger than one micrometer.
Copyright: Simmchen Group

TU Dresden Freigeist fellow Dr Juliane Simmchen is investigating with her multidisciplinary junior research group the motion of synthetic microswimmers in liquids. Her goal is to enable these inanimate microparticles to move in a certain direction of their own accord and thus, in future, to be used in sensor technology or biological cleaning. “Actually, it’s a bit like playing computer games in the laboratory,” the chemist describes her extraordinary research work in an interview with the Volkswagen Foundation.

The Simmchen group is working with the so-called “Janus particles.” These consist of a body of titanium dioxide with two differently coated sides: one side with a catalytically active layer of nickel and gold, the other side remains untreated. Titanium dioxide is used as a whitening agent, for example in wall paint, but it also reacts with light. As a result, Janus particles are photocatalytic, which means that as soon as light hits them, chemical reactions occur that set off a movement.

The group has now observed and analyzed an extremely unusual phenomenon in the motion of Janus particles: as soon as the particles leave an illuminated zone in the microscope, they turn around by themselves and swim back – a behavior that is actually only known from microorganisms. But how can such complex behavior be triggered in synthetic microswimmers?

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