Travel into the darkness: Half-coated Janus particles, named after the roman double-faced godness, are autonomously navigating towards a light source. Researchers based in Stuttgart find such phototactic behaviour, also known from living microorganisms, also in synthetically fabricated microswimmers.

Many microorganisms, with phytoplankton and zooplankton as prominent examples, display phototactic behaviour, i.e. the ability to perform directed motion within a light gradient. Here, we experimentally demonstrate that sensing of light gradients can also be achieved in a system of synthetic photo-activated microparticles being exposed to an inhomogeneous laser field. We observe a strong orientational response of the particles due to diffusiophoretic torques which in combination with an intensity-dependent particle motility eventually leads to phototaxis. Because the aligning torques saturate at high gradients, a strongly rectified particle motion is found even in periodic asymmetric intensity landscapes. Our results are in excellent agreement with numerical simulations of a minimal model and should similarly apply to other particle propulsion mechanisms. Since light fields can be easily adjusted in space and time, this also allows to extend our approach to dynamical environments.

When we subject a diluted suspension of microswimmers to a one-dimensional linear intensity gradient, we observe directed particle motion towards low intensities within a few seconds.

Particle current in periodic asymmetric light profiles