As previously described, the synthesized TiO 2 microparticles form with a crack due to the release of accumulated stress from the solvent vapors (Figure , inset SEM image). In combination with the photocatalytic properties, this geometry allows these Pac-Man TiO 2 particles to generate an asymmetric fluid flow around them and propel autonomously. In comparison to traditional Janus particle micromotors, this fabrication yields active particles without any asymmetrization step or the requirement of expensive noble metals. Here, Particle Image Velocimetry would give great insights, revealing the flow field and the origin of motion, but the colloids are rather small, making the tracer selection extremely tedious and all existing techniques have been optimized for particles attached to the substrate, which is even more difficult for the Pac-Man morphology.