A Janus distribution of Pt, which catalyses the decomposition of H 2 O 2 , on the surface of the microparticles creates asymmetric gradients around the Pt-SiO 2 particles. These in turn develop asymmetric flow fields, which result in the self-phoresis of the particles . To evaluate our galvanic approach to microswimmer fabrication, we perform particle tracking experiments in fuel-rich environments. We track the trajectories of the bulk-Pt-functionalised and Janus-Ptfunctionalised microparticles and quantify their short-time mean squared displacements (MSD, Figure ). We then evaluate the particle velocities from their short-time MSDs using ∆x 2 = 4D T ∆τ + V 2 ∆τ 2 . This analysis requires that rotational diffusion does not influence the motion of the microswimmers, i.e. that τ << τ R , where τ R is the characteristic time of rotation (see Text S4 for discussion). We therefore compute the MSD over 2s (20 data points at 10 FPS), noting that τ R is approximately 50s.