  Living microorganisms in confined systems typically experience an affinity to
populate boundaries. The reason for such affinity to interfaces can be a
combination of their directed motion and hydrodynamic interactions at distances
larger than their own size. Here we will show that self-propelled Janus
particles (polystyrene particles partially coated with platinum) immersed in
droplets of water and hydrogen peroxide tend to accumulate in the vicinity of
the liquid/gas interface. Interestingly, the interfacial accumulation occurs
despite the presence of an evaporation-driven flow caused by a solutal
Marangoni flow, which typically tends to redistribute the particles within the
droplet's bulk. By performing additional experiments with passive colloids
(flow tracers) and comparing with numerical simulations for both particle
active motion and the fluid flow, we disentangle the dominating mechanisms
behind the observed interfacial particle accumulation. These results allow us
to make an analogy between active Janus particles and some biological
microswimmers concerning how they interact with their environment.
