Interfacial agglomeration of self-propelled Janus particles in evaporating droplets*

Living microorganisms are often found in confined systems, where they typically experience an affinity to populate boundaries, or at least their vicinities. For example, in the case of model microorganisms as the bacteria E. coli, the reasons for such affinity to interfaces is 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 on the vicinity of the liquid/gas interface. Interestingly, the interfacial accumulation occurs despite the presence of a Marangoni flow in the evaporating droplet, which tends to redistribute the particles within the bulk. By performing additional experiments with passive colloids behaving as flow tracers and by comparing with numerical simulations for the fluid flow and for the particle dynamics, we will disentangle the dominating mechanisms behind the interfacial particle accumulation observed. Our results show new unexpected parallelisms between active Janus particles and some biological microswimmers in the way they interact with their environment.