Ionic Diffusiophoresis of Active Colloids via Galvanic Exchange Reactions.

In order to move toward realistic applications by extending active matter propulsion reactions beyond the classical catalytic hydrogen peroxide decomposition, we investigate the self-propulsion mechanism of Janus particles. To address the influences of ionic species, we investigate Janus particles driven by a galvanic exchange reaction that consumes and produces ions on one hemisphere. Our galvanophoretic experiments in the regime of thin Debye layers confirm that even the simplest models in active matter are still full of important surprises. We find a logarithmic speed dependence on the fuel concentration, which cannot be explained using the classic ionic self-diffusiophoretic framework. Instead, an approach based on the Poisson-Nernst-Planck equations yields a better agreement with the experiments. We attribute the discrepancy between the two models to the breakdown of two key hypotheses of the ionic self-diffusiophoretic approach.