  Colloidal particles partially coated with platinum and dispersed in H2O2
solution are often used as model self-propelled colloids. Most current data
suggest that neutral self-diffusiophoresis propels these particles. However,
several studies have shown strong ionic effects in this and related systems,
such as a reduction of propulsion speed by salt. We investigate these ionic
effects in Pt-coated polystyrene colloids, and find here that the direction of
propulsion can be reversed by addition of an ionic surfactant, and that
although adding pH neutral salts reduces the propulsion speed, adding the
strong base NaOH has little effect. We use these data, as well as measured
reaction rates, to argue against propulsion by either neutral or ionic
self-diffusiophoresis, and suggest instead that the particle's propulsion
mechanism may in fact bear close resemblance to that operative in bimetallic
swimmers.
