Impact of tracer particles on the electrolytic growth of hydrogen bubbles

The thermocapillary effect at gas bubbles growing at micro-electrodes seems well understood. However, the interfacial flow measured in the upper bubble part decays faster than found in first simulations by Massing et al. ["Thermocapillary convection during hydrogen evolution at microelectrodes," Electrochim. Acta 297, 929 (2019)]. Recently, Meulenbroek et al. attributed the origin of the difference to the influence of surfactants being present in the electrolyte ["Competing Marangoni effects from a stagnant cap on the interface of a hydrogen bubble attached to a microelectrode," Electrochim. Acta 385, 138298 (2021)]. Surprisingly, the presence of tracer particles added to the electrolyte for measuring its flow was not yet considered. Our recent experiments reveal that varying the small amount of tracer particles added influences the bubble shape, its dynamics, and also the electrolyte flow nearby. We therefore present a model to describe the particle attraction to and the particle dynamics at the bubble interface, which allows us to quantify the impact. Corresponding simulations are validated against measurements for different bulk particle concentrations and show a good agreement of the tangential velocity profile at the bubble interface caused by thermo- and solutocapillary effects. Depending on the particle concentration, parts of the upper bubble interface are found to become stagnant. The results allow a deeper insight into the complex phenomena of electrolytic gas evolution and further put attention to a careful application of particle-based measurement techniques in gas-liquid systems.