  The formation, growth and detachment of gas bubbles on electrodes are
omnipresent in electrolysis and other gas-producing chemical processes. To
better understand their role in the mass transfer efficiency, we perform
experiments involving successive bubble nucleations from a predefined
nucleation site which consists of a superhydrophobic pit on top of a
micromachined pillar. The experiments on bubble nucleation at these spots
permit the comparison of mass transfer phenomena connected to electrolytically
generated H$_2$ bubbles with the better-understood evolution of CO$_2$ bubbles
in pressure-controlled supersaturated solutions. In both cases, bubbles grow in
a diffusion-dominated regime. For CO$_2$ bubbles, it is found that the growth
rate coefficient of subsequent bubbles always decreases due to the effect of
gas depletion. In contrast, during constant current electrolysis, the bubble
growth rates are affected by the evolution of a boundary layer of dissolved
H$_2$ gas near the flat electrode which competes with gas depletion. This
competition results in three distinct regimes. Initially, the bubble growth
slows down with each new bubble in the succession due to the dominant depletion
of the newly-formed concentration boundary layer. In later stages, the growth
rate increases due to a local increase of gas supersaturation caused by the
continuous gas production and finally levels off to an approximate steady
growth rate. The gas transport efficiency associated with the electrolytic
bubble succession follows a similar trend in time. Finally, for both H$_2$ and
CO$_2$ bubbles, detachment mostly occurs at smaller radii than theory predicts
and at a surprisingly wide spread of sizes. A number of explanations are
proposed, but the ultimate origin of the spreading of the results remains
elusive.
