## 3 Results and Discussion
### Bubble Growth and Departure Regimes
Interestingly, the bubble on the left-hand side (marked with green borderline) starts to depart from the surface at the 0.044 s mark but is pulled down towards the surface due to coalescence with another bubble seen at the back of the imaging plane (i.e., see the snapshot at 0.023 s). After the 0.050 s mark, this bubble starts departing again. Bubble interaction in the vertical direction has previously been studied by [PERSON] et al. [30], where the first detached bubble was attached towards the electrode surface while the second bubble just departed from the electrode, exhibiting a sudden reversal in motion, followed by coalescence with the first bubble. As the left bubble departs, all of the nucleating microbubbles start feeding the bubble on the right-hand side (marked with red borderline), which grows in size and starts departing after the 0.063 s mark. Within 0.001 s (between 0.080 s and 0.081 s marks), the two bubbles merge into a single one that finally completely departs from the surface. While Figure 14 only shows one case, similar behavior is frequently observed at 2.9 V operation, where the actual time and size of departure depend on the spatiotemporal distribution of intermediate (-50 \(\upmu\)m) bubbles. The growth of a single coalesced bubble at 3.0 V is presented in Figure 15. Two intermediate coalesced bubbles appear, and they merge between the 0.015 and 0.016 s mark. This bubble nearly departs at 0.055 s but is pulled back this time due to multiple simultaneous coalescences with smaller bubbles that grow atop the electrode; bubble motion compared with location at previous time stamp is denoted with red arrows. Bubble motion reversals and contactless interaction between two hydrogen bubbles during water electrolysis were previously studied in-depth by [PERSON] et al. [30]. They reported that the most prominent scenario of bubble interaction consists of a sudden reversal of the first detached bubble's motion, causing the bubble to return to the electrode's surface and coalesce with the second bubble, attributing this effect primarily to thermocapillary (Marangoni) forces. When the thermal boundary layer on top of the second bubble (i.e., bubble at the bottom) can touch the bottom of the first bubble (i.e., bubble at the top), bubble reversal motion at specified conditions sets in. Their conclusion was that thermocapillary migration is at the origin of H\({}_{2}\) bubble motion reversal, and the interaction of electrogenerated bubbles needs to be taken into account during water electrolysis. Our observations, while not refuting the role of Marangoni forces, emphasize that an extended contact line, formed by a dense layer of microbubbles bridging the electrode and the larger bubble, also plays a significant role. This extended contact line increases  
Figure 9:0 Growth of a single coalesced hydrogen bubble at 3.0 V. Coalesced bubble center position relative to the previous time stamp is denoted by red arrows. the pinning force, which in turn may counteract buoyancy and contribute to the reversal of bubble motion. Due to the limitations of our current visualization techniques, we are unable to separate the influence of Marangoni forces from that of the extended contact line. Nevertheless, our findings suggest that under certain conditions, both mechanisms may act in concert to influence bubble reversal dynamics. Our observations suggest that bubbles emerging from the electrode increase in size and merge with a larger bubble above. After coalescence, the larger bubble experiences a downward force due to the extended contact line between the electrode and the multitude of microbubbles bridging the gap between the surface and the larger bubble. Since the pinning force is directly proportional to the total contact line length, it becomes significantly stronger than buoyancy in this case, resulting in the observed downward movement. As the bubble continues to grow, buoyancy increases, while the microbubble cloud on the electrode partially merges due to ongoing electrolysis.