Shear-flow-driven bubble growth and detachment under microgravity

Bubble removal is a central limitation for water electrolysis under microgravity, where buoyancy no longer provides a robust detachment mechanism. Here, we study shear-flow-driven detachment of hydrogen and oxygen bubbles growing at Pt microelectrodes during DLR parabolic-flight campaigns. Microscopic shadowgraphy, synchronized current measurements, tracer-particle velocimetry and finite-element calculations were combined to determine how imposed flow modifies bubble detachment near the electrode. In the no-flow microgravity reference state, bubbles remain close to the microelectrode and grow slowly on a microbubble carpet, thereby shielding the electrode and allowing low currents only. Imposed shear can displace the mother bubble, partially unshield the electrode, and increase the mean current. However, the removal process is not governed by the Reynolds number alone. Calculations for a representative microgravity configuration show that thermocapillary Marangoni stress generated by localized Joule heating near the electrode may produce a hydrodynamic force of the same order as the drag force by the imposed shear flow. This result provides a mechanistic basis for the long residence observed in the selected microgravity configuration. In parallel, coalescence with newly nucleated microbubbles provides intermittent perturbations that can assist the detachment of smaller bubbles or disturb the near-wall configuration of larger bubbles, thereby promoting electrode rewetting. Pulsating flow provides a second removal pathway by combining lateral displacement, electrode rewetting, accelerated microbubble growth, and coalescence-induced jump-like motion.