Mechanism of the zigzag and spiral bubble ascension: The alternating steering and continuous chase effects of the side reflux on the bottom surface

The zigzag and spiral trajectories observed during bubble ascent in quiescent water are revisited through a combined experimental and numerical investigation. A strong coupling is identified among the bypass flow, bubble shape evolution, and lateral path. In particular, the flow along the bubble’s lower surface induces periodic shape transitions—from a backslash (“ \”) to a “V” and then to a forward slash (“ /”), corresponding to lateral deflections in motion. While alternating steering from the “ V”-shaped interface contributes to the zigzag pattern, further analysis reveals that the bypass flow splits at the stagnation point, forming a pair of counter-rotating streams. Their competition near the bubble’s bottom induces transverse internal flow and rolling torque, which are identified as the primary drivers of path instability. This mechanism provides an alternative to classical explanations based on vortex shedding, which is shown here to be a consequence rather than a cause. Moreover, the orientation of the competing surface flows determines the trajectory type: in-plane competition results in zigzag motion, while out-of-plane interaction leads to spiraling. These findings suggest a unified framework linking shape dynamics, interfacial flow, and bubble trajectory.