Hydrodynamic mechanisms of deformable bubble pair interactions at varying Weber and Reynolds numbers

The two-bubble system provides an ideal framework for investigating the fundamental interaction mechanisms within bubble swarms. Using a coupled front-tracking and proportional-integral-derivative control simulation framework, we systematically investigate the hydrodynamic interactions between bubble pairs across a wide range of Reynolds numbers (Re), Weber numbers (We), and spatial configurations. Our results reveal a stark dichotomy in interaction regimes dictated by the Weber number. At low Weber numbers, where bubbles remain nearly spherical, tandem pairs exhibit a drag reversal: with increasing Reynolds number, the interaction transitions from a shielded, attractive state to an inertia-driven repulsive one, while side-by-side pairs experience attraction. In contrast, at high Weber numbers, pronounced bubble deformation leads to persistent attractive interactions for tandem pairs across all tested Reynolds numbers, whereas side-by-side configurations become repulsive. Furthermore, a critical inclination angle range of 30°–45° is identified, marking a transition in the interaction regime, characterized by a reversal in lift force direction. The rotational dynamics also show strong Weber number dependence: at low We, the torque consistently promotes horizontal alignment, consistent with predictions for spherical bubbles, whereas at high We, the torque reverses within the same critical angle range, actively promoting vertical alignment for highly deformable bubbles. These findings provide new insights into the dynamics of bubble pairs and advance our understanding of multiphase flow behavior in bubble swarms.