Time-resolved analysis of coalescence-induced bubble oscillations at the Onset of Nucleate Boiling in a subcooled mini-channel

Bubble coalescence plays a fundamental role in nucleate boiling, as it redistributes interfacial energy and modifies vapor organization locally. This study investigates the post coalescence dynamics of wall-attached bubbles in subcooled mini-channel boiling using deionized water with high-speed imaging at 10,000 Hz. Advances in computer vision enable the extraction of time-resolved quantitative bubble behavior using CNN segmentation with Kalman tracking. Thanks to the implemented tracking processing, the focus is on the short-term deformation and oscillatory relaxation of merged bubbles and their relation to coalescence symmetry and detachment. Directly after coalescence, bubbles undergo a strong shape deformation characterized by high ellipticity, followed by damped oscillations. Under the experimental conditions, an empirical transition near Λ ≈ 0.11 , based on the parent-bubble diameters, was observed between post-coalescence detachment and sliding events. The deformation intensity and oscillation damping then depend mainly on parent bubbles symmetry. Indeed, nearly symmetric coalescence produces larger deformation amplitudes and more sustained oscillatory motion, favoring detachment, whereas asymmetric interactions result in weaker and more rapidly damped oscillations, favoring sliding. The dominant oscillation frequencies range from 500 to 700 Hz and are consistent with early-time dynamics mechanisms despite significant deformation and wall attachment, with a mean dimensionless frequency K ∗ ≈ 0.49, close to the theoretical Rayleigh–Lamb for quadrupolar oscillations. These results are consistent with coalescence-induced detachment mechanisms and the work extends previous studies on bubble detachment by quantifying post-coalescence oscillation frequency, damping at ONB.