Interfacial mass transfer enhancement induced by bubble bouncing and shape oscillations

Induced shape oscillations during bubble bouncing on substrates have been predicted to enhance mass transfer. However, the underlying physical mechanisms, including how bubble shape oscillations enhance mass transfer and whether this is governed by amplitude (low-order modes) or frequency (high-order modes), remain insufficiently understood. In this work, we hypothesize that bubble bouncing with shape oscillations enhances mass transfer by promoting interface renewal induced by circulation and concentration boundary layer separation. Within this framework, we further propose that the enhancement is primarily governed by low-order oscillation modes.

A combination of optical methods with high spatiotemporal resolution - planar laser-induced fluorescence, particle image velocimetry and shadowgraphy - is employed to quantify the dissolved oxygen concentration field, the surrounding flow field and the bubble morphology. Additionally, a shape decomposition method is developed to analyze the oscillation modes of the bubble.

Bubble bouncing accompanied by shape oscillations enhances mass transfer by approximately 20%, compared with the predictions of the classical model dominated by convection for moving bubbles. This enhancement results from the circulation and the concentration boundary layer separation, both driven by relatively large-amplitude oscillations of low-order modes during bubble bouncing, which promote interface renewal, as revealed by the spatiotemporal evolution of the flow and concentration fields. Building on these findings, an extended Sherwood number formulation is proposed, which takes bubble bouncing with shape oscillations in the low-order mode into account.