Interfacial mass transfer enhancement induced by bubble bouncing and shape oscillations.

Area of Science:

• Fluid dynamics
• Mass transfer phenomena
• Bubble dynamics

Background:

• Shape oscillations during bubble bouncing are predicted to enhance mass transfer.
• The precise mechanisms and governing factors (amplitude vs. frequency) remain unclear.

Purpose of the Study:

• To investigate how bubble shape oscillations enhance mass transfer.
• To determine if low-order or high-order modes are primarily responsible for this enhancement.
• To elucidate the role of interface renewal, circulation, and boundary layer separation.

Main Methods:

• Utilized high spatiotemporal resolution optical methods: planar laser-induced fluorescence, particle image velocimetry, and shadowgraphy.
• Quantified dissolved oxygen concentration, flow fields, and bubble morphology.
• Developed a shape decomposition method to analyze bubble oscillation modes.

Main Results:

• Bubble bouncing with shape oscillations increased mass transfer by approximately 20% compared to classical models.
• Observed interface renewal driven by circulation and concentration boundary layer separation, linked to low-order, large-amplitude oscillations.
• Spatiotemporal evolution of flow and concentration fields confirmed these mechanisms.

Conclusions:

• Bubble shape oscillations, particularly low-order modes, significantly enhance mass transfer during bouncing.
• The enhancement mechanism involves interface renewal via circulation and boundary layer separation.
• Proposed an extended Sherwood number formulation accounting for bubble bouncing with low-order mode oscillations.