Microbubble Generation via Combined Saffman-Taylor and Plateau-Rayleigh Instabilities

Microbubble generation via rapid depressurization of supersaturated water through a 8 mm wide by 2 mm long by 0.1 mm deep Hele-Shaw passage was explored experimentally using micro shadowgraphy. The flow topology consists of a series of regularly sized cellular cavities, characteristic of viscous fingering in a Hele-Shaw flow. Cells develop in a region of separated flow at the sharp leading edge of the confined flow. Each cell is stable and pinned to the wall. Small-scale structures stretch into ligaments downstream and destabilise due to Plateau-Rayleigh instability, with consequent microbubble pinch-off providing a coherent bubble stream from each cell. The influence of cavitation number (σin j) and Reynolds number (Re) on instability formation and microbubble production is investigated by varying the differential pressure between the passage inlet and outlet. Optimum conditions for bubble generation were found for σ_inj = 0.28 − 0.65 and coincide with the maximum in the number of cells forming along the passage leading edge. Polydisperse bubble populations were generated for all conditions. Bubble production rates of up to 2.2 MHz were observed, with a dominant bubble size of ≈ 6µm. Further decrease in σ_inj hindered bubble generation due to disappearance of the laminar instability as the flow in the passage transitions to a turbulent regime.