Vibrational interaction of two vapor-rich microbubbles in degassed water*

In this study, we clarify the change in vibration frequency due to the vibrational interaction between two self-oscillating microbubbles. When degassed water is locally heated, a vapor-rich bubble of about 10 μm in diameter is generated above the heat source. This bubble can generate flows of up to 1 m/s and are therefore expected to be used as micropumps [1]. The vapor-rich bubbles self-oscillate at a stable frequency for a long period of time because of intense evaporation and condensation cycles on the local heating point. In this study, two bubbles are generated simultaneously by irradiating two laser spots on a photothermal conversion thin film. The distance between the two laser spots is systematically varied using a spatial light modulator. Then, a 5 Mfps high-speed camera is used to capture the change in bubble oscillation as a function of the distance between the two bubbles. When the distance between the bubbles is varied without changing the laser intensity, the frequency of the bubbles varies from 0.5 to 0.8 MHz depending on the distance between the bubbles. Particularly when the distance between the bubbles is close, the bubbles' oscillations is counterphase-synchronized. This coupled behavior is well reproduced by the extended Rayleigh-Plesset model, which incorporates pressure interactions.

[1] K. Namura et al., Sci. Rep. 7, 45776 (2017).