  Self-oscillations underlie many natural phenomena such as heartbeat, ocean
waves, and the pulsation of variable stars. From pendulum clocks to the
behavior of animal groups, self-oscillation is one of the keys to the
understanding of synchronization phenomena and hence the collective behavior of
interacting systems. In this study, we consider two closely spaced bubbles
pulsating in the kHz range in response to ultrasonic excitation. A
translational bouncing motion emerges from their interaction with a much lower
frequency than the bubble pulsation frequency. Our analysis reveals that the
observed bubble bouncing exhibits the main features of self-oscillation, such
as negative damping and the emergence of a limit cycle. These results highlight
unexpected nonlinear effects in the field of microbubbles and give insights
into the understanding of synchronization in large bubble clouds.
