Curvature driven acoustics and the physical dynamics of a bubble bursting in a droplet*

We investigate the sounds of bubbles bursting in liquid droplets placed on different wettability surfaces. A high-speed camera is synchronized with an acoustic air microphone and a piezoelectric contact microphone, to capture the physical and acoustic characteristics. The bubble pops, a jet forms, and subsequent capillary waves travel along the droplet toward the contact microphone and back again.  Our observations reveal that the peak frequency of the sound recorded by the air microphone during the initial bubble pop, varies with the bubble size irrespective of droplet volume as expected. However, the initial popping amplitude of the signal recorded by the contact microphone is influenced by the droplet shape, which is determined by surface wettability. Superhydrophobic surfaces exhibit a higher frequency magnitude from the emanating jet after bursting compared to hydrophilic surfaces, a result of droplet curvature. The frequency of the induced capillary wave motion is dependent on surface wettability and bubble size for a given droplet volume. In this regard, the spectrogram results can distinguish the frequency pattern of the capillary waves on a hydrophilic versus a superhydrophobic surface.