Vincent Gourmandie (Laboratoire Matière et Systèmes Complexes, Université Paris Cité)

Drop impact: effects of surfactants and acoustic of resonating Bubbles

When a droplet impacts a liquid surface, an air cavity forms underwater, occasionally leading to the entrapment of a bubble during sudden retraction. The precise mechanism responsible for bubble formation remains unclear. It seems that bubble formation occurs within a specific range of drop diameters and fall heights. Another observation from the literature is that this phenomenon ceases when surfactants are introduced into the liquid. However, this effect has not yet undergone systematic study. Using a water-ethanol mixture and water-surfactant (SDS) solutions, we conducted experiments to investigate the impact of surface tension on bubble entrapment.

This entrapment is accompanied by powerful acoustic emissions, a phenomenon familiar to anyone dealing with a dripping faucet. While the frequency has been well-documented since Minnaert in 1933, the amplitude of the acoustic signal has not yet been explored. We employed drop impact and an underwater nozzle to generate bubbles, recording the acoustic signals with a hydrophone and capturing the birth of the bubbles with a high-speed camera. Our findings suggest that the acoustic amplitudes can be effectively explained by considering the bubbles as harmonic oscillators with initial velocities measurable from the recorded videos.