Fast dynamics of surfactant probed by the acoustics of a drop impact

Adding a surfactant to water leads to changes in the outcome of a water drop impacting on the solution such as the dynamics of the Rayleigh jet, and the same is true for the bubbles entrainment. The resulting acoustic signal is, therefore, modified in the presence of a surfactant and is found to be related to the fast dynamics features of the latter. To this end, the airborne acoustic signal is synchronized with hydrodynamic images, recorded by a high-speed camera, of a water drop impacting aqueous solutions with varying concentrations of three different surfactants. It is found that the starting time of the acoustic events shows a maximum around the third of the critical micellar concentration independently of the surfactant chemistry. This feature is related to the variation of the Rayleigh jet maximum height resulting mainly from a subtle balance between an increase in viscosity and concentration in addition to Marangoni flows that affect the acceleration of the Rayleigh jet and modify its initial speed.