A drop impacting on a liquid surface may entrain a bubble resulting in sounds from the bubble oscillation in addition to that of the initial drop impact. This subject has received considerable interest with respect to the underwater sound produced by rain. The previous optical and acoustical studies have extensively examined the role of drop size and impact velocity as well entrainment formation mechanisms. However, the role of surfactants on the liquid surface has received less attention especially with respect to sound production. One previous study [1] of two different surfactants (Kodak Photoflo and a sulfo detergent) reports that these suppress entrainment resulting in diminished or negligible bubble noise. However, we have found bubble entrainment and acoustic emissions associated with the addition of a heptane layer on the water surface. Using a synchronized hydrophone and a high-speed camera, acoustical and optical data of the bubble drop dynamics were obtained. Particle image velocimetry (PIV) was also utilized to measure the surface velocity produced by the drop. These results show a significant difference in the drop rebound and subsequent fluid column dynamics, which results in a dual acoustic emission for the surfactant case.
Visualization of a bubble entrainment by a drop impingement onto liquid surface