Influence of surfactant kinetics on rapid interface creation via microjet impact on liquid pools

Hypothesis: We posit that the adsorption kinetics of surfactants critically influence cavity dynamics during the rapid formation of interfaces on submillisecond timescales. We propose that standard surfactants such as sodium dodecylsulfate (SDS), while significantly reducing surface tension at equilibrium, do not significantly alter cavity dynamics. In contrast, ultrafast surfactants such as Surfynol, with adsorption rates that match or exceed the rate of new interface formation, would allow for the formation of deeper cavities that last longer. Experiments: We conducted experiments using a submillimeter jet impacting a liquid pool much larger than the jet dimensions, containing surfactant that create transient cavities which retract and close within milliseconds. Two surfactants with distinct adsorption kinetics were compared: sodium dodecylsulfate (SDS) and Surfynol 465. The induced dynamics, retraction and closure of the cavity, due to their characteristic submillisecond times, have been captured using ultrafast photography. Findings: Our experiments reveal the difference between the effects of sodium dodecylsulfate (SDS), a surfactant with moderately fast adsorption kinetics, and Surfynol 465, a surfactant with ultrafast adsorption kinetics. For SDS, the collapse pathway is nearly indistinguishable from that of pure water, suggesting negligible dynamic surface tension reduction. In contrast, Surfynol allows the emergence of deeper cavities that persist longer in the liquid pool. To quantify these effects, we applied a harmonic oscillator model that describes cavity retraction in the deep seal regime. The model fits the experimental data well, and the extracted damping values correlate with the expected dynamic surface tensions.