Impact of drops of surfactant solutions on small targets

The collisions of drops of surfactant solutions (dioctyl sulfosuccinate sodium salt (DOS) and trisiloxane oxypropylene polyoxyethylene (Silwett L77)) with small disc-like targets were studied both experimentally and theoretically. Upon impact, the drops spread very fast beyond the target in the shape of a thin lamella surrounded by a thick rim. No significant difference between water and surfactant solutions was observed in the early stage of the impact. But the collapse stages were very different. In particular, the lamellas of solutions of Silwett L77 disintegrated owing to a spontaneous nucleation of holes, giving to the lamella a web-like structure prior to its break-up. In contrast, lamellas of DOS solutions collapsed like water lamellas, except that the maximum diameter and the lifetime of the lamella of the most concentrated DOS solution were significantly increased compared with pure water and other surfactant solutions. A theoretical analysis shows that the observed instability effects in the lamella and the increase in the size and lifetime of the lamella can be caused by the coupling between liquid inertia and Marangoni stresses.