PLIF Studies on Surfactant Distribution during Drop/Interface Coalescence*

In this work, High-speed Laser Induced Fluorescence was used for the first time to detect the distribution of the fluorescent surfactant NBD-PC during the coalescence of aqueous drops with liquid/liquid interfaces. The aqueous phase was a 46% glycerol solution while the organic phase was a 5 cSt silicone oil. To vary the surfactant concentration on the interface, the drop and the corresponding interface were allowed to stand for different times (0 ~ 900 min) before the experiments. After the drop contacted the interface, the surfactants on the contacted region were swept away by the draining liquid in the trapped film and peaked at a barrier ring area, 0.75R_h away from the centre, where R_h is the horizontal drop radius. Once the thin film retracted after the rupture, the surfactant concentration peaked at the neck tip and continued to increase during the film retraction. At the later coalescence stages, the drop took the shape of a cylinder due to the capillary waves on the drop surface. The surfactant concentration was found to be low at the top of the cylinder where the interface was stretched. As the liquid cylinder started to shrink at the final stages the surfactant concentration at the top increased significantly.