An experimental investigation of effects of surfactants on spilling breakers

The effects of surfactants on spilling breakers were studied experimentally. The spilling breakers were generated mechanically with a dispersive focusing technique and five different wave-maker motion conditions. Three surfactants (Triton X-100, Sodium Dodecyl Sulfate (SDS) and hemicyanine) were used and a separate series of experiments was performed with each. The ambient dynamic surface properties of the solutions were measured in situ with a Langmuir trough that incorporates a longitudinal wave device and a Whilhelmy plate. The histories of the crest profiles of breakers were measured with a photographic technique.; In clean water, as the waves approach breaking, a bulge with a round surface forms on the forward face. The leading edge of the bulge (called the toe) is a point of high upward curvature and capillary waves form upstream of the toe. As the process continues the flow separates at the toe and ripples are generated between the toe and the crest as the toe moves rapidly down the wave face.; This process dramatically changes in several ways depending on the surfactant and its concentration. Generally, at low concentrations of surfactant, the breaking process also starts with a bulge on the forward face, but the bulge is flatter. The amplitude and number of the capillary waves are reduced. As the concentration of surfactant is increased but kept below the critical micella concentration (CMC), the size of the bulge diminishes and the capillary waves decrease and eventually disappear. The ripples vary with the concentration of surfactant.; When the concentration of Triton X-100 is above the CMC, no bulge or capillary waves appear and a small plunging jet occurs at the crest. This jet event is qualitatively similar to that found in numerical calculations without surface tension. A jet also appears at the highest concentration of SDS when additional surfactants of unknown chemical composition were also present in the tank. In this case, the bulge forms without capillary waves and a small jet issues from the surface of the bulge at a point just above the toe. A similar jet was found in the hemicyanine solution.