Simultaneous measurement of free-surface velocity and surfactant concentration via a common laser probe

A new technique was developed to simultaneously measure the velocity and surfactant surface concentration at an air/water interface via a common laser probe. The surfactant concentration is measured using the nonlinear optical technique of reflected second-harmonic generation (SHG), utilizing an Nd:YAG laser. The transmitted portion of the same laser beam provides illumination for velocity measurements at the interface using the technique of boundary-fitted digital particle image velocimetry (DPIV). The combined SHG and DPIV system was utilized to scan along the free surface of a uniform water flow with a surface barrier trapping hemicyanine, an insoluble monolayer. Measurements were made downstream of the Reynolds ridge, which marks the contamination front and the leading edge of a free-surface boundary layer.