Velocity field and surfactant concentration measurement techniques for free-surface flows

 Noninvasive measurement techniques were developed to obtain the surfactant concentration and the velocity field of a vortex pair interacting with a contaminated free surface. The optical method of second-harmonic generation (SHG) was utilized to determine the time-varying concentration of a surfactant at a point on the surface, and the established technique of digital particle image velocimetry (DPIV) was used for the measurement of the velocity field. The evolution of the initially uniform, insoluble monolayer along with the vorticity field are described. One of the present limitations is the temporal resolution of the concentration measurements which is of order 5 Hz. The extension of the SHG technique to high Reynolds number, turbulent flows is discussed. Received: 24 January 1996 / Accepted: 17 July 1996