Imaging surfactant concentration distribution at the air/water interface

The surface specific spectroscopic probes of reflected second harmonic generation (SHG) and reflected sum frequency generation (SFG) have been successfully employed to measure surfactant monolayer concentration gradients on the ocean surface. Reflected SHG was adapted for areal measurements of monolayer concentrations using an intensified pulse gated CCD camera for detection and a dichromatic image splitter to allow for signal normalisation and scaling. As a preliminary test prior to field use, the imaging probe was used to measure the surfactant concentration gradients for an insoluble monolayer on a channel flow downstream of the Reynolds ridge. The resulting concentration measurements had a temporal resolution of 3 nanoseconds and spatial resolution of 0.21 millimetres or less per pixel.