Second harmonic generation and fluorescence images from surfactants on hanging droplets

Second harmonic generation (SHG) has been observed for the first time from the surface of hanging water droplets coated with surfactant molecules. SHG from 4-(4-dimethylaminostyryl)-octadecylpyridinium bromide surfactants is observed in both the specularly reflected and refracted transmitted directions and suggests that the molecules are not lying completely flat at the water–air interface. A fluorescence imaging technique is used to verify that rod-like surfactants are lying approximately flat on the surface. Decreasing the size of the hanging droplet results in an increase in surfactant density and SHG, similar to Langmuir–Blodgett film techniques used on flat liquid surfaces.