An oscillating bubble device for direct measurement of molecular exchange processes at the air-liquid interface in the medium frequency range

An experiment for the investigation of the exchange dynamics of adsorbed and dissolved surfactantsat the air–water interface is described. The experiment combines two established techniques: themethod of the oscillating bubble generates in a well-defined fashion a nonequilibrium state by aperiodic compression and expansion of the surface layer. The state of the adsorption layer is theninvestigated by surface second harmonic generation ~SHG!. SHG is a second order nonlinear opticaltechnique, which probes only the adsorbed molecules. Contribution of the bulk is widely suppressedand information on the orientational order of the surfactant and the corresponding number densityof the adsorbed species are gained. These data are of utmost importance to assess the establishedmodels of Gibbs adsorption layer. The combination of the nonlinear optical technique with a rapidlyoscillating bubble imposes severe experimental hurdles such as the synchronization of the laserpulses with the state of the bubble or problems arising from a bad signal to noise ratio. Theexperimental setup discussed in this article overcomes these problems and outlines all relevantdesign parameters