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

Abstract

An experiment for the investigation of the exchange dynamics of adsorbed and dissolved surfactants at the air–water interface is described. The experiment combines two established techniques: the method of the oscillating bubble generates in a well-defined fashion a nonequilibrium state by a periodic compression and expansion of the surface layer. The state of the adsorption layer is then investigated by surface second harmonic generation (SHG). SHG is a second order nonlinear optical technique, which probes only the adsorbed molecules. Contribution of the bulk is widely suppressed and information on the orientational order of the surfactant and the corresponding number density of the adsorbed species are gained. These data are of utmost importance to assess the established models of Gibbs adsorption layer. The combination of the nonlinear optical technique with a rapidly oscillating bubble imposes severe experimental hurdles such as the synchronization of the laser pulses with the state of the bubble or problems arising from a bad signal to noise ratio. The experimental setup discussed in this article overcomes these problems and outlines all relevant design parameters.