Surface fluctuation spectroscopy: A novel technique for characterizing liquid interfaces

Abstract

Surface fluctuation spectroscopy is a dynamic light scattering technique which involves the detection of thermal fluctuations (capillary waves) that cover every liquid interface. The advantages of this technique over conventional mechanical methods are discussed. The signal resolution and precision are improved by using a grating heterodyne technique coupled with a digital correlator. Detailed calculations showed that, for the measured power spectrum (the Fourier transform of the correlation function) to take the Lorentzian shape, it is necessary that certain optical conditions be met. These conditions are demonstrated with experimental data collected for ethanol at the ambient temperature as a function of the wave number. Once the optical parameters are adjusted correctly, the experimental results show excellent agreement with the literature values. We have also demonstrated that, using microcomputer technology, the entire system can be automated. The instrument is easy to operate and can be used either for research purposes or on a routine basis. The implication of this instrument as a powerful analytical tool for a wide spectrum of liquid interfaces is also discussed.