THERMALLY EXCITED LIQUID SURFACE-WAVES AND THEIR STUDY THROUGH THE QUASI-ELASTIC SCATTERING OF LIGHT

A simple apparatus employing a semiconductor diode laser and PIN photo diode has been used to investigate thermal capillary waves on liquid s urfaces. These waves act as a weak, time-varying diffraction grating f or the incident laser light; the diffracted light can be heterodyned w ith the light directly reflected from the liquid surface to extract fl uid properties (surface tension and viscosity). In this paper we prese nt a discussion of the phenomenon of surface waves and describe the co nstruction of an apparatus to observe them. Results of measurements wi th this apparatus for the surface tension and viscosity of water with and without oil films and of benzyl alcohol at different temperatures demonstrate the effectiveness of the technique as well as environmenta l effects on liquid properties. This problem provides rich ground for a study of wave and thermal phenomena as well as an introduction to a variety of experimental techniques. (C) 1996 American Association sf P hysics Teachers.