Wide band spectroscopy of liquid surface ripplon

A light scattering technique was developed to measure the propagation of liquid surface waves over a wide frequency range. Thermal fluctuation of liquid surface displacement excites a surface tension wave, called the ripplon, whose behavior gives information on dynamic properties of the liquid surface. A laser beam is scattered by the ripplon, and the power spectrum of the scattered light is detected with the optical beating technique, which yields the frequency and damping constant of the ripplon. Previous measurements of the ripplon have been limited to frequencies below 100 kHz. The range of measurements is extended up to 6 MHz. The dispersion relation and the damping constant were obtained over the ripplon wave number (K) range of 4×10⁴<K<3×10⁶ m^-1 for some pure liquids including water, methanol, and ethanol. These results are in good agreement with the prediction of hydrodynamic theory