Measurement of the surface tension of water using microwave backscatter from gravity-capillary waves

A technique developed for measuring the surface tension of fluid based on the resonant scattering of microwave radiation from gravity-capillary waves on the surface of the fluid is discussed. The method was implemented by generating monochromatic water waves in a tank, illuminating them with microwave radiation, and then tuning the water-wave frequency until Bragg resonance was observed, that is, until the backscattered power was observed to reach its maximum value. At resonance, the wavelength and wave frequency can be calculated from the physical parameters of the system and from the Doppler shift of the scattered radiation. Laboratory experimental results indicate an accuracy on the order of ±1.6 dyn/cm for the surface-tension measurement. This accuracy is in agreement with an error model based on the width of the Bragg resonance line. The technique is, in principle, nonintrusive and thus can be used for observing the properties of surfactants on water surfaces