Study on High Frequency Dynamics of Liquid Surface by Laser Induced Surface Vibration Spectroscopy

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We developed a new technique of picking up the liquid surface in a non-contact manner by a cw-laser radiation. The momentum change of light at the laser transmission through the air-liquid interface appears as the radiation pressure, which deforms the liquid surface into the shape determined by the balance between the Laplace force of the curved surface and the radiation pressure. The displacement of the liquid surface is inversely proportional to the surface tension, which was measured by an optical probe. The dynamic response of the liquid surface deformation was theoretically derived under the periodical modulation of the radiation pressure. The experimentally observed spectra were in good agreement with the theory giving the dynamic properties of the liquid surface.Objectives of microscope were employed to excite the high frequency surface vibration. The laser spot was focused on the liquid surface within the diameter of several microns and the surface vibration was excited over the frequency of 1 MHz.We also tried to excite the wave propagating on the liquid surface with linear a wavefront. The laser light was linearly focused on to the liquid surface with a cylindrical lens with the width of several microns and the length of 200 microns. Coherent wave was successfully excited in the frequency range from 100 Hz up to several tens of kHz. We can accurately determine the mechanical properties on and near the surface, such as surface tension and shear viscosity, by measuring the surface wave propagation.The technique of the laser induced surface deformation (LISD) has a potential as the measurement tool of the surface dynamic properties, such as the time dependent surface tension and surface visco-elasticity.