Laser heterodyne apparatus for measurements of liquid surface properties—Theory and experiments

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Abstract
An apparatus has been developed, which measures the laser light scattering from thermally excited surface capillary waves of liquids. Such measurements do not cause any mechanical or thermal disturbances of the surface under study (a low‐power cw He‐Ne laser is used). This is especially important in the study of film‐covered surfaces which are biologically interesting. Our heterodyne technique apparatus uses a grating to create a well‐controlled local oscillator light beam. This is the main difference between our apparatus and those of previous workers. Compared with these apparatuses the sensitivity and resolution are considerably improved in the measured spectra, from which liquid properties such as surface tension and viscosity are calculated. A quantitative analytical theory is developed which permits optimization of the measuring set up and it is demonstrated that the predicted resolution and sensitivity are obtained in the practical measurements. It is demonstrated with higher accuracy than in earlier light scattering measurments that for a number of liquids the standard capillary wave theory correctly predicts the measured spectra. The surface tension and viscosity of water as calculated from our measurements agree within 2% and 10%, respectively, with the values known from standard mechanical measurements.

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