| Abstract: Surface acoustic waves (SAW) is a kind of elastics wave, the propagating energy of which concentrates on the medium (solid or liquid) surface. SAW travels along the medium surface, which carries a large number of surface information. Therefore SAW research has already brought to more and more attentions. Compared with the bulk, the liquid surface has much difference in composition, structure, force, energy state, and so on. It is the difference that makes the liquid surface take on some characteristics. One can obtain some parameters of liquid by means of researching them. Optical method is an important technique to detect the substance properties. Of course, this method is a good choice for the surface research as well. When the optical effects, including diffraction, reflection, occur on the liquid surface, many fantastic phenomena will emerge, which may be used to measure some parameters of liquid. Optical methods have been adopted extensively in many fields, for example, Fluid Mechanics, Ocean Optics. In this thesis, we apply the optical method to study the surface of liquid and the liquid surface wave.In our research, the major phenomena and conclusions were obtained as follows:1. In experiment, highly visible stationary diffraction patterns from low-frequency liquid surface waves were obtained, based on the light diffraction. The asymmetrical position and intensity were also observed. The diffraction pattern is processed by a computer and the figure of the diffraction intensity versus diffraction distance is plotted, which indicates the degree of asymmetry increases with the heighten order of diffraction.2. In theory, the relationship between the diffraction intensity and surface wave has been derived. The approximate conditions of the surface wave diffraction and the half angular width were analyzed, which explains the asymmetrical distribution well. The theoretical and experimental results are in good agreement.3. An optical method of measuring liquid viscosity, based on the light diffraction from these low frequency liquid surface waves, was developed. The dependence of the liquid viscosity on the SAW attenuation coefficient and wave number has been theoretically derived. Further more, the correlation of the SAW attenuation coefficient and wave number with the diffraction pattern has been deduced. The experimental setup was given and highly visible diffraction patterns were observed. The diffraction patterns and the diffraction intensity at different propagation distances were obtained, respectively. And then, the attenuation curve of the SAW was plotted. The result of surface tension and water viscosity measured experimentally is 53.4×10-3N·m-1 and 0.730×10-3N·m/s2, respectively. In comparison with the widely accepted values 72.8×10-3N·m-1 and 1.018×1013 N·m/s2, the values of measurement is small. In general, the impurity of air will be absorbed by the liquid to decrease the value of surface tension, in the other words, the value of viscosity will be decreased. In comparison with capillary method, this technique is real time and non-contact. When the widely accepted values of surface tension is used into the equation of the viscosity, the calculated values is agreement with the widely accepted values. This indicated the technique is feasible.
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