The Characters Of Laser Diffraction Fringes From Liquid Surface Waves At Low-frequency And Measurement In Its Parameters By Means Of Laser Diffraction

Compared with the bulk, the liquid surface has much difference in composition, structure, energy state, and so on. It is the difference that makes the liquid surface take on some special characteristics. These characteristics will be involved in the liquid surface waves. So, we can get the parameters through detecting the liquid surface waves.The liquid surface wave is a kind of elastic wave which travels along the liquid surface. An effective measure must be set up if you want to obtain the characters of the liquid or liquid surface waves. Researchers are favor of the optic means which possess the lossless, no contacted, real-time and prompt characters. Since the laser has been invented, it was been widely used in studying the surface waves. The research of high frequencies surface waves are mostly based on acoustic-optic diffraction theory. Based on the theory, the diffraction angle will be widening along with the surface waves frequencies accretion. So, this kind of experiment is focused on Ultrasonic wave mainly.In 1979, G.Weisbuch put forward the idea that liquid surface waves was detected by laser diffraction, and an optic way of detecting liquid surface tension has been come true. We have taken special research on the liquid surface waves at low frequencies. In these experiments, we declined the surface wave's frequencies to tens Hz in our experiments and achieved the goal well.The liquid surface wave's characters have been studied by laser diffraction in this paper. Through the analysis of the laser diffraction field, the dependence of the diffraction intensity upon the surface wave amplitude and the relation of fringes distribution with surface wavelength were derived. We can measure the liquid surface wavelength,amplitude and liquid surface tension base on the relation. In our research, the major phenomena and conclusions were obtained as follow:1. We class the surface waves and sum up the acoustic-optic effect. There are two kinds of diffraction when the light incidence on the crystal which was acted by sound wave, Raman-Nath diffraction and bragg diffraction. The phenomenon of Raman-Nath diffraction can be observed higher order fringes rather than the bragg's in which you only can find the zero and the first order diffraction fringes obviously. 2. Making use of the momentum conservation equation, Navier-Stokes equation (N-S equation) was deducted. The N-S equation was simplified based on the reversion force traits of the surface waves at low frequencies. The surface wave's equation has been obtained through answering equation N-S equation. From the laplace equation and the potential function, the liquid dispersion relation also has been obtained at the same time.3. Light diffraction effect from the low frequencies liquid surface waves. Highly visible stationary laser diffraction patterns from liquid surface waves at a few tens Hertz were obtained experimentally, and the disappearance of diffraction spot was also observed, including zero fringes. The dependence of the diffracted field upon the surface waves were derived by theory of Fourier optics. Based on the relations of the diffraction field and the surface wave, some relations were conformed such as: The diffraction fringes spatial distribution versus surface wavelength, diffraction intensity versus surface wave amplitude, and so on.4. Measurement in the parameters of the low frequencies surface waves by means of laser diffraction. In order to detect the liquid surface waves validly, an appropriate way should be chosen. We measure the liquid surface tension, surface wave's amplitude, surface wavelength by means of laser diffraction. In our experiment, we test the liquor surface tension of sodium oleate liquor, ethanol liquor and sodium chloride liquor and get the relation of surface tension versus the liquor concentration; the result will be helpful for studying surfactant. The experimental results indicate another result that the liquid surface wave amplitude declined exponentially with the spreading distance and the damping coefficient, which varied lineally with frequency, has dispersion characteristics.