Liquid Surface Properties Of The Optical Visualization

Optical technique is important to probe the substance properties. Of course, this method is a good choice for the liquid surface research. When the optical effects, including interference, diffraction, reflection and refraction, occur on the liquid surface, some parameters of liquid surfaces can be measured. Visualization of liquid surfaces by means of the optical method includes two aspects: one is study on the dynamic liquid surface, in other words, probing of capillary waves and liquid surface properties by laser diffraction. The other is study on the static liquid surface, in so many words, measurement of the curved liquid surfaces and liquid surface properties by laser boundary reflection and glancing incidence. Visualization of liquid surfaces by means of the optical method is investigated in detail in this dissertation. The main work can be summarized as follows.(1) The theory of investigating capillary waves using optical diffraction in a case of oblique incidence is found. When the laser beam obliquely impinges on capillary waves at a certain angle, steady and visible diffraction spots are formed. The capillary wave acts as a sinusoidal reflection phase grating for the laser beam, and the theory is further developed from the theory of Fourier Optics. Missing order and asymmetry of diffraction patterns are investigated in some detail, both theoretically and experimentally. A combination of simulation and experiment is proposed to trace out light diffraction from capillary waves at a certain angle. The theoretical analysis and experimental results are in good agreement.(2) Measurement of the wavelength, the amplitude, and the dispersion relation of capillary waves by laser diffraction. The wavelength of capillary waves is real time determined with great precision by measuring the distance of diffraction spots. The amplitude of capillary waves is calculated by measuring the diffraction intensity ratio of different order. The data give the dispersion relation of capillary waves and provide an accurate method for determining the relation between the surface tension and the temperature, and the standard value and experimental results are in good agreement. The dispersion relation of liquid film surfaces is discussed. The relation between angular frequency and wave number at a fixed film thickness and the relation between wave number and film thickness at a fixed angular frequency are verified with experiments, and experimental data agree well with theoretical curves. A new method for measurement of liquid film thickness is presented in a no-contact way.(3) Study on the relation between the diffraction intensity and the angle of incidence. The theory of laser diffraction from capillary waves at a different angle of incidence is found from the theory of Fourier Optics, and the relation between the diffraction intensity and the angle of incidence is obtained. When the laser beam obliquely impinges on capillary waves at a different angle, steady and visible diffraction spots are formed. The diffraction intensity and the distance of diffraction spots vary with the change of an angle of incidence, and there is the phenomenon of missing orders in the diffraction spectrum. Asymmetry of diffraction patterns increases with the increasing of an angle of incidence. The theoretical and experimental results are in good agreement.(4) Measurement of liquid surface tension, surface pressure of insoluble monolayer surface film and velocity of capillary waves by laser diffraction. Surface tension of distilled water and kerosene are measured using laser diffraction, subsequently, the theory of dispersion relations is well consistent with the experimental data. The analytic expression of the surface tension and the temperature was derived by the non-linear least square fit to the data. Surface tension of distilled water decreases with the increasing temperature. The approximate relation between surface tension and temperature is linear. When laser falls on the insoluble monolayer surface film capillary waves, in the experiment steady and visible diffraction patterns are obtained. Surface pressure of Benzene is measured carefully. Phase velocity and group velocity of capillary waves are investigated. Velocity of capillary waves increases with the increasing frequency of capillary waves, velocity of capillary waves decreases with the increasing wavelength of capillary waves, and the relation is obviously nonlinear. At last, the velocity of capillary waves is measured at a different temperature, the analytic expression of the velocity and the temperature was derived by the non-linear least square fit to the data. Velocity of capillary waves decreases with the increasing temperature, and the approximate relation between velocity and temperature is linear.(5) Boundary reflection from curved liquid surfaces and its application. A simple technique for investigating the curved liquid surface (CLS) was developed, which is based on analyzing the reflection patterns from the CLS. When a collimated light beam vertically illumined the CLS which is caused by the effect of the wetting a smooth flat plate, a rod, a double parallel flat and a columned glass tube, the special reflective patterns, which correspond to the boundary light reflection from a deformed liquid surface, are experimentally obtained. In fact, there are the visible interference fringes in the reflective patterns, and the width of patterns varies with the change of the decreasing width of the incident beam. By analyzing the relation between the reflective patterns and the CLS, the analytic expressions of the slope and height of the CLS around a smooth flat plate and a double parallel flat are derived. According to the same method, the analytic expressions of the slope of the CLS around a rod and a columned glass tube are derived. Furthermore, the contact angle and the surface maximal height of the sample liquid are obtained by the analytic expression. It is believed that the boundary reflection principle is capable of real time describing the measurement of the characterization of CLS.(6) A novel contact angle measurement by laser glancing incidence method and tilting plate method. When an expanded and collimated laser beam impinges on the CLS around a smooth flat plate at glancing incidence, special reflective patterns of a strip-shape dark region in the center and the bright region on both sides are observed. The relation of the dark region width and the maximal height of the CLS is derived theoretically. The contact angle of distilled water on the glass slides is calculated directly by utilizing the dark area width of the reflection pattern. A contact angle measurement by traditional tilting plate method is improved. When a collimated light beam vertically illumined the CLS, the distribution of the reflected light from the CLS will change with the surface deformation. It is possible to measure the surface deformation by determining the change of the reflective light from the CLS. A contact angle is direct calculated by the angle of rotation of a plate.