| Liquid diffusion coefficient (D) is a fundamental data in the study of mass-transfer process, the calculation of mass-transfer rate and the design and development of chemical equipment. Its accurate measurement is important in Physics, Chemistry and Biology fields. The spatially resolving ability of liquid-core cylindrical lenses (LCL) for measuring refractive index (RI) can be used to observe the diffusion process and measure liquid D values. The measurement of liquid RI with high accuracy is the precondition for measuring D values, therefore, the rational design of LCL for improving RI measurement accuracy is important for measuring liquid D values. This dissertation puts forward several new methods for measuring the D values of liquids based on an asymmetric liquid-core cylindrical lens (ALCL), designed by ourselves.The main research contents and results of the dissertation are consisted of six aspects as following:First of all, parameters for evaluating the quality of LCLs for measuring liquid RI were analyzed, including depth of field (DOF), RI sensitivity, RI resolution and spherical aberration (SA). The DOF formulae for a microscope equipped with an electronic ocular were deduced in detail. A new view, related to the geometric DOF and physical DOF satisfying "OR" instead of "SUM" in mathematical, was proposed. A capillary measurement system was designed to verify the derived results. SA of LCL consisting of four refractive surfaces was deduced based on the ray tracing method. The parameter analysis provided theoretical basis of the lens design.Secondly, the focal length formulas of capillary, symmetric liquid-core cylindrical lens (SLCL) and ALCL were derived and the advantages and disadvantages of these three LCLs for measuring liquid RI values were compared. Capillary is able to measure RI of micro-quantity liquid, the liquid amount is less than 5μL, while the measurement accuracy can only achieve 0.001 RIU. The SLCL measurement accuracy is approaching 0.0002RIU, which is better than capillary, but the SA of SLCL filled with liquid is about 300μm, which is larger enough to reduce the image quality. ALCL only enlarges the curvature radius of the fourth interface of SLCL, but produces a noticeable effect, that is, the RI measurement accuracy is superior to 0.0002RIU and the SA is down to less than 5 um, which lays the foundation for measuring D values accurately.Thirdly, a new method for measuring liquid D values was proposed:"instantaneous image analytical method". This method chose ALCL as the diffusion pool and key imaging element, and the spatial distributions of RI and concentration could be obtained by measuring and analyzing one instantaneous image. And then the liquid D value could be worked out rapidly based on the Fick’s second law. This method reduced the measurement time greatly from 5000-20000s in traditional methods to less than seconds. The infinite dilution D value of ethylene glycol (EG) diffusing in water at 298.15K was measured using both "equivalent RI of thin liquid layer method" and "instantaneous image analytical method", the measuring results obtained by these two methods were approximate and were well consistent with the literature values.Fourthly, a new way for measuring mutual D values in different concentrations was put forward. In this method, the initial concentration difference between two diffusion solutions is not required to be small enough any longer. If only a suitable thin liquid layer in diffusion cell is selected, a reasonable measured D value can be obtained. Different concentrations of EG diffusing in water were performed to prove this method.Fifthly, the effects of diffusing substance category, concentration and temperature on diffusion process were verified by the "equivalent RI of thin liquid layer method". The infinite dilution D values of EG diffusing in pure water at different temperatures were measured. The experimental results showed that the higher temperature, the larger D values, conforming to the Arrhenius equation. The infinite dilution D values of triethylene glycol (TEG) and glycerol diffusing in water at 298.15K were also measured. The results showed that the larger molecule (TEG) has a lower diffusion rate. The D values of EG with different concentrations diffusing in water were measured. The results indicated that a dense diffusion solution leads to a low diffusion rate. The advantage for measuring D values visually based on the ALCL were strengthened by comparing the diffusion images appearing in different diffusion processes.Finally, based on the Collins formula in diffraction theory, using FFT techniques and Matlab software running in a PC, the dynamic diffraction patterns of apertures of ALCL, in which two different solutions is mutual diffusing, is calculated in this paper. Based on the scalar diffraction theory, Fraunhofer pinhole diffraction and single slit diffraction were simulated using Matlab software at first, the simulation images were similar with the experimental results, which proving the correctness of the scalar diffraction theory and the programs we wrote. And on this basis, we simulated the diffusion process occurring in the ALCL and compared the simulated images with the experimental ones:the simulation results were well consistent with the experimental images when the concentrations of liquid layers were lower; while, there were obvious discrepancies when the concentrations of liquid layers were higher. This result gave another proof for the effects of the selected thin liquid layers RI on the D calculation results, stressing the importance of selecting a suitable thin liquid layer in diffusion cell. |
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