Rapid measurement of the diffusion coefficient of liquids using a liquid-core cylindrical lens:a method for analysing an instantaneous diffusive picture

This paper studies the equivalent refractive index method and other methods to measure the liquid diffusion coefficient. Based on this, a quick method to measure the liquid diffusion coefficient is proposed, i.e. using a specially designed asymmetric liquid-core cylindrical lens as both diffusive pool and imaging element. By means of this system with the liquid-core cylindrical lens to measure the diffusion coefficient, we can eliminate the spherical aberration and improve the accuracy in refractive index measurement. Based on the spatially resolving ability of the cylindrical lens in measuring the refractive index, only one instantaneous diffusive picture is required. Depending on the correspondence between the image width and the refractive index, we thus can quickly calculate the diffusion coefficient D by the Ficks second law. Then the diffusive process of ethylene glycol in water at 25℃ is investigated by this method. We calculate the diffusion coefficient between 660-3000 s with the method to analyse an instantaneous diffusion picture. At the beginning, injection will cause the liquid turbulent, and thus create a larger diffusion coefficient. In the course of diffusion, the influence of turbulence on the diffusion coefficient gradually decreases, but the image narrowing can make inaccurate results. Therefore, this method is required to be used at an appropriate time and an appropriate position to reduce experimental errors. After repeated experiments we can conclude that, between 1500-2700 s we may select the appropriate measurement of location for measuring liquid diffusion coefficient by the method to analyze an instantaneous diffusive picture. This not only can avoid the effect of turbulence but also avoid the effect of fewer sampling points. Compared with other methods reported in the literature, the results show that this method is characterized by short time (~20 ms) in data acquisition, faster measurement ( 1 s), high-accuracy (relative error 3%), and easy operation, thus providing a new method for measuring the diffusion coefficient of liquids rapidly.