Optical measurement of concentration-dependent diffusion coefficient by tracing diffusion image width

Objective: Repeated experiments at different concentrations are necessary to measure the concentration-dependent liquid diffusion coefficient D(C), this process is arduous and time consuming using traditional methods. Methods: In order to overcome the limitation, the self-designed liquid-core cylindrical lens were used as the diffusion cell and imaging element, considering one observed altitude on the focal plane of the self-designed liquid-core cylindrical lens, the variation in the diffusion image width reflects the experimental time-varying concentration profile C^ez, t), D(C) over the whole concentration range was calculated by the comparison between calculated concentration profiles Cⁿ(z, t)s and above experimental values by changing the observed altitudes. Result: The D(C) of urea aqueous solution was measured with D(C) = 1.3872 × (1–0.0399 ×C-0.0031 ×C²-0.0002 ×C³) × 10⁻⁵ cm²/s, this is similar to literature values and verifiable via image simulations. Comparing with traditional measurement method, this approach requires one experiment and significantly reduces experimental workloads, further improving D(C) measurement technology.