A Frequency-Domain Soret Forced Rayleigh Scattering Instrument with Lock-in Detection for Measuring Soret and Mass Diffusion Coefficients in Binary Liquid Mixtures

A frequency-domain Soret forced Rayleigh scattering (SFRS) instrument was developed by integrating a lock-in detection scheme with the holographic grating excitation technique of SFRS, or thermal diffusion forced Rayleigh scattering (TDFRS), to measure the Soret coefficient S _T and mass diffusion coefficient D ₁₂ in binary liquid mixtures. The performance of the developed system was validated using benchmark binary mixtures ( n -dodecane, isobutylbenzene, and 1,2,3,4-tetrahydronaphthalene) at a temperature of T  = 298.2 K, showing good agreement with the benchmark values. Based on an uncertainty evaluation in accordance with the Guide to the expression of uncertainty in measurement (GUM), the expanded uncertainties (coverage factor k  = 2) were within 4.4% for D ₁₂ , 3.1% for S _T , and 5.3% for the thermodiffusion coefficient D _T . This approach enables mass transport measurements with a relatively simple optical setup and a heating laser power approximately two orders of magnitude lower than that typically used in conventional time-domain SFRS. By offering a distinct measurement principle with minimal thermal perturbation and inherent robustness against stray light, this frequency-domain method is expected to be a promising tool for exploring complex systems.