Measuring the Refractive Index, Density, Viscosity, pH, and Surface Tension of Potassium Thiocyanate (KSCN) Solutions for Refractive Index Matching in Flow Experiments

Refractive index matching (RIM) methods are widely used in combination with optical flow measurement techniques such as particle image velocimetry and laser Doppler velocimetry to investigate fluid flows. In applying RIM, matching the refractive indices of the fluid and solid eliminates the problem of refraction/reflection at the solid-liquid interface. An experimental analysis of the effects of salt mass fraction and temperature on the refractive index of potassium thiocyanate (KSCN) solutions is conducted to quantify its performance as a RIM solution that is used in flow experiments. This enables the researchers to control the refractive index of the test medium under different test conditions by manipulating the KSCN mass fraction in the solution. Empirical correlations are developed by fitting the refractive index results, which highlight a nonlinear dependency on mass fraction and a linear dependency on temperature. The refractive index of the KSCN solution is found to be much less sensitive to changes in salt mass fraction and temperature compared to sodium iodide (NaI) solutions, a salt solution more commonly used for RIM studies. For a demonstration case, the refractive index of a KSCN solution was adjusted to align with borosilicate glass, a common transparent material used in two-phase experiments. The refractive index of the solid is a function of the quality and mass fraction of the ingredients used in the batch manufacturing operation and the process itself. For the batch of borosilicate beads used, a 0.624 (kg/kg) KSCN solution provided a perfect index matching rendering the glass beads transparent. Using this approach, the refractive index of a solid with a small and complex shape could be measured. In flow experiments, in addition to the refractive index, other fluid properties are influential and might need to be matched with the application. A discussion is also provided on the density, viscosity, surface tension, and pH of KSCN solutions to enable fluid dynamic scaling of experimental flow conditions through a set of correlations, developed to predict the properties of different KSCN solutions.