Fluid Composition Analysis in Segmented Microflows via Refractive Index Measurement with Digital In-Line Holography

A novel application of Digital In-Line Holography (DIH) is reported for the quantitative in situ measurement of the refractive index of transparent droplets within segmented microfluidic flows. This method addresses a key analytical gap in hydrometallurgical recycling processes, particularly in liquid-liquid extraction systems where conventional UV-vis spectroscopic techniques are ineffective for nonabsorbing phases. A custom-fabricated microfluidic device was developed to generate monodisperse droplets under controlled conditions, and a DIH-based optical setup was implemented to capture the diffraction patterns. By combining image analysis and Lorenz-Mie theory, simultaneous determination of droplet size and refractive index was achieved, with a typical a precision of Δn ∼ 10^-2, limited primarily by droplet axial positioning. Excellent agreement was observed with benchtop refractometry, demonstrating the method's suitability for monitoring two-phase process variables such as acidity in both the dispersed phase and the continuous phase. The approach holds promise for extension to absorbing media, allowing compositional analysis in complex multiphase systems under both static and dynamic conditions and is particularly relevant for advanced hydrometallurgical applications, such as nuclear fuel reprocessing and critical metal recycling.