Simultaneous measurement of refractive index, diameter and colloid concentration of a droplet using rainbow refractometry

Abstract

In the present study, droplets of colloidal suspensions or emulsions are characterized using rainbow refractometry according to diameter, relative refractive index and concentration of the dispersed phase. The position and angular spacing of the rainbow are used to retrieve the relative refractive index and droplet size. For the measurement of colloid concentration, a novel method using the intensity ratio of the p = 2 (second-order refraction) and p = 0 (reflection) scattering components present in the rainbow signal is introduced. The experimental system, comprising a monodisperse droplet generation system and standard rainbow refractometry, has been used for validation of the technique. Distilled water mixed with monodisperse polystyrene nanoparticles at different volume concentrations (CV = 0 − 0.3%) were tested, providing ground truth values of concentration. The measured relative refractive index and droplet diameter agree well with results obtained using the extended effective medium approximation for turbid media and with the known experimental values. The measured highly sensitive relation found to exist between intensity ratio attenuation and colloid concentration is numerically verified using a Monte Carlo ray tracing method.