Novel Approach for Reliable Determination of the Refractive Index of Particles in the Liquid Phase Using a Hybrid Flow Particle Tracking Method.

We developed a novel method for the reliable determination of the refractive index (RI) of particles under flow conditions: the hybrid flow particle tracking (FPT) method, i.e., simultaneous observation of Brownian motion and light scattering intensities for individual particles under flow conditions. After determination of the size of individual particles from their diffusive motion by Stokes-Einstein assumption, the RI was determined using the relationship between the size and corresponding light scattering intensity according to Rayleigh/Mie theory. However, there were two problems in the establishment of the hybrid FPT method. First, the calculated size of the individual particles from their diffusion motion has considerable error because of the randomness in the diffusion trajectory owing to the Brownian motion. Second, the observed light scattering intensities of the particles varied depending on the position of the particles in the incident beam because of the spatial light intensity profile of the incident beam. After resolving these two adverse effects, we successfully obtained reliable RIs for the different types of particles. This established method will contribute to the simultaneous determination of the size and types of particles in a flow system.