Characterization of dense suspensions using frequency domain photon migration

Abstract

Interparticle interactions determine the microstructure, stability, rheology, and optical properties of concentrated colloidal suspensions involved in paint, paper, cosmetic, and pharmaceutical industries, etc. Frequency domain photon migration (FDPM) involves modeling the photon transport in a multiple scattering medium as a diffusion process in order to simultaneously determine isotropic scattering and absorption coefficients from measured amplitude attenuation and phase shift of the propagating photon density wave. Using FDPM, we investigated the impact of electrostatic interaction upon the optical properties and structure of dense charged suspensions. We demonstrated that electrostatic interactions among charged polystyrene latex may significantly affect the light scattering properties and structure of dense suspensions at low ionic strength (<0.06 mM NaCl equivalent) by actual FDPM measurement. We showed that the structure factor models addressing electrostatic interaction can be used to describe the microstructure of charged suspensions and quenched scattering due to electrostatics, and demonstrated that FDPM has the potential to be a novel structure and surface charge probe for dense suspensions. We also showed that the FDPM measured isotropic scattering coefficients may respond to the change in effective particle surface charge, and displayed the potential of using FDPM for probing particle surface charge in concentrated suspensions. We presented that the interference approximation implies a linear relationship between the absorption coefficient and volume fraction of suspension. We illustrated that FDPM measured absorption coefficient varies linearly with suspension volume fraction and affirmed the interference approximation from a perspective of light absorption. The validation of the interference approximation enables us to develop the methodology for estimating absorption efficiencies and imaginary refractive indices for both particles and suspending fluid simultaneously using FDPM. We further demonstrated a novel application of FDPM measured absorption coefficients in determining pigment absorption spectra, and displayed the potential of using FDPM as a novel analytical tool in pigment and paint industry.