Characterization of concentrated colloidal suspensions using time-dependent photon migration measurements

A method is presented to characterize concentrated suspensions of particles using frequency-domain photon migration in order to demonstrate its potential for on-line monitoring in the pharmaceutical and chemical based industries. In this study, the isotropic scattering coefficient was determined from FDPM measurements conducted at wavelengths between 448 and 835 nm using surfactant stabilized polystyrene latices of 195–335 nm mean diameter, 7–19% polydispersity and concentrations between 1 and 40% by volume. Upon using the Percus–Yevick model for static structure of monodisperse spheres to account for hindered scattering in the inverse solution, determination of the volume fraction and mean particle diameter were considerably improved size at solids volume fractions of 10% or greater. Owing to (i) the relative refractive index of polystyrene in water, (ii) the inaccuracy of the first order particle interaction model for suspensions with polydispersity >8%, and (iii) the minimal number of wavelengths used, it was difficult to recover the variance of the unimodal particle size distributions. Nonetheless, this work demonstrates the use of time-dependent measurements of multiply scattered light to characterize suspensions with solids volume fractions greater than 10%.