Characterization of concentrated colloidal suspensions with frequency domain photon migration technique
Abstract
Direct characterization of colloidal suspensions at high volume fractions is difficult by currently available optical methods due to confounding effects of multiple scattering and particle interactions. In this work, the frequency domain photon migration (FDPM) technique, based upon multiple light scattering, is extended for characterization of dense (interacting) colloidal suspensions. By using FDPM measurement of scattering properties, we demonstrate that the photon diffusion model, derived by the assumption of neglecting particle interactions (i.e., non-interacting suspensions), can be extended to estimate the optical properties in interacting suspensions as long as the length scales for multiple scattering and particle interactions are magnitudes different. Since the FDPM-derived scattering properties contain the information of near-field coherence effects arising from particle interactions and local microstructure, it can be employed to study particle interactions in dense (interacting) suspensions. We investigate the particle interactions in mono-, bi- and poly-disperse colloidal suspensions at high volume fractions by comparing FDPM measurements with theoretical predictions. The results show that the volume exclusion interactions between colloids are predominant in dense suspensions, which can be modeled by hard sphere Percus-Yevick (HSPY) approximation. Using these tested forward models that are suitable for dense suspensions, we successfully recover the particle size in mono- and bi-disperse systems and particle size distribution (PSD) in polydisperse systems at high volume fractions (up to 0.4). Recovered particle sizes and PSDs agree well with the dynamic light scattering (DLS) results measured at diluted samples (∼0.001 volume fraction). This work greatly extends our previous work for particle sizing of non-interacting (<0.01 volume fraction) to interacting polydisperse suspensions. Finally, we develop a novel linear inverse algorithm for recovery of free form PSD and volume fraction of non-interacting polydisperse suspensions from FDPM measurement. This algorithm was tested using synthetic as well as actual FDPM measurements. Compared to previous work, this algorithm avoids the arbitrary assumption of a priori PSD form. Also, the algorithm is faster and more suitable for in situ monitoring of PSD and volume fraction in industry.
Degree
Ph.D.
Advisors
Sevick-Muraca, Purdue University.
Subject Area
Chemical engineering
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