Characterising the sedimentation of bidisperse colloidal silica using analytical centrifugation

Abstract

This study investigates the settling behaviour of bidisperse colloidal silica suspensions in two different size ratios (100:500 nm and 500:800 nm) with various mixture ratios and volume fractions, using an analytical photocentrifuge. For dilute systems, translation of settling rate profiles to size distributions resulted in more accurate measurements for both monodisperse and bidisperse systems than using dynamic light scattering, in comparison to SEM. However, a critical limitation was also observed, as distinction between upper (smaller particle) and lower (larger particle) interfaces could only be measured for total volume fractions <∼0.02. For 500:800 mixtures, results were compared with Richardson-Zaki and modified Batchelor model predictions. The Batchelor model proved more accurate at predicting the lower interface rate, while both models over predicted the upper interface, likely due to increased drag that was most evident with higher fractions of smaller particles. For 100:500 cases, higher centrifugal rotation speeds were required to settle the 100 nm particles, and the influence of Brownian motion was evident. It was also found that the smaller particles obtained a lower initial settling rate than its terminal velocity, due to induced anisotropic effects or backflow from the 500 nm fraction, which was again more evident with a greater proportion of smaller particles.