Diffusing Wave Spectroscopy Measurements of Colloidal Suspension Dynamics

Diffusing wave spectroscopy (DWS) is used to measure the dynamics of charged silica particles between the volume fractions 0.065 ≤ ϕ ≤ 0.352 (weight percentages from 12.7 to 55.8 wt %). The short-time diffusivity averaged over the scattering vectors sampled by DWS $\overline{D}$(ϕ) decreases with an increasing concentration. An effective hard-sphere model that accounts for hydrodynamic interactions and a double-layer repulsion fits the values up to an effective volume fraction ${\varphi }_{eff}=\varphi {\hat{b}}^3\approx 0.6$, where $\hat{b}$ is the excluded shell radius normalized by the particle radius $\hat{b}$ = b/a = 1.3. While DWS measurements of diffusivity are sensitive to repulsive interactions, we show that they are relatively insensitive to attraction, such as those due to secondary minima in the interaction potential or weak depletion interaction.