Diffusion in concentrated dispersions: a study with fiber-optic quasi-elastic light scattering (FOQELS)
A fiber-optic, quasi-elastic light-scattering instrument is described using single-mode fiber optical components, including a novel slanted exit face optode. The setup operates with homodyne signal detection. It enables the characterization of diffusion processes in concentrated dispersions up to volume concentrations of 50%.
The performance of the instrument is exemplified with results obtained from latex spheres with diameters of 226 nm and 404 nm at volume fractions from Φ=0.01 to Φ=0.5. The correlation functions are analyzed according to the second order cumulants method and the “Contin”-procedure yielding an average and a distribution function of the short-time self-diffusion coefficient,D eff s , respectively.
At high ionic strength the concentration dependence ofD eff s /D0 is found to be in close agreement with theoretical predictions based on a multi-body interaction model of hard spheres up to Φ=0.45. With decreasing ionic strength the negative slope of the virial expansion tends to increase, presumably due to enhanced repulsive electrostatic interactions.
The described technology offers new experimental means for on-line remote control sensing of particle size in concentrated disperse systems.
Key wordsDynamic light scattering fiber-optics latex dispersions self-diffusion
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