, Volume 269, Issue 7, pp 704-712

Diffusion in concentrated dispersions: a study with fiber-optic quasi-elastic light scattering (FOQELS)

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Abstract

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 /D 0 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.

Presented at the 34. Hauptversammlung der Kolloidgesellschaft e.v., Bochum, Oct. 1–4, 1989