Application of fiber optic dynamic light scattering

The application of traditional dynamic light scattering (DLS) technique to characterize colloidal dispersions is limited to highly dilute samples to avoid problems associated with multiple scattering.;Fiber optic dynamic light scattering (FODLS) and diffusing wave spectroscopy (DWS) techniques have been investigated to extend the application of traditional DLS to highly concentrated colloidal dispersions.;A FODLS system using single mode optical fiber has been set up for characterization of turbid dispersions. Concentrated monodisperse polystyrene latex dispersions and commercial silica-based CMP slurries were used as samples to evaluate this FODLS system under various conditions. The scattered light signals from the particles were characterized and analyzed by autocorrelation function and power spectrum analysis. Relevant LabVIEW Virtual Instruments (VIs) have been developed to control the measurements and analysis.;The experimental results in this work clearly indicated that despite the high turbidity of the samples, single scattering signals dominate the multiple scattering ones in FODLS when using single mode fibers. It was found that even using a slanted surface fiber optode, the scattered light was mixed with strong local oscillation signals so that the detection is of "pure" heterodyne mode for this system.;Complex effects of particle size, concentration and interaction on the measured diffusion coefficients have been confirmed in this work. Both self-diffusion coefficient and collective diffusion coefficient were probed by FODLS. One has to account for the influence of particle interaction in concentrated dispersion to use FODLS as accurate particle sizing equipment.;The power spectrum density of heterodyne signals has linear relationship with the scattering intensity. The measured power spectrum densities were found increasing with particle numbers, and thus particle concentration, in the scattering volume. Mathematical models were developed to describe the relationship between the integrated power spectrum density and the particle concentration. This provided a unique method to obtain the information of particle concentration by FODLS data.;Because the multiply scattered light is too weak and divergent to effectively focus on a single mode fiber. The fiber optic diffusing wave spectroscopy setup did not yield meaningful measurements.