Diffusing wave spectroscopy applied to material analysis and process control

Lloyd, Christopher James (2002). Diffusing wave spectroscopy applied to material analysis and process control. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000d467


Diffusing Wave Spectroscopy (DWS) was studied as a method of laboratory analysis of submicron particles, and developed as a prospective in-line, industrial, process control sensor, capable of near real-time feedback. No sample pre-treatment was required and measurement was via a noninvasive, flexible, dip in probe.

DWS relies on the concept of the diffusive migration of light, as opposed to the ballistic scatter model used in conventional dynamic light scattering. The specific requirements of the optoelectronic hardware, data analysis methods and light scattering model were studied experimentally and, where practical, theoretically resulting in a novel technique of analysis of particle suspensions and emulsions of volume fractions between 0.01 and 0.4. Operation at high concentrations made the technique oblivious to dust and contamination. A pure homodyne (autodyne) experimental arrangement described was resilient to environmental disturbances, unlike many other systems which utilise optical fibres or heterodyne operation.

Pilot and subsequent prototype development led to a highly accurate method of size ranking, suitable for analysis of a wide range of suspensions and emulsions. The technique was shown to operate on real industrial samples with statistical variance as low as 0.3% with minimal software processing.

Whilst the application studied was the analysis of TiO2 suspensions, a diverse range of materials including polystyrene beads, cell pastes and industrial cutting fluid emulsions were tested. Results suggest that, whilst all sizing should be comparative to suitable standards, concentration effects may be minimised and even completely modelled-out in many applications. Adhesion to the optical probe was initially a significant problem but was minimised after the evaluation and use of suitable non stick coating materials. Unexpected behaviour in the correlation in the region of short decay times led to consideration of the effects of rotational diffusion coefficient. The inherent instability of high density suspensions instigated high speed analysis techniques capable of monitoring suspensions that were undergoing rapid change as well as suggesting novel methods for the evaluation of the state of sample dispersion.

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