Diffusing wave spectroscopy applied to material analysis and process control
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
Whilst the application studied was the analysis of Ti02 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.