Probing complex dynamics with spatiotemporal coherence-gated DLS

We discuss the specific features of fiber-based implementations of optical sensing techniques based on spatiotemporal coherence-gated dynamic light scattering (DLS). This sensing approach has a number of unique capabilities such as an effective isolation of single scattering, a large sensitivity, and high collection efficiency, and it can also operate over a wide range of optical regimes while providing means for proper ensemble averaging. We review a number of applications in which these specific characteristics permit recovering information beyond the capabilities of traditional light-scattering-based techniques.

In situ characterization of structural dynamics in swelling hydrogels

Characterizing the structural morphology and the local viscoelastic properties of soft complex systems raises significant challenges. Here we introduce a dynamic light scattering method capable of in situ, continuous monitoring of structural changes in evolving systems such as swelling gels. We show that the inherently non-stationary dynamics of embedded probes can be followed using partially coherent radiation, which effectively isolates only single scattering contributions even during the dramatic changes in the scattering regime. Using a simple and robust experimental setup, we demonstrate the ability to continuously monitor the structural dynamics of chitosan hydrogels formed by the Ag(+) ion-triggered gelation during their long-term swelling process. We demonstrate that both the local viscoelastic properties of the suspending medium and an effective cage size experienced by diffusing probe particles loaded into the hydrogel can be recovered and used to describe the structural dynamics of hydrogels with different levels of cross-linking. This characterization capability is critical for defining and controlling the hydrogel performance in different biomedical applications.

Hydrodynamic and Colloidal Interactions in Concentrated Charge-Stabilized Polymer Dispersions

Hydrodynamic and colloidal interactions are explored in concentrated, charge-stabilized colloidal dispersions by measuring the dependence of rheology (e.g., low and high-shear viscosity, high-frequency viscosity, and modulus) and self-diffusivity on salt content, particle size, and concentration. Model, sulfonated polystyrene lactices of varying diameter are prepared and investigated by shear rheology, high-frequency torsional resonance, electrophoresis, titration, and dynamic light scattering. The high-frequency and high-shear viscosity both are dominated by hydrodynamic interactions, but are shown not to be identical, due to the microstructure distortion resulting from high shear rates. The short-time self-diffusion is also shown to be insensitive to direct particle interactions, but has a different concentration dependence than the high-frequency viscosity, further illustrating a predicted violation of a generalized Stokes-Einstein relationship for these properties. The apparent colloidal surface charge is extracted from the high-frequency elastic modulus measurements on concentrated dispersions. The surface charge is in good agreement with results from critical coagulation concentration measurements and perturbation theories, but disagrees with electrophoretic mobility experiments. This indicates that the effective surface charge determined by torsional high-frequency measurements is a more reliable predicter of the salt stability of charge-stabilized dispersions, in comparison to zeta-potentials determined from electrophoretic mobilities. Further, we demonstrate by direct comparison that measurements of the apparent plateau modulus by rotational rheometry underestimate the true, high-frequency modulus and provide unreliable estimates for the surface charge. Copyright 2000 Academic Press.

Development of a Heterodyne Photon Correlation Spectroscopy Measuring Probe for Highly Concentrated Dispersions

A new FOQELS setup is described for use in concentrated dispersions. A fiber-to-fiber coupler is introduced as an extra local oscillator source. This results in a reliable determination of the diffusion coefficient of concentrated dispersions where the heterodyne mode appertains at every concentration. This system is compared with others and results are presented of measurements of a 176-nm latex dispersion and a 230-nm titanium-dioxide dispersion. The diffusion coefficients measured with the new setup clearly differ from those measured with a single optical fiber probe, which are within the mixed homodyne-heterodyne regime; when taking this degree of mixing into account, the latter results can be transferred to the value of the true diffusion coefficients. For prolonged and robust measurement of highly concentrated dispersion, however, direct measurements with heterodyne probes arepreferred. Copyright 1998 Academic Press.

Compact Fiber Optic Dynamic Light Scattering System

Fiber optic dynamic light scattering (FODLS) is fast developing as a technique for sizing particles in concentrated dispersions. Fiber optic systems have the potential to be made into online particle size measuring equipment in process industries. Two different laser light sources were used to study monodisperse polystyrene latex dispersion with different particle sizes and volume fractions. The apparent diffusion coefficients were a function of particle size, volume fraction, and signal to noise ratios. The apparent diffusion coefficient data showed that FODLS probes collective diffusion for smaller particles and self diffusion for larger particles. It was observed that for a polydisperse concentrated dispersion, both self and collective diffusion were probed by FODLS. This means that the apparent diffusion coefficient can be used only as a relative quantity. Copyright 1997 Academic Press. Copyright 1997Academic Press