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In a dynamic light scattering experiment, we have investigated the time intensity correlation function and the profile of the transmitted laser beam for organic dispersions of light absorbing colloidal particles containing tiny gold clusters. The correlation functions have been found to show a superposition of an exponential decay, corresponding to Brownian motion of the tracers, and well-defined oscillations. These oscillations are caused by convection due to local heating of the sample by the incident laser beam, which has been confirmed independently via measurements of the local temperature within the sample. It will be shown how the particle convection velocity, which is the order of 1 mm/s, can be obtained quantitatively from the oscillating correlation functions. The profile of the transmitted beam allowed us to determine the Soret coefficient, which is a measure for the thermal diffusion of the particles. This article shows how tracer diffusion, convection, and thermal diffusion can be determined simultaneously by one single experiment, laser light scattering of light absorbing colloidal particles in dispersion.

We have investigated diffusion and thermal diffusion properties of light-absorbing colloidal polyorganosiloxane microgels containing tiny nanometer-sized gold clusters dispersed in toluene. Transient holographic gratings allow for very subtle perturbations in the linear regime where Soret feedback is negligible. Gold-doped colloids of different size and crosslink ratios show different Soret coefficients but identical thermal diffusion coefficients D(T). Undoped colloids tend to aggregate, but a consistent interpretation is obtained if an identical D(T) is assumed for the doped, the undoped, and the aggregated particles. Previously reported Soret feedback measurements on similar systems incidentally yielded comparable Soret coefficients. We show, however, that they suffer from strong convective perturbations.

Forced Rayleigh scattering (FRS) was used to study diffusional processes in block copolymer/homopolymer blend systems forming spherical microdomains. Materials used were an asymmetric polystyrene-block-polyisoprene copolymer (M n = 74 000, 21 wt % polystyrene) labeled with photosensitive dyes (o-nitrostilbene or fluorescein) at the end of polystyrene block chains (b-PS) and two kinds of homopolyisoprene (HPI) (M n = 23 000, M n = 90 000). Using the lower molecular weight HPI for blending with the block copolymer, b-PS spheres with coronas of polyisoprene block chains (b-PI) strongly swollen by the matrix HPI are obtained (wet-brush system), whereas the HPI matrix having higher M n yields much less swollen coronas of b-PI (dry-brush system). The differences between these two systems in structural and dynamical behavior were found by small-angle X-ray scattering and FRS studies, respectively. To exclude chain release from the spherical micelles during the measurements, diffusion has been studied close to the T g of b-PS microdomains. Unexpectedly, FRS signals differed strongly if measured well below or slightly above the T g of PS microdomains in both intensity and corresponding diffusional processes.

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