Translational and Rotational Diffusion Coefficients of Gold Nanorods Dispersed in Mixtures of Water and Glycerol by Polarized Dynamic Light Scattering

Abstract

Polarized dynamic light scattering (DLS) gives access to orientation-averaged translational and rotational diffusion coefficients of anisotropic particles dispersed in fluids in a single experiment. As the combination of both diffusivities contains information on the morphology of the particles, their simultaneous and accurate measurement for the same sample and thermodynamic state is beneficial for particle characterization. For nontransparent model suspensions of gold nanorods in water and water–glycerol mixtures, a scattering geometry in reflection direction was realized, which minimizes multiple scattering and allows using low laser powers to avoid laser heating. Furthermore, a heterodyne detection scheme was guaranteed by superimposing much stronger reference light to the scattered light. This ensures an unambiguous data evaluation and reduces the uncertainties for the rotational and the translational diffusivity, where the latter is accessible with smaller uncertainty. For the water-based suspensions, both diffusivities agree well with the stick hydrodynamic theory for rods and show an Andrade-type behavior in the studied temperature range from 271 to 323 K. The measured results for both diffusivities, particularly for the rotational diffusivity, indicate a breakdown of the stick boundary conditions for dynamic viscosities larger than 4 mPa·s.

Effective Thermal Conductivity of Nanofluids: Measurement and Prediction

“…For the characterization of particle size and its distribution as well as the dispersion stability of the nanofluids in the same fluid state as it is investigated in connection with the effective thermal conductivity, the diffusion coefficients were obtained by dynamic light scattering (DLS). For details on the principles of the technique [52] and its application for the determination of several thermophysical properties of various types of fluids [53][54][55][56] and dispersed systems [57][58][59], the reader is referred to the respective literature. In the following, only the information relevant for the present investigations is given.…”

“…In the present study, the same experimental setup as applied in our previous work [59] has been used to perform heterodyne DLS experiments under vertical polarization directions of the incident and the detected light. For the concentrated nanofluids which are strongly light-absorbing in most cases, only a detection of the scattered light in reflection direction was possible.…”

“…For the data evaluation detailed in our previous work on highly diluted nanoparticle dispersions [59], the CFs measured simultaneously by the linear-tau and multitau correlator were represented by least-squares fitting. In all cases, the sum of two or three exponentially decaying signals associated with the corresponding mean decay times τ C were needed to obtain residuals between the measured and fitted data, which are free of systematic behaviors.…”

“…The refractive index data are calculated at the average temperatures of the related DLS measurement series with an estimated relative expanded uncertainty of 0.5 % and 2 % for the highly diluted and concentrated nanofluids. For each thermodynamic state, the final diffusion coefficients were obtained by averaging the data determined from the individual CFs recorded with the two correlators for five different individual measurements using an uncertainty-based weighting scheme [59]. As expanded uncertainties of the D data, the double standard deviations of the values obtained from the individual measurements and weighted in the same manner were calculated.…”

Progress in Multidimensional Particle Characterization

“…The scattering vector is q. In particular, polarized DLS in a heterodyne detection scheme was shown to provide the orientation-averaged translational and rotational diffusion coefficients of gold nanorods in a single experiment (Berger Bioucas et al, 2019). The scattering geometry of the detection reduces multiple scattering effects and thus allows lower laser powers and hence reduced laser heating.…”