Diffusion measurements in fluids by dynamic light scattering

In the course of the last thirty years, light scattering techniques have been used with increasing effort and attention for the measurement of the thermophysical properties of pure fluids and fluid mixtures. Here, an introduction is given to dynamic light scattering (DLS) as a valuable tool for the measurement of diffusion processes. First, the basic principles of the method and its experimental realization are presented in some detail. A survey on various applications is given, which especially are related to the determination of transport and other thermophysical properties of transparent fluids. Selected measurements and results are shown for the determination of the thermal diffusivity a in pure fluids and fluid mixtures. For the latter the experimental conditions can turn out to be more complex. The measurement of the mutual diffusion coefficient D12 in binary fluid mixtures, however, is also treated with the simultaneous determination of a which can be realized under certain conditions. In this context beside results for fluids of technical interest, e.g., alternative refrigerants, typical measurements within the critical region for a binary fluid mixture along the critical isochor and a critical separation system are highlighted. Beside a and D12 the application of light scattering from bulk fluids on a molecular level gives also information about sound speed cS, sound attenuation DS, and Landau-Placzek ratio S. For these properties some results from DLS are renewed for the hydrofluorocarbons 1,1,1,2-tetrafluoroethane and pentafluoroethane. Additionally, the measurement of the dynamic viscosity η based on the determination of the diffusion coefficient DP of suspended seed particles is demonstrated for selected pure fluids. The application of DLS to a liquid-vapor interface, also called surface light scattering (SLS), for the simultaneous determination of liquid kinematic viscosity ν and surface tension σ is demonstrated for the important and, thus, well-documented reference fluid toluene, pure refrigerants and their mixtures, and a high viscosity fluid. These measurements demonstrate an excellent performance of the SLS-technique regarding both the achievable high accuracy and the application over a wide range of viscosity

Translational and rotational diffusion coefficients in nanofluids from polarized dynamic light scattering

Nanofluids representing nanometer-sized solid particles dispersed in liquids are of interest in many fields of process and energy engineering, e.g., heat transfer, catalysis, and the design of functionalized materials [1]. The physical, chemical, optical, and electronic properties of nanofluids are strongly driven by the size, shape, surface potential, and concentration of the nanoparticles. For the analysis of diffusive processes in nanofluids allowing access to, e.g., particle size and its distribution, dynamic light scattering (DLS) is the state-of-the-art technique. It is based on the analysis of microscopic fluctuations originating from the random thermal movement of particles in the continuous liquid phase at macroscopic thermodynamic equilibrium. For anisotropic particles or particle aggregates, besides translational diffusion also rotational diffusion occurs. To obtain the sum of the orientation-averaged translational (DT) and rotational (DR) diffusivities by depolarized DLS [2], a homodyne detection scheme is usually applied which can hardly be fulfilled in the experimental realization. Furthermore, the experiments are restricted to limited ranges for temperature, particle concentration, and viscosity

An industrial reference fluid for moderately high viscosity

In industrial practice, there is a demand for a reference standard for viscosity that is established for a readily available fluid to simplify the calibration of industrial viscometers for moderately high viscosities [(50 to 125) mPa · s]. Diisodecyl phthalate (DIDP) has been suggested as that reference fluid, and a number of studies of its properties have been carried out in several laboratories throughout the world, within the auspices of a project coordinated by the International Association for Transport Properties. That project has now progressed to the point where it is possible to collate the results of studies of the viscosity of the fluid by a number of different techniques, so as to lead to a proposed standard reference value which will be included in the paper. To support this recommended value, the various measurements conducted have been critically reviewed, and the sample purity and other factors affecting the viscosity have been studied. Density and surface tension measurements have also been performed. This paper does not describe the individual viscosity determinations carried out in independent laboratories because these are the subject of individual publications, but it does describe the ancillary studies conducted and their relevance to the viscosity standard. In addition, the paper contains recommended values for the viscosity of liquid DIDP. The samples of DIDP to which the recommended values refer are isomeric mixtures available commercially from certain suppliers, with a minimum purity by gas chromatography of 99.8 %. The recommended values result from a critical examination of all the measurements conducted to date and are supported by careful arguments dealing with the likely effects of the isomeric content of the sample as well as of other impurities. The proposed reference standard is intended particularly to serve an industrial need for a readily available calibration material with a viscosity close to that required in practical situations. To that end, the recommended value has an overall relative uncertainty of approximately 1 %. It is therefore not intended to supersede for the reference value for the viscosity of water at 20 °C, which is known much more accurately, but rather to complement it