This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or "nanofluids," was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band ͑Ϯ10% or less͒ about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are ͑small͒ systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. ͓J. Appl. Phys. 81, 6692 ͑1997͔͒, was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

With the aim of providing reliable benchmark values, we have measured the Soret, thermodiffusion and molecular diffusion coefficients for the ternary mixture formed by 1,2,3,4-tetrahydronaphthalene, isobutylbenzene and n-dodecane for a mass fraction of 0.8-0.1-0.1 and at a temperature of 25°C. The experimental techniques used by the six participating laboratories are Optical Digital Interferometry, Taylor Dispersion technique, Open Ended Capillary, Optical Beam Deflection, Thermogravitational technique and Sliding Symmetric Tubes technique in ground conditions and Selectable Optical Diagnostic Instrument (SODI) in microgravity conditions. The measurements obtained in the SODI installation have been analyzed independently by four laboratories. Benchmark values are proposed for the thermodiffusion and Soret coefficients and for the eigenvalues of the diffusion matrix in ground conditions, and for Soret coefficients in microgravity conditions.

Soret coefficients of aqueous solutions of methyl, ethyl, isopropyl, and n-butyl alcohols and of solutions of methyl and ethyl alcohols in benzene have been measured in the Soret coefficient measurement (SCM) experiment under microgravity conditions on board the orbital platform EURECA at a mean temperature of 37.5 °C. These reference measurements will allow us to improve the understanding of the ground based results and to calibrate the ground based setup. This point is discussed with the values that we obtained by the flow cell technique and for systems that exhibited a hysteretic loop in the Schmidt-Milverton plots in the Be ´nard cell at a mean temperature of 37 °C.

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Measurements of Soret coefficients of tin alloys have been performed in the MFA-067 experiment of the automatic retrievable satellite EURECA. They complete the set of measurements performed in tin dilute alloys in microgravity manned laboratories. They allow also for an accurate determination of the isotopic thermomigration and for a better evaluation of the accuracy of microgravity measurements of the thermodiffusion factors. These data are compared to nonequilibrium condensed matter theories. A reasonable agreement has been obtained with the revised Enskog theory, and no electronic contribution could be identified.

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