Analysis of Maxwell’s mean-field (or effective-medium) theory reveals two limiting bounds — an upper and a lower bound — for thermal conductivity in binary composite systems. The lower and the upper bounds correspond to continuous conduction paths through the base medium and the dispersed medium, respectively (assuming that the dispersed medium has a higher thermal conductivity). Extensive comparisons to experimental data show that most of the reported thermal conductivity data on nanofluids, solid composites and liquid mixtures fall between the limiting Maxwell bounds. For a nanofluid, this indicates that the effective thermal conductivity is largely dependent on the geometrical configuration and the connectivity of the dispersed nanoparticle phase. The lower bound corresponds to a colloidal configuration of well-dispersed nanoparticles with the continuous conduction path provided by the base medium while the upper bound represents a linear, fractal-like nanoparticle arrangement with the continuous conduction path provided by the nanoparticles.

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