Can nanofluids exhibit "anomalous" thermal conductivity?

Nanofluids are suspensions of nanometer-sized solids or fluids in a base fluid and first rose to prominence when "anomalously" large enhancements of the thermal conductivity were reported for metallic suspensions. Controversy and a lack of repeatability in the literature prompted an international benchmark exercise to verify experimental nanoparticle results, many of which were found to lie within the classical limits of series and parallel resistance prompting some to say the controversy is over and that classical bounds encompass all results.

In this talk, it is demonstrated that the binary hard sphere model near the ideal gas limit exhibits truly anomalous thermal conductivity. This result is used to find experimental data in the literature (He-H2 gas mixtures) which also exhibit this effect. Non-equilibrium molecular dynamics and results from kinetic theory are used to illustrate that both enhancement and dehancement outside series/parallel bounds are possible, albeit for slightly nonphysical size and mass ratios. A transient effect is also demonstrated at more typical molecular size/mass ratios which may be linked to the difficulties in experimental repeatability but further investigation using coupled kinetic theory and hydrodynamics is required to verify the effect and to determine if it can be exploited in experiment.