Are nanofluids really as cool as some make them out to be?

Engineers at the Massachusetts Institute of Technology (MIT) and the Politecnico di Milano have confirmed what others have said about the hype surrounding nanofluids. That is, the liquids do not have the theoretical cooling capabilities claimed by some.

Colloidal suspensions of nanoparticles were first produced in the early 1990s, and early experiments showed that they possessed thermal conductivities much higher than predicted by the mean-field theory of James Clark Maxwell.

In an attempt to explain the anomalous conductivity, so-called “microconvection theory” gained favour among a number of experts. This is based on the hypothesis that the random motion of diffusing nanoparticles is an efficient source of fluid convection, and can increase the heat transfer capability of the surrounding fluid.

MIT graduate student Wesley Williams and his colleagues recently tested one prediction of microconvection theory – that lighter particles will increase thermal conductivity – and have shown experimentally that the observed conductivities are far lower than predicted by the theory. In fact, the results match closely the predictions of the Maxwell theory.

So were the initial experiments poorly performed? “This is a point not touched upon in the literature for fear of making enemies,” says Williams. “The most unfortunate part of the measurement technique is that it is much more likely to measure values higher than the actual values, than lower. This is due to natural convective behaviour in the system as well as other things.”

As for the hype, and the endless proliferation of nano-nouns, Williams quotes the late Richard Feynman: “You can know the name of a bird in all the languages of the world, but when you’re finished, you’ll know absolutely nothing whatever about the bird.”

University of Illinois materials scientist David Cahill comments: “This contribution from the MIT group is a valuable addition to the growing number of experiments and computer simulations that question the idea that a suspension of well-dispersed nanoparticles can significantly enhance the thermal conductivity of a fluid.”

Further reading: Mean-Field Versus Microconvection Effects in Nanofluid Thermal Conduction, Eapen et al., Phys. Rev. Lett. 99, 095901 (2007).

Article first published in Nanomaterials News.