"Nanofluids could be ideal coolants for future electronic devices but for such applications, the fluids need to have large thermal conductivities," explained team leader John Philip. "This is why we need to develop novel nanofluids."

The materials studied by the researchers are a colloidal suspension of single-domain superparamagnetic Fe3O4 nanoparticles between 3 and 10 nm in size that are magnetically polarizable – that is, they respond to a weak magnetic field. The particles are capped with a monolayer of surfactant molecules so that they do not agglomerate – something that is crucial for the experiments subsequently performed.

In the absence of a magnetic field, the magnetic moments of the particles are oriented in random directions. When a field is applied, the particles align in the direction of the field when the magnetic dipolar interaction energy between the particles overcomes the thermal energy of the particles. The dipolar interaction energy depends on the distance between neighbouring particles and their mutual orientation.

Chain-like structure
As soon as the dipolar interaction becomes sufficiently strong, the magnetic particles form a chain-like structure as they line up in the direction of the applied field. The thermal conductivity of the nanofluid then increases because heat can then flow very efficiently along the chain, says Philip.

The increase in the thermal conductivity in these fluids is extremely high – several hundred times that of traditional nanofluids. What is more, the increase is perfectly reversible and can be tuned from high to low values by applying the magnetic field either parallel to the direction of particle chains or perpendicular to them. These properties mean that the nanofluids could be ideal for use as "intelligent" coolants, states Philip.

Programming the magnetic field strength
"Depending on the heat load of the fluid, you can simply programme the magnetic field strength to achieve higher cooling," he told nanotechweb.org. "All you need is a feedback control circuit in a device that automatically senses and varies the magnetic field strength depending on the amount of cooling needed." The set-up could be used to cool computer chips, for instance, or cool down MEMS and NEMS devices.

The team now plans to further develop and test these fluids through microchannels to test the cooling efficiencies under flow. "We are also working on developing a new class of nanofluid that can work at very high temperatures and react quickly," revealed team member P D Shima.

The current work is reported in The Journal of Physical Chemistry C.