Nanofluids are suspensions of nanoparticles in a pure liquid for enhancement of single-phase and two-phase heat transfer. The nanoparticles are typically made of ceramic material, such as alumina or silica, but can be of other materials, diamond, nanoparticles of metals, oxides, carbides, nitrides, or nanotubes.
Nanofluids exhibit enhanced thermal properties such as higher thermal conductivity, heat transfer coefficients, magnetic pumping applications and capillary properties compared to the conventional base fluids. As a cooling fluid, nanofluid can provide considerable cooling in any cooling system. Hence, there is considerable interest in the use of nanofluids for any process that uses process heat requiring cooling.
Certain nanofluids can be made to conduct heat extremely well when a magnetic field is applied to them. This phenomenon can be used to cool down miniature devices like micro- and nano-electromechanical systems, and computer chips.
Researchers at the Indira Gandhi Centre for Atomic Research in Tamilnadu, India have developed novel nanofluids which could be ideal coolants for future electronic devices due to large thermal conductivities.
The materials studied by the researchers are a colloidal suspension of single-domain super paramagnetic Fe3O4 nanoparticles between 3 and 10 nm in size that are magnetically polarizable responding to a weak magnetic field. The particles are capped with a monolayer of surfactant molecules so that they do not agglomerate.
Principle
In any particles the magnetic moments are oriented in random directions, but when a magnetic field is applied, the particles align in the direction of the field and overcome the thermal energy of the particles due to the magnetic dipolar interaction energy depending on the distance between neighboring particles and their mutual orientation. 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.
The increase in the thermal conductivity in these fluids is several hundred times that of traditional nanofluids and is perfectly reversible using which it 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 according to the researchers.
By incorporating a feedback control circuit in a device, it can automatically sense and vary the magnetic field strength depending on the amount of cooling needed in components such as computer chips or MEMS and NEMS devices.
Liquids embedded with nanoparticles have high stability when exposed to electric fields and this property can lead to new types of miniature camera lenses, cell phone displays, micro scale and nanoscale actuator device applications, fluidic devices, digital display devices, optical devices and micro electromechanical systems (MEMS) such as lab-on-chip analysis systems.
Since the boiling characteristics in pool boiling are similar to flow boiling, nanofluids have opened up exciting possibilities of raising chip power in electronic components or simplifying cooling requirements for space applications. Further nanofluid pool boiling is transient and has shown that this is due to the growth of the nano coating over the heated surface during the course of the pool boiling.