A nanofluid is a fluid containing nanometer-sized particles, called nanoparticles. These fluids are engineered colloidal suspensions of nanoparticles in a base fluid. The nanoparticles used in nanofluids are typically made of metals, oxides, carbides, or carbon nanotubes. Common base fluids include water, ethylene glycol and oil.
Nanofluids have novel properties that make them potentially useful in many applications in heat transfer, including microelectronics, fuel cells, pharmaceutical processes, and hybrid-powered engines, engine cooling/vehicle thermal management, domestic refrigerator, chiller, heat exchanger,in grinding, machining and in boiler flue gas temperature reduction. They exhibit enhanced thermal conductivity and the convective heat transfer coefficient compared to the base fluid. Knowledge of the rheological behaviour of nanofluids is found to be very critical in deciding their suitability for convective heat transfer applications Nanofluids also have special acoustical properties and in ultrasonic fields display additional shear-wave reconversion of an incident compressional wave; the effect becomes more pronounced as concentration increases.
In analysis such as computational fluid dynamics (CFD), nanofluids can be assumed to be single phase fluids. However, almost all of new academic paper use two-phase assumption. Classical theory of single phase fluids can be applied, where physical properties of nanofluid is taken as a function of properties of both constituents and their concentrations. An alternative approach simulates nanofluids using a two-component model.
The spreading of a nanofluid droplet is enhanced by the solid-like ordering structure of nanoparticles assembled near the contact line by diffusion, which gives rise to a structural disjoining pressure in the vicinity of the contact line. However, such enhancement is not observed for small droplets with diameter of nanometer scale, because the wetting time scale is much smaller than the diffusion time scale.
Nanofluids are produced by several techniques they are, 1.Direct Evaporation (1 step), 2.Gas condensation/dispersion (2 step), 3.Chemical vapour condensation (1 step), 4.Chemical precipitation (1 step). Several liquids including water, ethylene glycol, and oils have been used as base fluids. Although stabilization can be a challenge, on-going research indicates that it is possible. Nano-materials used so far in nanofluid synthesis include metallic particles, oxide particles, carbon nanotubes, graphene nano-flakes and ceramic particles.
Smart cooling nanofluids
Realizing the modest thermal conductivity enhancement in conventional nanofluids, a team of researchers at Indira Gandhi Centre for Atomic Research Centre, Kalpakkam developed a new class of magnetically polarizable nanofluids where the thermal conductivity enhancement up to 300% of basefluids is demonstrated. Fatty-acid-capped magnetite nanoparticles of different sizes (3-10 nm) have been synthesized for this purpose. It has been shown that both the thermal and rheological properties of such magnetic nanofluids are tunable by varying the magnetic field strength and orientation with respect to the direction of heat flow. Such response stimuli fluids are reversibly switchable and have applications in miniature devices such as micro- and nano-electromechanical systems. In 2013, Azizian et al. considered the effect of an external magnetic field on the convective heat transfer coefficient of water-based magnetite nanofluid experimentally under laminar flow regime. Up to 300% enhancement obtained at Re=745 and magnetic field gradient of 32.5 mT/mm. The effect of the magnetic field on the pressure drop was not as significant.
In nanofluids, it is recognized that nanoparticles do not follow the fluid streamlines passively. In fact, there are some reasons that induce a slip velocity between the nanoparticles and the base fluid. Movements of nanoparticles has significant impact on rheological and thermophysical properties of the nanofluids. Therefore, investigating the nanoparticles motion is critical for evaluating the performance of nanoparticles inclusion to the base fluid as a heat transfer medium. Since the nanoparticles are very small ( 100 nm), Brownian and thermophoretic diffusivities are the main slip mechanisms in nanofluids, as Buongiorno  declared. Brownian diffusion is due to random drifting of suspended nanoparticles in the base fluid which originates from continuous collisions among the nanoparticles and liquid molecules. Thermophoresis induces nanoparticle migration from warmer to colder region (in opposite direction of the temperature gradient), making a non-uniform nanoparticle volume fraction distribution.
In fact, theoretical models estimated that nanoparticles are non-homogeneously distributed. The level of non-uniformity is completely depend on thermal boundary conditions, the nanoparticle size, shape, and material. Rigorous readers encouraged to find more interesting results in open literature.
Response stimuli nanofluids for sensing applications
Researchers have invented a nanofluid-based ultrasensitive optical sensor that changes its colour on exposure to extremely low concentrations of toxic cations. The sensor is useful in detecting minute traces of cations in industrial and environmental samples. Existing techniques for monitoring cations levels in industrial and environmental samples are expensive, complex and time-consuming. The sensor is designed with a magnetic nanofluid that consists of nano-droplets with magnetic grains suspended in water. At a fixed magnetic field, a light source illuminates the nanofluid where the colour of the nanofluid changes depending on the cation concentration. This color change occurs within a second after exposure to cations, much faster than other existing cation sensing methods.
Such response stimulus nanofluids are also used to detect and image defects in ferromagnetic components. The photonic eye, as it has been called, is based on a magnetically polarizable nano-emulsion that changes colour when it comes into contact with a defective region in a sample. The device might be used to monitor structures such as rail tracks and pipelines.
Magnetically responsive photonic crystals nanofluids
Magnetic nanoparticle clusters or magnetic nanobeads with the size 80–150 nanometers form ordered structures along the direction of the external magnetic field with a regular interparticle spacing on the order of hundreds of nanometers resulting in strong diffraction of visible light in suspension.
Nanofluids are primarily used for their enhanced thermal properties as coolants in heat transfer equipment such as heat exchangers, electronic cooling system(such as flat plate) and radiators. Heat transfer over flat plate has been analyzed by many researchers. However, they are also useful for their controlled optical properties. Graphene based nanofluid has been found to enhance Polymerase chain reaction efficiency. Nanofluids in solar collectors is another application where nanofluids are employed for their tunable optical properties.
American scientific publishers launched a journal called Journal of Nanofluids specially for nanofluids. Journal of Nanofluids covers research areas on molecular fluid, nanofluids, and related technologies.
- Stephen U. S. Choi
- Sarit Kumar Das
- Slavko Kralj
- Thalappil Pradeep
- Jacopo Buongiorno
- Dhananjay Yadav
- Robert A Taylor 
- Aminreza Noghrehabadi
- Mehdi Bahiraei
- Argonne National Laboratory
- Flow battery
- Fluid dynamics
- Heat transfer
- Nanophase material
- Surface-area-to-volume ratio
- Taylor, R.A.; et al. "Small particles, big impacts: A review of the diverse applications of nanofluids". Journal of Applied Physics. 113 (1): 011301-011301-19. Bibcode:2013JAP...113a1301T. doi:10.1063/1.4754271.
- Buongiorno, J. (March 2006). "Convective Transport in Nanofluids". Journal of Heat Transfer. American Society Of Mechanical Engineers. 128 (3): 240. doi:10.1115/1.2150834. Retrieved 27 March 2010.
- "Argonne Transportation Technology R&D Center". Retrieved 27 March 2010.
- Minkowycz, W., et al., Nanoparticle Heat Transfer and Fluid Flow, CRC Press, Taylor & Francis, 2013
- Das, Sarit K.; Stephen U. S. Choi; Wenhua Yu; T. Pradeep (2007). Nanofluids: Science and Technology. Wiley-Interscience. p. 397. Retrieved 27 March 2010.
- Kakaç, Sadik; Anchasa Pramuanjaroenkij (2009). "Review of convective heat transfer enhancement with nanofluids". International Journal of Heat and Mass Transfer. Elsevier. 52: 3187–3196. doi:10.1016/j.ijheatmasstransfer.2009.02.006. Retrieved 27 March 2010.
- S. Witharana, H. Chen, Y. Ding; Stability of nanofluids in quiescent and shear flow fields, Nanoscale Research Letters 2011, 6:231 http://www.nanoscalereslett.com/content/6/1/231/
- Chen, H.; Witharana, S.; et al. (2009). "; Predicting thermal conductivity of liquid suspensions of nanoparticles (nanofluids) based on Rheology". Particuology. 7: 151–157. doi:10.1016/j.partic.2009.01.005.
- Forrester, D. M.; et al. (2016). "Experimental verification of nanofluid shear-wave reconversion in ultrasonic fields". Nanoscale. doi:10.1039/C5NR07396K.
- Maiga, Sidi El Becaye; Palm, S.J.; Nguyen, C.T.; Roy, G; Galanis, N (3 June 2005). "Heat transfer enhancement by using nanofluids in forced convection flows". International Journal of Heat and Fluid Flow. 26: 530–546. doi:10.1016/j.ijheatfluidflow.2005.02.004.
- Kuznetsov, A.V.; Nield, D.A. "Natural convective boundary-layer flow of a nanofluid past a vertical plate". International Journal of Thermal Sciences. 49 (2): 243–247. doi:10.1016/j.ijthermalsci.2009.07.015.
- Wasan, Darsh T.; Nikolov, Alex D. "Spreading of nanofluids on solids". Nature. 423: 156–159. doi:10.1038/nature01591.
- Lu, Gui; Hu, Han; Duan, Yuanyuan; Sun, Ying. "Wetting kinetics of water nano-droplet containing non-surfactant nanoparticles: A molecular dynamics study". Appl. Phys. Lett. 103: 253104. doi:10.1063/1.4837717.
- "Heat Transfer in Nanofluids—A Review". Heat Transfer Engineering. 27: 3–19. doi:10.1080/01457630600904593.
- "A review on preparation methods and challenges of nanofluids". International Communications in Heat and Mass Transfer. 54: 115–125. doi:10.1016/j.icheatmasstransfer.2014.03.002.
- Heysiattalab, S.; Malvandi, A.; Ganji, D. D. (2016-07-01). "Anisotropic behavior of magnetic nanofluids (MNFs) at filmwise condensation over a vertical plate in presence of a uniform variable-directional magnetic field". Journal of Molecular Liquids. 219: 875–882. doi:10.1016/j.molliq.2016.04.004.
- Malvandi, Amir (2016-06-01). "Anisotropic behavior of magnetic nanofluids (MNFs) at film boiling over a vertical cylinder in the presence of a uniform variable-directional magnetic field". Powder Technology. 294: 307–314. doi:10.1016/j.powtec.2016.02.037.
- Malvandi, Amir (2016-05-15). "Film boiling of magnetic nanofluids (MNFs) over a vertical plate in presence of a uniform variable-directional magnetic field". Journal of Magnetism and Magnetic Materials. 406: 95–102. doi:10.1016/j.jmmm.2016.01.008.
- J. Philip, Shima.P.D. & B. Raj (2006). "Nanofluid with tunable thermal properties". Applied Physics Letters. 92: 043108. doi:10.1063/1.2838304.
- Shima P.D.and J. Philip (2011). "Tuning of Thermal Conductivity and Rheology of Nanofluids using an External Stimulus". J. Phys. Chem. C. 115: 20097–20104. doi:10.1021/jp204827q.
- Azizian, R.; Doroodchi, E.; McKrell, T.; Buongiorno, J.; Hu, L.W.; Moghtaderi, B. "Effect of magnetic field on laminar convective heat transfer of magnetite nanofluids". Int. J. Heat Mass. 68: 94–109. doi:10.1016/j.ijheatmasstransfer.2013.09.011.
- Malvandi, A.; Moshizi, S. A.; Soltani, Elias Ghadam; Ganji, D. D. (2014-01-20). "Modified Buongiorno’s model for fully developed mixed convection flow of nanofluids in a vertical annular pipe". Computers & Fluids. 89: 124–132. doi:10.1016/j.compfluid.2013.10.040.
- Bahiraei, Mehdi (2016-11-01). "Particle migration in nanofluids: A critical review". International Journal of Thermal Sciences. 109: 90–113. doi:10.1016/j.ijthermalsci.2016.05.033.
- Bahiraei, Mehdi (2015-09-01). "Effect of particle migration on flow and heat transfer characteristics of magnetic nanoparticle suspensions". Journal of Molecular Liquids. 209: 531–538. doi:10.1016/j.molliq.2015.06.030.
- Malvandi, A.; Ghasemi, Amirmahdi; Ganji, D. D. (2016-11-01). "Thermal performance analysis of hydromagnetic Al2O3-water nanofluid flows inside a concentric microannulus considering nanoparticle migration and asymmetric heating". International Journal of Thermal Sciences. 109: 10–22. doi:10.1016/j.ijthermalsci.2016.05.023.
- Bahiraei, Mehdi (2015-05-01). "Studying nanoparticle distribution in nanofluids considering the effective factors on particle migration and determination of phenomenological constants by Eulerian–Lagrangian simulation". Advanced Powder Technology. Special issue of the 7th World Congress on Particle Technology. 26 (3): 802–810. doi:10.1016/j.apt.2015.02.005.
- Pakravan, Hossein Ali; Yaghoubi, Mahmood (2013-06-01). "Analysis of nanoparticles migration on natural convective heat transfer of nanofluids". International Journal of Thermal Sciences. 68: 79–93. doi:10.1016/j.ijthermalsci.2012.12.012.
- Malvandi, A.; Moshizi, S. A.; Ganji, D. D. (2016-01-01). "Two-component heterogeneous mixed convection of alumina/water nanofluid in microchannels with heat source/sink". Advanced Powder Technology. 27 (1): 245–254. doi:10.1016/j.apt.2015.12.009.
- Malvandi, A.; Ganji, D. D. (2014-10-01). "Brownian motion and thermophoresis effects on slip flow of alumina/water nanofluid inside a circular microchannel in the presence of a magnetic field". International Journal of Thermal Sciences. 84: 196–206. doi:10.1016/j.ijthermalsci.2014.05.013.
- Bahiraei, Mehdi; Abdi, Farshad (2016-10-15). "Development of a model for entropy generation of water-TiO2 nanofluid flow considering nanoparticle migration within a minichannel". Chemometrics and Intelligent Laboratory Systems. 157: 16–28. doi:10.1016/j.chemolab.2016.06.012.
- Mahendran, V. "Spectral Response of MagneticNanofluid to Toxic Cations". Appl. Phys.Lett. 102: 163109. doi:10.1063/1.4802899.
- Mahendran, V. (2012). "Nanofluid based opticalsensor for rapid visual inspection of defects in ferromagnetic materials". Appl. Phys. Lett. 100: 073104. doi:10.1063/1.3684969.
- "Nanofluid sensor images defects". nanotechweb.org. Retrieved 8 June 2015.
- He, Le; Wang, Mingsheng; Ge, Jianping; Yin, Yadong (18 September 2012). "Magnetic Assembly Route to Colloidal Responsive Photonic Nanostructures". Accounts of Chemical Research. 45 (9): 1431–1440. PMID 22578015. doi:10.1021/ar200276t.
- http://nanos-sci.com/technology.html Properties and use of magnetic nanoparticle clusters (magnetic nanobeads)
- "Advances in Mechanical Engineering". hindawi.com. Retrieved 8 June 2015.
- Phelan, Patrick; Otanicar, Todd; Taylor, Robert; Tyagi, Himanshu (2013-05-17). "Trends and Opportunities in Direct-Absorption Solar Thermal Collectors". Journal of Thermal Science and Engineering Applications. 5 (2): 021003–021003. ISSN 1948-5085. doi:10.1115/1.4023930.
- Hewakuruppu, Yasitha L.; Dombrovsky, Leonid A.; Chen, Chuyang; Timchenko, Victoria; Jiang, Xuchuan; Baek, Sung; Taylor, Robert A. (2013-08-20). "Plasmonic "pump–probe" method to study semi-transparent nanofluids". Applied Optics. 52 (24): 6041–50. PMID 24085009. doi:10.1364/ao.52.006041.
- Lv, Wei; Phelan, Patrick E.; Swaminathan, Rajasekaran; Otanicar, Todd P.; Taylor, Robert A. (2012-11-21). "Multifunctional Core-Shell Nanoparticle Suspensions for Efficient Absorption". Journal of Solar Energy Engineering. 135 (2): 021004–021004. ISSN 0199-6231. doi:10.1115/1.4007845.
- Otanicar, Todd P.; Phelan, Patrick E.; Taylor, Robert A.; Tyagi, Himanshu (2011-03-22). "Spatially Varying Extinction Coefficient for Direct Absorption Solar Thermal Collector Optimization". Journal of Solar Energy Engineering. 133 (2): 024501–024501. ISSN 0199-6231. doi:10.1115/1.4003679.
- "Enhancing the efficiency of polymerase chain reaction using graphene nanoflakes - Abstract - Nanotechnology - IOPscience". iop.org. Retrieved 8 June 2015.
- "Nanofluid optical property characterization: towards efficient direct absorption solar collectors". springer.com. Retrieved 8 June 2015.
- "Nanofluid-based optical filter optimization for PV/T systems". Light: Science. 1: e34. doi:10.1038/lsa.2012.34. Retrieved 8 June 2015.
- "Journal of Nanofluids". aspbs.com. Retrieved 8 June 2015.
- "MIE - NewsStephenChoiMemoriam". uic.edu. Retrieved 8 June 2015.
- "Untitled Document". iitm.ac.in. Retrieved 8 June 2015.
- "Robert A Taylor - Google Scholar Citations". scholar.google.com.au. Retrieved 2016-03-18.
- nanofluids.ir Archived November 13, 2013, at the Wayback Machine.