Ramesh and Prabhu Nanoscale Research Letters 2011, 6:334 http://www.nanoscalereslett.com/content/6/1/334 metals (Al, Cu), nonmetals (graphite, carbon nanotubes), layered (Al+Al2O3, Cu+C), PCM and functionalized nanoparticles and the base fluids includes are water, Ethylene or tri-ethylene glycols, oil, polymer solutions, bio-fluids and other common fluids [10]. There are mainly two techniques used to produce nanofluid: the single-step and two-step method. Latter method is extensively used in the synthesis of nanofluids in which nanoparticles was first produced and then dispersed in the base fluids [8]. The properly prepared nanofluids are expected to give the benefits of (i) higher heat conduc- tion, (ii) more stability, (iii) microchannel cooling with- out clogging, (iv) reduced chances of erosion and (v) reduction in pumping power [11]. The addition nano- particles to the conventional fluids result in anomalous change in thermo-physical properties of the fluid. Apart from that, the addition of nanoparticles affect the boil- ing behaviour at the surfaces as they fill up the disconti- nuity at the surfaces and probably affect the critical heat flux. Nanofluids can be considered to be the next gen- eration heat transfer fluids as they offer exciting new possibilities to enhance heat transfer performance com- pared to pure liquids. They are expected to have differ- ent properties related to heat transfer as compared to conventional fluids [8]. Nanofluids offer completely dif- ferent behaviour of wetting kinetics and heat removal characteristics and these characteristics could be exploited in industrial heat treatment for quenching. The present article reviews important thermo-physical properties, wetting and boiling heat transfer characteris- tics of the nanofluids. The importance of using nano- fluids as effective quench media for hardening process during heat treatment is highlighted. Discussion Thermophysical properties of nanofluids Thermal conductivity Experiments on nanofluids have indicated that the addi- tions of small volume fraction of nanoparticles into the base fluid have significant impact on the effective ther- mal conductivity of the fluid. Choi coined the term nanofluid in 1995 and proposed that the thermal con- ductivity of the base fluid can be increased by adding low concentration of nanoparticles of materials having higher thermal conductivity than the base fluid [12]. The transient hot wire method, the steady-state parallel- plate technique and the temperature oscillation techni- que are the different techniques employed to measure the thermal conductivity of nanofluids [8]. Eastman et al. showed 60% improvement in thermal conductivity by suspending 5% volume of nanocrystalline copper oxide particles in water [13]. Wang et al. observed that the effective thermal conductivity of ethylene glycol increases by about 26 and 40% when approximately 5 Page 3 of 15 and 8 vol.% of Al2O3 nanopowders are added, respec- tively [14]. Choi measured thermal conductivity enhancement of 150% for MWCNT’s dispersed in poly- alphaolefin [15] and Marquis observed upto 243% incre- ments in CNT nanofluids [16]. The summary of enhancement ratio of the thermal conductivity of water by addition of different nanoparticles is listed in Table 1 [13,14,17-42]. There are no general mechanisms to explain the behaviour of nanofluids so far and the possi- ble mechanisms for the increment of thermal conductiv- ity of the nanofluids are as follows [43-63]: I. Brownian motion of nanoparticles: The Brownian motion of nanoparticles at the molecular and nanos- cale level was a key mechanism governing the ther- mal behaviour of nanoparticle-fluid suspensions [45]. The random motion of nanoparticles suspended in the fluid results in continuous collisions between the particles and molecules of bulk liquid thereby trans- port energy directly by nanoparticles. The impact of Brownian motion was more effective at higher tem- peratures [46]. The micro convection/mixing effect of the base fluid in the immediate vicinity of the nanoparticles caused by the Brownian motion was an important reason for the large thermal conductiv- ity enhancement of nanofluids [47]. However, the Brownian motion contribution to the thermal con- ductivity of nanofluid was very small and cannot be responsible for extraordinary thermal transport properties of nanofluids [43,48-50]. II. Liquid layering around nanoparticles: The ordered layering of liquid molecules at the solid par- ticle surface forms solid-like nanolayer. This layer acts as a thermal bridge between the solid nanoparti- cles and the base liquid and plays an important role in the enhanced thermal conductivity of nanofluids [51-54]. The effective thermal conductivity increases with increase in nanolayer thickness. Especially in small particle size range, the effects of particle size and nanolayer thickness become much more obvious, which implies that manipulating nanolayer structure might be an effective method to produce highly thermally conductive nanofluids [55]. Although the presence of an interfacial layer may play a role in heat transport, it is not likely to be solely responsible for enhancement of thermal con- ductivity [43]. By using molecular dynamics simula- tions, Xue et al. demonstrated that the layering of the liquid atoms at the liquid-solid interface does not have any significant effect on thermal transport properties [58]. III. Nature of the heat transport in the nanoparticles: When the nanoparticle size becomes very small, the mean free path of phonon is comparable to the size