Ramesh and Prabhu Nanoscale Research Letters 2011, 6:334 http://www.nanoscalereslett.com/content/6/1/334 50% compared with PAO at 0.6% concentration by weight [88]. Zhou et al. showed that specific heat capa- cities of nanofluids vary with the base fluids, the size and volume concentration of nanoparticles [89]. Vajjha and Das measured the specific heat of three nanofluids containing Al2O3, SiO2 and ZnO nanoparticles. The first two were dispersed in a base fluid of 60:40 by mass of ethylene glycol and water and the last one in deionized water. Experiments were conducted at different particle volume concentration and different temperatures. They developed a general specific heat correlation as [90]: Cpnf = Cpbf A ∗ T T0 + B ∗ (C + ϕ) Cps Cρbf . Density The density of the nanofluids can estimated from the mixture theory [81]: ρnf = ϕρp + (1 − ϕ)ρw. where j is the volume fraction of the nanoparticles, rp is the density of the nanoparticles and rw is the density of the base fluid. Sundar et al. estimated the densities of nanofluids at different temperatures. The density was found to decrease with increase in temperature [92]. Similarly Harkirat measured the density of Al2O3 nano- particles dispersed in water using specific gravity bottles at different ranges of temperature (30 to 90°C) and dif- ferent concentrations of nanofluids (1 to 4%). He observed that density of nanofluids is higher than the base fluids and increase with increase in volume fraction of nanoparticles from 1 to 4%. The density of nanofluids decreases with increase in temperature upto about 80°C. Beyond this value, densities of 1 to 4% nanofluids remained nearly constant but still were more than that of water [93]. Surface tension Surface tension is defined as the force acting over the surface of the liquid per unit length of the surface per- pendicular to the force. Surface tension has a significant influence on the boiling process since bubble departure and interfacial equilibrium depends on it [94]. Surface tension of nanofluids prepared by without addition of any surfactant was found to differ minimally whereas addition of surfactant during preparation of nanofluids affect significantly [78,95,96]. The surfactant behaves like an interfacial shell between the nanoparticles and base fluids and modifies the surface tension of nano- fluids [97]. Surface tension decreases with increases in concentration of nanoparticle and temperature [98-100]. It clears from the above study, the addition nanoparti- cles to the base fluids would result in a change in (3) (4) Page 8 of 15 thermophysical properties of the base fluids. A wide spectrum of microstructure and mechanical properties can be obtained for a given steel component by control- ling the cooling rate (Figure 3) [101]. In order to attain the fully quenched structure (martensitic structure), the component must be quenched below the nose of the TTT curve called critical cooling rate. This critical cool- ing rate is not a constant for all materials and addition of alloying elements to the steel shift the nose of TTT curve (Figure 4) [102]. Therefore, the heat treaters need different types of quenching media to provide varying critical cooling rate. Table 1 shows for the same base fluid, addition different nanoparticle materials at differ- ent concentrations yield varying thermal conductivities. Jagannath and Prabhu observed peak cooling rates vary- ing from 76°C/s to 50.8°C/s by addition of Al2O3 nano- particles of concentration 0.01 to 4% by weight into water during quenching of copper probe [103]. The standard cooling curve analysis by Gestwa and Przyłecka observed that addition 1% of Al2O3 nanoparticles to the 10% polymer water solution results cooling speed increases from 98 to 111°C/s [104]. Babu and Kumar also observed different cooling rates with the addition of different concentration of CNT into water during quenching of stainless steel probe [105]. Further, the addition of nanoparticles not only changes the peak cooling rate but also results in change of the six cooling curve characteristics. Hence, the change in thermophysi- cal properties of base fluids with addition of nanoparti- cles can be utilized to prepare fluids having different cooling properties by controlling the particle volume concentration, particle material, particle size, particle shape and base fluid. Synthesis of quenching media hav- ing varying cooling severity would greatly benefit the heat treatment industry. Wetting characteristics of Nanofluids The presence of nanoparticles affects the spreading and wettability of base fluids because of additional particle- particle, particle-solid and particle-fluid interactions [106]. Two important phenomena for the enhancement of wetting behaviour of nanofluid are (i) solid like order- ing of nanoparticles in the vicinity of three-phase con- tact region and (ii) deposition of nanoparticles during boiling. Simulations study by Boda et al. on hard spheres in a wedge-shaped cell reported formation of new layers of hard spheres between the walls of the wedge [107]. Wasan and Nikolov directly observed the particle-struc- turing phenomenon in the liquid film-meniscus region by using reflected-light digital video microscopy [108]. The layering arrangement of the particles gives rise to an excess pressure in the film, the structural disjoining pressure which has an oscillatory decay profile with the film thickness. A result of such a structure force is that