Ramesh and Prabhu Nanoscale Research Letters 2011, 6:334 http://www.nanoscalereslett.com/content/6/1/334 whereas the measured hardness profile shows a continu- ous line in the case of sample quenched in the polymer solution having total wetting time of 1.5 s (Figure 5) [2]. Thus, the type of the wetting process significantly affects the cooling behaviour of the quenchant and hardness profile of the quenched samples. Vafaei et al. measured the contact angle of nanofluid sessile droplets and showed that the contact angle depends strongly on nanoparticle concentration and for the same mass con- centration smaller size nanoparticles lead to larger changes in contact angle [110]. Sefiane et al. observed that advancing contact line velocity increases to a maxi- mum as the concentration increases up to 1% and then decreases as the concentration is increased further. They explained that the enhanced wetting is attributed to a pressure gradient within the nanofluid which is created due to the nanoparticles forming a solid-like ordering in the fluid ‘wedge’ in the vicinity of the three-phase con- tact line and agglomeration of nanoparticles at higher concentration reduces the degree of enhanced wetting [106]. The surface wettability study by Kim et al. mea- sured the static contact angle of sessile droplets for pure water and nanofluids on clean surfaces and nanoparti- cle-fouled surfaces. They found dramatic decrease of the contact angle on the fouled surfaces and concluded that the wettability was enhanced by the porous layer on the surface, not the nanoparticles in the fluid [111]. Another study by Mehta and Khandekar measured static contact angles of sessile droplets showed that the wettability of Page 10 of 15 laponite nanofluid on copper substrate was indeed much better than both alumina nanofluid and pure water [112]. These studies imply that the use of nano- particles in the conventional quenching media would result in enhancement of wettability. The enhanced wet- ting characteristics of nanofluids can be adopted to pro- mote the Newtonian wetting and improve the spreading process during quench heat treatment of components. Boiling heat transfer characteristics of nanofluids The alteration of thermophysical properties, especially the enhancement of the thermal conductivity, of the nanofluid and different heat transfer mechanisms are expected to have a significant effect on heat transfer characteristics. Xuan and Li [18] listed the following five reasons for improved heat transfer performance of the fluid by suspending nanophase particles in heating or cooling fluids: (i) the suspended nanoparticles increase the surface area and the heat capacity of the fluid, (ii) the suspended nanoparticles increase the effective (or apparent) thermal conductivity of the fluid, (iii) the interaction and collision among particles, fluid and the flow passage surface are intensified, (iv) the mixing fluc- tuation and turbulence of the fluid are intensified and (v) the dispersion of nanoparticles flattens the transverse temperature gradient of the fluid. Experiments on two phase (boiling) heat transfer of nanofluid shows different behaviour. Das et al. conducted experiments to study the pool boiling in water-Al2O3 nanofluid with different (a) (b) Figure 5 Surface hardness profile calculated from the measured wetting time tB and the specific calibration curve for the material related to the distance from the lower end of the sample and compared to the measured hardness profile. Sample: 100Cr6 dia 25 mm × 100 mm, bath: (a) distilled water, (b) polymer solution.