Kim Nanoscale Research Letters 2011, 6:415 http://www.nanoscalereslett.com/content/6/1/415 Page 13 of 18 Figure 11 Static contact angles of 5-μL sessile droplets on stainless steel surfaces. (a) Pure water droplet on surface boiled in pure water, (b) alumina nanofluid droplet on surface boiled in pure water, (c) pure water droplet on surface boiled in alumina nanofluid, (d) alumina nanofluid droplet on surface boiled in alumina nanofluid [46]. Kim et al. [59] also found that a porous layer of nano- particles significantly improved the surface wettability. On the other hand, they were the first to show that the effect of wettability alone cannot explain additional increases beyond the attainable CHF value when the contact angle approaches zero. By focusing on the role of capillarity in the CHF behavior of nanofluids, they showed that capillarity causes the liquid to rise on the nanoparticle-deposited surfaces in accordance with where Rc and cosθ represent the microscopic structures and surface wettability of the nanoparticle layers, respec- tively. Capillary flow during boiling supplies fresh liquid to the dry region beneath the vapor bubbles, delaying the irreversible growth of hot spots and CHF. Kim and Kim [60] used capillarity to characterize a completely wetted nanoparticle-coated surface. They showed that the esti- mated heat-flux gain due to capillary liquid supply along Lc,max = 2σRccρogsθ , (3) the porous layer was of the same order of magnitude as that due to wettability enhancement (Figure 12). They concluded that the significant CHF enhancement of nano- fluids during pool boiling is a consequence not only of increased surface wettability, but also of improved capillar- ity resulting from the surface deposition of nanoparticles. A consensus explanation of the cause of CHF enhancement in nanofluids seems to be obtainable via an intense study focused on the effect of the nanoparti- cle layer. In other words, the CHF of a nanofluid is enhanced by its improved ability to actively wet the hea- ter surface, thanks to the porous structure of the thin nanoparticle sorption layers. Exploration of the mechanism of CHF enhancement in nanofluids Several recent studies have tried to link existing CHF the- ories and experimental results in nanofluids, for example, Kim et al. [46] and Golubovic et al. [19], to incorporate the improved surface properties caused by nanoparticle