Kim Nanoscale Research Letters 2011, 6:415 http://www.nanoscalereslett.com/content/6/1/415 Page 9 of 18 account for the enhancing effect of nanoparticles on liquid-to-vapor phase-change heat transfer. The two underlying roles of nanoparticles during boiling To interpret the mechanism of CHF enhancement in nanofluids, two kinds of hypotheses on the roles of nanoparticle during nanofluid boiling were suggested in the early stage of research. Vassallo et al. [6] (one of the initial studies in which significant CHF enhancement in nanofluids was observed) reported that a major deposition of nanoparti- cles (about 0.15-0.2 mm thick) occurs on the heater sur- face during nanofluid boiling, suggesting some possible interactions between the nanoparticles and the heated surface at high heat fluxes. Soon afterward, Milanova and Kumar [32] and Bang and Chang [47] confirmed that nanoparticles precipitate on the surface during nucleate boiling, forming a layer whose morphology depends on the nanoparticle material, and suggesting some surface effects on CHF phenomena such as the trapping of liquid near the heater surface due to porous structures and the breakup of voids near the surface. Figure 7 shows a SEM picture of NiCr wire after deposi- tion of silica nanoparticles during nanofluid boiling. Sefiane [48] suggested an alternative approach to clar- ify the mechanism by which the presence of nanoparti- cles affects heat transfer and CHF during boiling. He demonstrated experimentally that the nanoparticles in Figure 7 SEM picture of NiCr wire after deposition of silica nanoparticles during nanofluid boiling [32].