Kim Nanoscale Research Letters 2011, 6:415 http://www.nanoscalereslett.com/content/6/1/415 Page 12 of 18 Figure 10 Images of nanparticle coating generated, on the heater surface [28]. Accordingly, nanofluid boiling itself, and specifically microlayer evaporation, is responsible for producing the nanoparticle layer on the surface. Kim et al. [46,55] investigated the surface effect on CHF enhancement of water-based nanofluids containing alumina, zirconia, and silica nanoparticles. In their research, the deposition of nanoparticles on the heater surface significantly improved the wettability, as mea- sured by the reduction of the static contact angle (see Figure 11). Note that no appreciable differences were found between pure water and nanofluids. They inferred that the buildup of a porous layer with oxide nanoparti- cles increases the adhesion tension gSV - gSL and the roughness factor r (the ratio of the effective contact area to the smooth contact area), and both effects lead to a pronounced reduction of the contact angle in cosθ = γSV − γSL r, σ (2) accordance the modified Young-Laplace equation [56], where θ and s are the contact angle and the surface tension, respectively. A systematic review of the preva- lent CHF theories then demonstrates that higher wett- ability can produce a CHF enhancement consistent in magnitude with the experimental observations of numerous researchers. Subsequently, a number of stu- dies focusing on the role of a nanoparticle layer, includ- ing Coursey and Kim [57], Liu and Liao [21], Golubovic et al. [19], and Jeong et al. [58], produced the same con- clusions as Kim et al. [46] in regard to the significant improvement of surface wettability and its role in the CHF enhancement of nanofluids.