Kim Nanoscale Research Letters 2011, 6:415 http://www.nanoscalereslett.com/content/6/1/415 drives the meniscus to advance and rewet the hot/dry spot). Kim et al. [46] semi-quantitatively showed that the improved wettability on the nanoparticle-fouled surface significantly increases the surface tension force to rewet the hot/dry spot, suggesting higher CHF. In this regard, they concluded that the hot/dry spot model incorporat- ing the micro-hydrodynamics of a liquid meniscus cor- roborates the link between increased wettability and CHF enhancement in nanofluids. In addition, Kim et al. [69] conducted sessile-drop wetting experiments focused on the effect of a nanoparticle layer on the stability of an evaporating meniscus. They found that an individual liquid meniscus is more stable on an alumina nanoparti- cle layer and hence can sustain the evaporation recoil force at a higher heat flux. The evaporative heat-flux gain attainable on the nanoparticle layer was of the same order of magnitude as the CHF increases in nano- fluids. Thus, these experimental results also supported the hot/dry spot theory based on the micro-hydrody- namics of a liquid meniscus. Several recent studies have demonstrated that the CHF model proposed by Kandlikar and Steinke [70] is reasonably well correlated with measured CHF data in nanofluids as a function of contact angle (see, for exam- ple, [40,71]). This model utilizes the evaporation momentum force and receding contact angle b as para- meters. Fundamentally, this model also focuses on the micro-hydrodynamics of the vapor-liquid interface of a single bubble at the heater surface based on the force balance at the solid-vapor-liquid triple-contact line. The accuracy of this model in predicting the CHF of nano- fluids supports the argument that the hot/dry spot model incorporating the micro-hydrodynamics of an evaporating meniscus is a plausible mechanism. Concluding remarks Over the past decade, a considerable amount of research has been carried out in the area of nucleate boiling criti- cal heat flux (CHF) in nanofluids. It is now known that in both pool and flow boiling, the CHF capability of conventional heat transfer fluids (such as water or alco- hol) is significantly improved by suspending nanoparti- cles in the base liquids even at small particle concentrations (less than 0.1 vol.%). The present review of available studies indicated that there is a general consensus in the key cause of CHF enhancement in nanofluid boiling: the thin nanoparticle layer formed on the heater surface, during nucleate boiling of nanofluids, increases the CHF via their improved ability to wet the heater surface. Although appropriate modifications of all the traditional CHF the- ories succeed in demonstrating approaches to and pos- sibilities for incorporating the impact of microscale Page 16 of 18 deposition of nanoparticles with nanoscale pores, a suf- ficiently definite theory to link the improved wettability and the increase of CHF on the nanoparticle layer has not yet emerged owing to the complexity of the phe- nomenon of CHF and to the lack of information about microscale two-phase flow underneath bubbles. It is very difficult to figure out the underlying mechanism leading to CHF from on relatively large-scale conven- tional nucleate boiling experiments, which only yield time- and space-averaged information of the complex phenomenon of CHF. In this regard, to understand the fundamental mechanism of CHF enhancement in nano- fluids, the efforts by researchers have to focus on obtaining the full details of two-phase heat transfer near the heater surface (for example, direct measurement of the time-dependent temperature and liquid-vapor phase distributions on the heater surface in high heat-flux nucleate boiling). Another area that merits further study is the effect of pressure and heater geometry. A systematic review of available data in literature revealed that the magnitude of the CHF enhancement in nanofluids is very strongly dependent on system pressure and heater geometry. These parametric effects must be carefully considered when assessing the potential of nanofluids for various industrial applications. For example, it is doubtable if nanofluids significantly enhance CHF even in the high- pressure condition, such as nucleate reactor core. Experiments are needed to extend the nanofluids’ usability to many high-flux systems with a wide diversity of heater geometry and pressure conditions. From a practical point of view, considering application of nanofluids to actual thermal-flow systems, good stabi- lity of nanoparticles is one of the critical necessary con- ditions, as indicated in the review of the microchannel flow boiling applications. Adding ionic additive to con- trol electrostatic condition of solution is one of the sim- plest options to improve dispersion stability of nanoparticles in nanofluids, but it can severely alter characteristic structures of nanoparticle deposition on a heater surface, resulting in the distorted nucleate boiling CHF performance. There is no systematic study avail- able in literature that describes the effects of additives on nucleate boiling CHF in nanofluids. Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST; grant no. 2010- 0018761). Authors’ contributions HK conducted the extensive literature review and drafted the manuscript. The author read and approved the final manuscript. Competing interests The author declares that they have no competing interests.