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Boiling of nanofluids is reported to exhibit conflicting heat transfer trends compared to base liquids. On one hand, a deterioration of heat transfer in nucleate boiling regime and on the other hand a spectacular enhancement of critical heat flux are reported [S. M. You etal, Appl. Phys. Lett. 83, 3374 (2003); D. Milanova and R. Kumar, ibid. 87, 233107 (2005)]. The analysis and interpretation of these trends have so far been focused on the effect of nanoparticles on surface roughness, nucleation site density, and the use of hydrodynamics model for critical heat flux. In this letter a basic experiment is performed to propose a different approach to understand the mechanism through which the presence of nanoparticles acts on heat transfer during boiling of nanofluids. Pure ethanol and ethanol with aluminum particles evaporating on a hot polytetrafluoroethylene surface are investigated. The results indicate that the nanoparticles promote the pinning of the contact line of the meniscus and sessile drops. The interpretation of the reported results on boiling of nanofluids could be considered in the light of these findings. Structural disjoining pressure stemming from the ordered layering of nanoparticles in the confined wedge of the evaporating meniscus [D. T. Wasan and A. D. Nikolov, Nature (London) 423, 156 (2003)] is thought to be the principal mechanism behind the observed pinning behavior. It is suggested that the analysis of boiling heat transfer of nanofluids must account for the important effect of nanoparticles on the contact line region through the structural disjoining pressure to provide accurate interpretation.