2.3.11. Exfoliation

The deposited layer of nanoparticles on the heating surface may still suffer a peeling action throughout the pool-boiling process. The boiling HTC decreases with increasing fraction of the nanoparticles and the exfoliated area will be increased. This was confirmed by Watanabe et al. [75], who performed experiments to evaluate the force of adhesion of the deposition layer and the effect of its partial detachment on the boiling heat transfer behavior. The researchers reported that the amount of nanoparticles deposited on the heating surface and the force of adhesion of the nanoparticle layer were influenced by the constitutive material of the nanoparticles. With the used nanoparticles, the mass of the deposited nanoparticles could be ordered as titanium oxide > alumina > silica and the force of adhesion was ordered as silica > alumina > titanium oxide. The degradation of the boiling heat transfer was particularly considerable for the titanium oxide with the largest deposited mass. With the peeling of the deposition layer, the settled mass of nanoparticles decreased and the equilibrium contact angle was enhanced. This fact rendered that the degradation of the HTC and increment of the CHF became diminished with the gradually increasing deposition layer detachment. Nevertheless, a larger HTC and a smaller CHF were observed as compared with the bare heating surface for highly exfoliated nanoparticle layers. The authors also stated that the detachment of the deposited layer of nanoparticles happened through a non-homogeneous manner. Moreover, the research team found that the peculiar boiling process at the edge of the remaining deposited layer caused the enhancement of the HTC and the decrement of the CHF.
