2.3.4. Capillary Wicking

Another important feature of the deposited layer of nanoparticles is the capillary wicking behavior. The formation of certain microstructures by the boiling-induced nanoparticle deposition may induce capillary wicking on the heating surface. For instance, in the work performed by Kim et al. [40], the wires covered by the nanoparticles of low fraction nanofluids do not induce a working fluid rise by capillary wicking since the formed microstructures on the heating surface at low concentrations are not sufficient to play the role of microflow pathways. The authors also confirmed that the material of the nanoparticles greatly influences the capillary wicking. In this sense, the oxide titanium nanoparticle-coated wire presented a maximum water capillary rise of 1.2 mm at maximum concentration, which then steeply increased for lower concentrations achieving 5.9 mm at the lowest concentration. Moreover, the alumina nanoparticle covered wire had a maximum water capillary rise value of 0.5 for all the tested concentrations. The fractal micro-scaled structures produced by the deposition of clustered titanium oxide nanoparticles induced fluid suction caused by the capillary wicking effect that increases the CHF of water and turns it considerably higher than that with nanofluids. Nevertheless, the much higher CHF of the heating surface covered by the nanoparticles is deteriorated by the use of nanofluids instead of water, given that the nanoparticles dispersed in the base fluids may clog the microflow pathways conducting the liquid bulk to the heat transfer surface through capillary wicking. Furthermore, the authors Kim and Kim [61] studied the influence of the surface wettability and capillarity of the boiling-induced nanoparticle deposition on the CHF. The authors reported that the deposition of nanoparticles during the boiling process induces capillary wicking on the porous deposition layer, whereby the supplied fluid delays the irreversible growth of dry patches. Moreover, the estimated heat flux gain based on the capillary liquid supply was of the same order of magnitude and consistent with the experimental CHF increase above the maximum CHF value of 1500 kW/m<sup>2</sup> obtained considering the wettability improvement. The researchers concluded that the appreciable CHF amelioration of nanofluid pool boiling is the result of not only improved surface wettability but also enhanced capillarity caused by the deposition of nanoparticles.
