*2.4. Peparation of Nanofluids*

The nanofluids used were prepared by dispersing different amounts of nanoparticles in the base fluid (twice-distilled water). The nanofluids used in this study were prepared and stabilized using ultrasonic vibration, without using surfactants or adjusting the pH value. TiO<sup>2</sup> nanoparticles were chosen for the study because of their favorable chemical and physical stability and their hydrophilicity, which was previously shown to enhance the CHF value during boiling experiments. Two nanoparticle size ranges were used to examine the effect of their size, especially relative to the size of laser-induced surface structures. One type of nanofluid was prepared using very small nanoparticles (sized below 10 nm,

much smaller than the majority of structures resulting from laser texturing), while the other type used nanoparticles that were two orders of magnitude larger, with diameters close to 500 nm (close to the scale of laser-induced surface structures). Furthermore, two nanofluids concentrations were tested for each type of nanofluid. Most studies typically report nanofluid concentrations in the range from 0.001 wt.% to 0.1 wt.%. Therefore, we opted to use the two extreme values, again separated by two orders of magnitude, in order to evaluate the effect of nanoparticle concentration on boiling performance.

One type of nanofluid was prepared using small nanoparticles, with a size of 4 to 8 nm (Carl Roth, Karlsruhe, Germany; ROTInanoMETIC ≥ 99.9%), and the second type was prepared using large nanoparticles, with a size of 490 nm (Nanografi Nano Technology, Ankara, 99.995+%). Both types of nanofluids were prepared at two different mass concentrations: 0.1 and 0.001 wt.%. The third type of nanofluid was prepared with 0.05 wt.% of the large-sized and small-sized nanoparticles, respectively. Twice-distilled water was degassed for 45 min via vigorous boiling before it was used to prepare the nanofluids to reduce the amount of entrapped gases and also reduce the need for further degassing in the boiling chamber, where deposition of nanoparticle on the immersion heater reduces the concentration of the nanofluid in an uncontrolled way. Nanofluids were sonicated for 1 h immediately after preparation in an ultrasonic bath (ASonic, Ultrasonic Cleaner-Pro 30, 40 kHz, 120 W). To stabilize the solution before use in the experiment, the nanofluid was again sonicated for 1 h. The performed experiments are summarized in Table 1.


**Table 1.** The characteristics of the performed experiments.

Overall, the used nanoparticle sizes and concentrations are based on the extreme values reported in the literature, and the corresponding extreme combinations, as used here, are expected to allow for the generalization of the results.
