*1.4. Scope and Aim of this Study*

There are few studies that investigate the boiling performance of nanofluids on premodified surface (e.g., laser-textured surfaces) [51,52], despite the seemingly great potential for concomitant and synergistic enhancement of boiling performance. To fill this knowledge gap, we investigated the pool boiling heat transfer performance of TiO2-water nanofluids prepared with two mass concentrations (0.001 and 0.1 wt.%) and with two different sizes of nanoparticles (small: 4–8, and large: 490 nm), on laser textured copper surfaces. Five consecutive measurements were performed on each laser-textured surface under pool boiling conditions. The surface morphology of deposited nanoparticles was analyzed using scanning electron microscopy (SEM), and the wettability changes were recorded through water contact angle (WCA) measurements. The results were analyzed through multiple comparisons to elucidate the effect of nanoparticle size and concentration on possible

additional enhancement or deterioration of boiling performance of the laser-textured copper surfaces. **2. Methods** 

nanofluids prepared with two mass concentrations (0.001 and 0.1 wt.%) and with two different sizes of nanoparticles (small: 4–8, and large: 490 nm), on laser textured copper surfaces. Five consecutive measurements were performed on each laser-textured surface under pool boiling conditions. The surface morphology of deposited nanoparticles was analyzed using scanning electron microscopy (SEM), and the wettability changes were recorded through water contact angle (WCA) measurements. The results were analyzed through multiple comparisons to elucidate the effect of nanoparticle size and concentration on possible additional enhancement or deterioration of boiling performance of the

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#### **2. Methods** *2.1. Sample Preparation and Amalysis*

#### *2.1. Sample Preparation and Amalysis* Samples for boiling experiments were prepared on high purity copper (>99.9% Cu).

laser-textured copper surfaces.

Samples for boiling experiments were prepared on high purity copper (>99.9% Cu). Each sample was first sanded using P1200 and P2000 grit sandpaper to achieve a surface roughness of approx. 0.15 µm. Afterwards, the samples were cleaned using isopropanol and lint-free wipes. One sample was tested without any further treatment, and it is denoted as REF (i.e., untreated reference sample). All other samples underwent direct laser texturing immediately before the boiling experiments. Each sample was first sanded using P1200 and P2000 grit sandpaper to achieve a surface roughness of approx. 0.15 μm. Afterwards, the samples were cleaned using isopropanol and lint-free wipes. One sample was tested without any further treatment, and it is denoted as REF (i.e., untreated reference sample). All other samples underwent direct laser texturing immediately before the boiling experiments. To perform the laser texturing, a nanosecond pulsed fiber laser was used (FL-mark-C

To perform the laser texturing, a nanosecond pulsed fiber laser was used (FL-mark-C with JPT Opto-electronics Co., Ltd. "M7 30 W" MOPA source, Shenzhen, China). The laser system is equipped with an OPEX F-Theta lens with a focal distance of 100 mm and working field of 70 <sup>×</sup> 70 mm<sup>2</sup> . A pattern of equidistant parallel lines (∆*x* = 60 µm) was used to create a channel-like microstructure on each sample. The pulse frequency was set to 110 kHz, the pulse duration to 45 ns, and the full power of 30 W was applied. With the focal beam diameter of ~25 <sup>µ</sup>m and laser beam quality parameter M<sup>2</sup> <sup>≤</sup> 1.3, the average laser pulse fluence was calculated to be ~56 J cm−<sup>2</sup> . with JPT Opto-electronics Co., Ltd. "M7 30 W" MOPA source, Shenzhen, China). The laser system is equipped with an OPEX F-Theta lens with a focal distance of 100 mm and working field of 70 × 70 mm2. A pattern of equidistant parallel lines (Δ*x* = 60 μm) was used to create a channel-like microstructure on each sample. The pulse frequency was set to 110 kHz, the pulse duration to 45 ns, and the full power of 30 W was applied. With the focal beam diameter of ~25 μm and laser beam quality parameter M2 ≤ 1.3, the average laser pulse fluence was calculated to be ~56 J cm−2.

The morphology and elemental composition of the samples were analyzed using scanning electron microscopy (ThermoFisher Scientific Quattro S, Waltham, MA, USA) and energy-dispersive X-ray spectroscopy (Oxford Instruments Ultim Max 65, Abingdon, UK). SEM images of the reference sample REF and laser-textured sample (LT) before exposure to boiling are shown in Figure 1. The morphology and elemental composition of the samples were analyzed using scanning electron microscopy (ThermoFisher Scientific Quattro S, Waltham, MA, USA) and energy-dispersive X-ray spectroscopy (Oxford Instruments Ultim Max 65, Abingdon, UK). SEM images of the reference sample REF and laser-textured sample (LT) before exposure to boiling are shown in Figure 1.

**Figure 1.** SEM images of the (**a**) untreated reference sample REF (blue frame) and (**b**) laser-textured sample LT (red frame) before exposure to boiling. **Figure 1.** SEM images of the (**a**) untreated reference sample REF (blue frame) and (**b**) laser-textured sample LT (red frame) before exposure to boiling.

The analysis of sample wettability was performed using a goniometer to record the static contact angles with water. Cleaning of the samples was performed with a UV/ozone cleaner (Ossila) to remove hydrocarbon contaminants after boiling/storage. Contact angles were measured: (i) before boiling, (ii) immediately after boiling experiments, (iii) 7 days after the boiling process, (iv) after UV/ozone cleaning (7 days after the experiments), and finally (v) three days after UV/ozone cleaning. Measurements were conducted using a goniometer (Ossila, ±1 ◦ ). Five drops were deposited onto different parts of the surface, and the contact angles were recorded and averaged.

#### *2.2. Boiling Performance Evaluation* The boiling performance evaluation of nanofluids on laser-textured copper surfaces

*2.2. Boiling Performance Evaluation* 

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and the contact angles were recorded and averaged.

The boiling performance evaluation of nanofluids on laser-textured copper surfaces was performed using a previously developed experimental setup shown in Figure 2 [53]. A glass cylinder with an inner diameter of 60 mm was placed between two stainless steel flanges, thus forming the boiling chamber. The latter was filled with 200 mL of the working fluid during experiments. As shown in Figure 2b, the sample was mounted on a copper heating block, and inserted through the bottom stainless-steel flange into the boiling chamber. PEEK bushing, a ring of flexible epoxy glue, and a silicone O-ring were used to ensure sealing, limit heat loss, and prevent parasitic boiling. Before every experiment, the working fluid was preheated and degassed with an immersion heater, which was controlled by a variable transformer. An immersion heater was also used to maintain the saturated state of the working fluid during the boiling experiment. The cartridge heaters, positioned inside the heating block, were used for the generation of heat and were also controlled by a variable transformer. All measurements were performed at atmospheric pressure, and the stability was confirmed by the stable temperature of the saturation. Vapor produced during measurements went to the water-cooled glass condenser and returned to the boiling chamber. was performed using a previously developed experimental setup shown in Figure 2 [53]. A glass cylinder with an inner diameter of 60 mm was placed between two stainless steel flanges, thus forming the boiling chamber. The latter was filled with 200 mL of the working fluid during experiments. As shown in Figure 2b, the sample was mounted on a copper heating block, and inserted through the bottom stainless-steel flange into the boiling chamber. PEEK bushing, a ring of flexible epoxy glue, and a silicone O-ring were used to ensure sealing, limit heat loss, and prevent parasitic boiling. Before every experiment, the working fluid was preheated and degassed with an immersion heater, which was controlled by a variable transformer. An immersion heater was also used to maintain the saturated state of the working fluid during the boiling experiment. The cartridge heaters, positioned inside the heating block, were used for the generation of heat and were also controlled by a variable transformer. All measurements were performed at atmospheric pressure, and the stability was confirmed by the stable temperature of the saturation. Vapor produced during measurements went to the water-cooled glass condenser and returned to the boiling chamber.

The analysis of sample wettability was performed using a goniometer to record the static contact angles with water. Cleaning of the samples was performed with a UV/ozone cleaner (Ossila) to remove hydrocarbon contaminants after boiling/storage. Contact angles were measured: (i) before boiling, (ii) immediately after boiling experiments, (iii) 7 days after the boiling process, (iv) after UV/ozone cleaning (7 days after the experiments), and finally (v) three days after UV/ozone cleaning. Measurements were conducted using a goniometer (Ossila, ±1°). Five drops were deposited onto different parts of the surface,

**Figure 2.** Dimensions of the sample including the position of thermocouples (**a**), cross section of the sample and the heating assembly (**b**), and experimental setup (**c**)*.* **Figure 2.** Dimensions of the sample including the position of thermocouples (**a**), cross section of the sample and the heating assembly (**b**), and experimental setup (**c**).

The temperatures inside the sample were measured by utilizing three K-type thermocouples spaced 5 mm apart from one another. The thermocouple closest to the boiling The temperatures inside the sample were measured by utilizing three K-type thermocouples spaced 5 mm apart from one another. The thermocouple closest to the boiling surface was positioned 5.3 mm from the top of the sample, as shown in Figure 2c. Two further K-type thermocouples were submerged into the boiling chamber at different heights to measure the temperature of the working fluid. A KRYPTONi-8xTH DAQ device was used for collecting all temperature signals as raw voltages. The temperature of each cold junction was recorded internally and used to offset the measurements to obtain correct temperature readings. The calculation of temperatures based on offset voltages was performed, utilizing NIST 9th degree polynomial. Data from the DAQ device were acquired using Dewesoft X3 software at a frequency of 10 Hz.
