## 2 Experimental Methods
### Experimental Apparatus  
Figure 7a,b show a photo and schematic diagram of the experimental setup used to study the pool-boiling heat transfer performance on the 3D-printed surfaces. The working fluid used in all experiments was de-ionized and de-gassed water. The main components of the experiment consist of the boiling chamber, primary and secondary heating sections, a coil condenser, a boiling visualization system, and a data acquisition system. The boiling chamber is made of cylindrical reinforced polycarbonate glass with a wall thickness of 3 mm, an outer diameter of 150 mm, and a height of 237 mm. The polycarbonate glass allows the boiling process inside the chamber to be visualized. Two-thirds of the vessel was filled with the working fluid before each experiment. The primary heating section consists of seven 600 W cartridge heaters (totaling 4200 W) installed at the bottom of a copper block. These heaters are controlled by two single-winding variac transformers (bolt sliders) powered by 200 V AC, allowing for precise regulation of the input heat flux to the test piece. However, as shown in Figure 7b, the measurement limit is reached when the temperature of the thermocouple located at the center of the cartridge heater section approaches the melting point of copper (1084.5 \({}^{\circ}\)C). The copper block is precisely designed with a hollow section at its top to fit the bottom part of the test piece with exact precision, ensuring negligible contact resistance and promoting uniform heat flow. To minimize heat loss from the sides and ensure that nearly all the generated heat is directed to the test section, the entire copper block is thoroughly insulated with glass wool. The design features, including copper block fitting and insulation, are crucial for minimizing radial heat losses and thereby limiting the influence of edge effects on the primary heat transfer surface. Figure 6: SEM images of the BJ3 DP surface. The secondary heating section maintains the working fluid's saturation temperature throughout the experiment. It comprises four 250 W cartridge heaters installed through protective tubes and immersed in the boiling chamber. Their output is controlled by a single small bolt slider (single winding Variac transformer) powered by 100 V AC. These heaters serve the crucial purpose of maintaining the bulk fluid at saturation and compensating for ambient heat losses from the chamber, thereby ensuring pure pool-boiling conditions with negligible sub-cooling. Several K-type thermocouples were used in the experiment. The fluid saturation temperature (\(T_{Sat}\)) was measured by a K-type thermocouple positioned at the top of the boiling chamber and immersed in the fluid. Four K-type thermocouples were embedded within the solid copper base of the test piece at different axial locations along the heat flow path to measure the temperature gradient. A linear regression based on these temperature measurements was used to determine the wall superheat (\(\Delta T=T_{wall}-T_{Sat}\)) at the boiling surface. These thermocouples were precisely calibrated to minimize errors during temperature measurements. Figure 7: (**a**) Photo image of the experimental setup; (**b**) schematic diagram of the experimental setup. A pressure gauge was installed on the flange at the top of the boiling chamber. An FP101A (Yokogawa Electric, Tokyo, Japan) absolute pressure gauge measures the vessel's pressure to maintain atmospheric pressure during experiments by controlling the condenser. For visualization, a high-speed camera model Nova S16 (Photron), recording at 16,000 frames per second (fps) with a shutter speed of 1/200,000 s and a resolution of 1024 \(\times\) 1024 pixels, was used to capture the boiling process. A lighting strobe on the camera side and a panel light (WTT 120120(8) from Nissin Electronics, Tokyo, Japan) on the opposite side were used as a backlight to ensure high image quality. A spiral-coil copper condenser with circulating coolant condensed the vapor to maintain atmospheric pressure.