## 2 Experimental Methods
### Test Piece Configuration and Geometric Parameters  
Five copper test pieces (Figure 1: (a) Plain, (b) Sintered Plain, (c) Large Lattice, (d) Small Lattice, and (e) Staggered) were prepared from a copper rod measuring 35 mm in diameter and 61.5 mm in length. The Plain test piece was for experiment integrity validation. The Sintered Plain test piece was fabricated from a 6 mm-thick sintered copper powder, sandwiched between two solid copper rod sections (Figure 2a), and served as a baseline for comparison against the porous test pieces. It was intended to confirm that the sintering process does not significantly alter the heat transfer characteristics of a uniform solid copper rod, implying that 3D-printed and sintered copper parts would exhibit similar heat transfer performance to solid copper. The three distinct 3D-printed porous copper test pieces, the Large Lattice, Small Lattice (both featuring a square lattice geometry), and the Staggered surface, which showcased a unique arrangement of square pores, are described here. Each was fabricated on a 35 mm diameter, 61.5 mm long copper rod. A 2 mm-thick porous copper section was 3D-printed and sintered onto this substrate using the binder-jetting 3D-printing (BJ3 DP) process, resulting in a final height of 63.5 mm for each test piece. The nominal geometric parameters for each porous structure, derived from their CAD designs, are detailed below and are visually represented in Figure 2b,c. These designs were specifically chosen to systematically investigate a spectrum of porous structures achievable through BJ3 DP, ranging from highly interconnected, uniform networks to more patterned, localized porous arrangements. This approach provides comprehensive insights into their respective boiling heat transfer characteristics, aiding in the identification of promising design principles for future optimization, particularly in enhancing capillary wicking and facilitating efficient vapor-liquid separation during boiling. Large Lattice surface (Figure 2b, left): This surface features a square lattice geometry designed to create a fully interconnected network of through pores. Figure 2: Surface characteristics of the BJ3 DP Surfaces. (**a**) Plain and Sintered Plain test pieces, (**b**) Large Lattice and Small Lattice, and (**c**) Staggered. * Pore dimensions (top and internal): The square pores have dimensions of 0.65 mm \(\times\) 0.65 mm. These pores are designed to be through pores, extending vertically from the heated surface down to the bottom of the 2 mm-thick printed structure;
* Horizontal pore spacing (strut width): 0.75 mm;
* Lateral interconnectivity: The pores also extend horizontally, connecting across the diameter of the cylindrical test piece, forming a continuous network in the lateral direction;
* Porous layer height: The overall height of the 3D-printed porous section is 2 mm;
* Vertical pore arrangement: Within the 2 mm height, layers of these through pores are vertically offset. The top layer of pores begins at a vertical depth of 0.25 mm from the heated surface, and subsequent vertical layers of pores are spaced 0.45 mm apart. The horizontal spacing between the pores remains 0.75 mm throughout the depth. Small Lattice surface (Figure 2b, right): Similar to the Large Lattice, this surface also utilizes a square lattice geometry with fully interconnected through pores but at smaller dimensions. * Pore dimensions (top and internal): The square pores have dimensions of 0.5 mm \(\times\) 0.5 mm. These pores are also designed as through-pores, extending vertically from the heated surface down to the bottom of the 2 mm-thick printed structure;
* Horizontal pore spacing (strut width): 0.75 mm;
* Lateral interconnectivity: Like the Large Lattice, the pores extend horizontally to connect across the diameter of the test piece, ensuring continuous lateral pathways;
* Porous layer height: The overall height of the 3D-printed porous section is 2 mm;
* Vertical pore arrangement: Within the 2 mm height, layers of these through pores are vertically offset.