## 4 Results and Discussion
### Experimental Determination of the Capillary Action
#### 4.1.4 Scaffold SLM  
This technique brings total control over the pore size or capillary radius, as previously mentioned, as well as over the tortuosity of the matrix. Tortuosity is defined as the ratio of the length of a streamline (flow path) between two points to the straight-line distance between those points.  
In this case, the configuration could be roughly described in terms of capillary conduits arranged parallel to each other. In the manufacturing process, the first step was to generate the CAD (Computer aided design) file of the wick to determine both its internal passages and its the external appearance. This file was then loaded into the 3D SLM printer. The main technical features are listed in Table 6.  
The printing parameters, such as laser intensity and action radius, powder flow rate and stainless-steel powder material, and printing depth were carefully adjusted. A general view of the whole wick is shown in Figure 9, as analyzed in [11].  
Two samples were built: a fine 90 capillary radius (Figure 10a) and a thick 180 capillary radius (Figure 10b), with porosities of 30% and 45%, respectively, the main characteristics of which are shown in Table 7.  
This arrangement reduces the tortuosity of the medium that would improve the pumping capacity. Despite the fact that SLM has the potential to manufacture complex cells with a better hydrodynamic behavior, a simple cell has been created in this work.  
The scaffold material was stainless-steel, and once again the working fluid was methanol. The load tests for both configurations offered the following results, as depicted in Figure 11a.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Feature** & **Value** \\ \hline
**Laser power** & 200 W \\ \hline
**Exposure time** & 286 \\ \hline
**Layer thickness** & 50 \\ \hline
**Scan speed** & 350 mm/s \\ \hline
**3D optics configuration** & Dual 1 \(\times\) 700 W \\ \hline \hline \end{tabular}
\end{table}
Table 6: Technological parameters using the selective laser melting (SLM) printer.  
Figure 9: Final result of the 3D printed wick.  
As demonstrated in the previous cases, the lower the pore size (finer pitch), the greater the pumping capacity. The maximum liquid height (\(H\)) for the 90 \(\upmu\)m scaffold (38 mm) was roughly 43% higher than it was for the 180 \(\upmu\)m scaffold (28 mm). Flow velocity at 30 mm height for the 90 \(\upmu\)m scaffold (0.25 mm/s) satisfied the application specification (0.031 mm/s), unlike the 180 \(\upmu\)m scaffold (0.0013 mm/s). Likewise, the 180 \(\upmu\)m scaffold was hardly able to pump the liquid 30 mm in height. Figure 11b shows the sudden drop in the forward speed for both cases, as well as the higher initial speed of the 90 \(\upmu\)m scaffold compared to that of the 180 \(\upmu\)m scaffold.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**D (\(\upmu\)m)** & \(\epsilon\) & **A (mm/s)** & **A\({}_{\mathbf{1}}\)** & **t\({}_{\mathbf{1}}\)** & **A\({}_{\mathbf{2}}\)** & **t\({}_{\mathbf{2}}\)** \\ \hline
90 & 0.30 & 0.2500 & 36.4042 & 50.213 & 6.4253 & 0.00626 \\ \hline
180 & 0.45 & 0.0013 & 25.4825 & 50.21 & 4.4976 & 0.00626 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Characteristics of scaffold samples and capillary action (methanol).  
Figure 10: Samples of scaffold internal structure; (**a**) fine capillary of 90 \(\upmu\)m and (**b**) thick capillary of 180 \(\upmu\)m.