## 3 Results and Discussions  
### Effect of Laser Fluence and Pulse Overlap on Microchannel Geometry  
Table 2 shows the design array with the corresponding performance measures of the microchannels fabricated by varying peak fluence and pulse overlap for 24 laser scans. Figure 3 shows the SEM image of the microchannel cross-sections with varying pulse overlap at the peak fluence of 169.76 J/cm\({}^{2}\). It was found that both peak fluence and pulse overlap had significant effects on the microchannel geometry. An increase in pulse overlap results in an increase in microchannel size and taper. The microchannels obtained at lower pulse overlaps were less deep and had minimum taper whereas at high pulse overlaps the microchannels were associated with high taper angle resulting in V-shaped channels as shown in Figure 3. In general, medium pulse overlap of 66.66% produced better channels. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multirow{2}{*}{**S.no**} & \multirow{2}{*}{ \begin{tabular}{c} **Peak Fluence** \\ **J/cm\({}^{2}\)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Pulse** \\ **Overlap \%** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Channel** \\ **Width \(\mu\)m** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Channel** \\ **Depth \%** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **TWE \(\mu\)m** \\ \end{tabular}  } & \multirow{2}{*}{\ \begin{tabular}{c} **BWE \(\mu\)m** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **D/S \(\mu\)m** \\ \end{tabular}  } \\ \hline
1 & 136.755 & 58.33 & 218.3 & 124.6 & 18.3 & 67.4 & 5.2 & 0.119 \\
2 & 169.765 & 58.33 & 227.5 & 194.4 & 27.5 & 74.2 & 8.1 & 0.189 \\
3 & 229.183 & 5  
Figure 4 shows the variation of microchannel geometry with increasing peak fluence at a fixed pulse overlap of 66.66%. It can be seen that low and medium peak fluence result in a flat surface at the bottom of the channel whereas higher peak fluence results in V-shaped microchannels. Both the laser peak fluence and pulse overlap have a significant effect on the channel width. The TWE increased with increase in peak fluence and the pulse overlap except for the 66.66% pulse overlap, which resulted in channels with TWE less than the lowest pulse overlap as shown in Figure 5. In general, low fluence and 66.66% pulse overlap resulted in microchannels with width closer to the target width. The increase in TWE with fluence and overlap is because at a higher fluence and overlap the energy density is higher which results in a high volume of molten material. This molten material is ejected out through the cross-section of the channel at high speed and erodes the top edge resulting in a greater width of the microchannel. Figure 6 shows the effect of peak fluence and pulse  
Figure 4: SEM images showing microchannel cross-sections with varying peak fluence at constant pulse overlap of 66.66%. Figure 3: SME images showing microchannel cross-sections with varying pulse overlap at laser fluence of 169.76 J/cm\({}^{2}\). overlap on the bottom width of the channel. BWE increases with increase in peak fluence and pulse overlap. At low peak fluence, the difference in BWE for both the pulse overlaps of 58.33% and 66.66% is minimum; however, this difference increases significantly with the increase in peak fluence. This is because at higher fluence levels, the depth of material removed is higher and as the depth increases, it makes the ejection of molten material difficult from the channel resulting in taper and lower bottom width. The pulse overlap of 75% resulted in V-shaped microchannels. This is because at higher overlap, the energy density is higher which results in a high volume of molten material and a high depth of cut. Moreover, once the molten metal has adhered to the wall of the channel, the laser loses the focus for the subsequent layers resulting in a V-shaped channel. As expected, the channel depth increases with increase in peak fluence and pulse overlap.