## 3 Results and Discussion
### Analysis of Droplet Spreading over Cu-SiC Surfaces  
#### 3.3.1 Analysis of Droplet Spreading over Cu-SiC Surfaces Processed by Abrasive Materials  
Figure 5 presents the change in the speed of the liquid front (three-phase contact line) in the time that a 10-\(\mu\)L water droplet spreads over the surfaces of Cu-SiC composite materials processed by abrasive materials. Positive values in Figure 5 correspond to the advancing liquid front (supplying liquid by a pump), negative values--receding liquid front (pumping out liquid by a pump). At the initial moment of time (0-2 s), it is technically impossible to record the values of the liquid front speed, so the dependencies do not start from zero (along the X-axis). It can be seen from Figure 5 that the liquid front speed at the initial moment of spreading increases. After a slight increase, the speed decreases monotonically. The operating time of the pump under supplying conditions at a flow rate of 5 \(\mu\)L/min was 120 s and at 100 \(\mu\)L/min, 5 s. The liquid supplying mode of the pump was replaced by  
Figure 5: Time dependences of the liquid front speed when a 10-\(\mu\)L water droplet spreads over the surfaces of Cu-SiC composite materials processed by abrasive materials. Liquid flow rate: (a)—5 \(\mu\)L/min; (b)—100 \(\mu\)L/min. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample** & **No. 2** & **No. 3** & **No. 4** & **No. 5** & **No. 6** & **No. 7** & **No. 8** \\ \hline \(\theta_{\text{steady}}\), \({}^{\circ}\) & 144.2 & 135.8 & 149.5 & 143.4 & 138.7 & 98.2 & 120.9 \\ \(a\), day & 21.4 & 16.6 & 15.9 & 17.1 & 15.5 & 21.8 & 21.8 \\ \(t\), day & 53 & 53 & 53 & 53 & 53 & 73 & 73 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Fitting coefficients and times of wetting stabilization of Cu-SiC samples modified by laser radiation. the liquid pumping out mode. When the pump stops in the supplying mode, the liquid front continues to move due to the action of inertia forces. Next, the pinning of the three-phase contact line occurs. It should be noted that during pinning, the pump is already operating in the liquid pumping out mode. The mechanism of the three-phase contact line pinning with a description of the forces acting on the droplet, as well as the causes due to roughness (asperities are energy barriers to the liquid front movement) are well studied in [20]. When the balance of forces is disturbed, the liquid front begins to move toward the previously wetted surface (the speeds are shown as conventionally negative in Figure 5) and the droplet contact diameter and height decrease. Figure 5 shows that the speeds of advancing liquid front (positive values) decrease on the samples in the order of increasing static contact angles: No. 8--No. 6--No. 7--No. 4--No. 3--No. 2--No. 5. It was concluded that the more hydrophilic the surface, the higher the speed of the liquid front. Based on the analysis of the liquid front speeds in the pumping out mode (negative values) (Figure 5), it was concluded that the surface wetting properties do not affect the liquid front speed over the pre-wetted surface. Figure 6 shows the time dependences of the dynamic contact angles obtained for water droplet spreading over the surfaces of Cu-SiC composite materials processed by abrasive materials. The advancing DCAs (I, Figure 6) registered when liquid supplied by the pump remains constant in time when the front of a liquid droplet moves over a previously unwetted surface. When switching the operating mode of the pump to pumping out liquid from a droplet formed on the surface (II, Figure 6), the receding DCAs decrease rapidly until the droplet is completely removed from the surface (for a flow rate of 5 mL/min). At a flow rate of 100 mL/min, the receding DCAs at Stage II remain constant for some time (2-3 s), despite a decrease in the droplet volume. Further DCAs also decrease. From Figure 7, it can be seen that SCAs lie in the range of \(\theta_{0\text{A}}\)-\(\theta_{0\text{R}}\).