## 3 Results and Discussion
### Analysis of Droplet Spreading over Cu-SiC Surfaces
#### 3.3.2 Analysis of Droplet Spreading over Cu-SiC Surfaces Processed by Laser Radiation  
The experimentally obtained change in the liquid front speed in time during the advancing and receding movement of water droplet contact line over the surfaces of Cu-SiC composite materials modified by laser radiation when the surfaces demonstrate hydrophobic properties are shown in Figure 8, and when the surface properties are stabilized--in Figure 9.  
It was found (Figure 8a) that the liquid front speed on the surfaces of samples No. 2-6 is significantly lower than that recorded on samples No. 7 and No. 8. It was concluded that at a low liquid flow rate (no more than 5 mL/min), the liquid front speed on the hydrophobic modified surfaces of Cu-SiC composite materials depends on the wetting properties. The worse the wetting properties, the lower the movement speed is. It was found (Figure 9a) that the liquid front speed when the surface wetting properties are stabilized decreases compared to that measured before stabilization (Figure 8a), which is also associated with the deterioration of their wetting properties. It should be noted that the sequence of speed change from sample to sample remained the same, as in the conditions when the wetting properties continued to change (Figure 8a). At a flow rate of 100 mL/min (Figure 9b), the speeds change significantly. On the samples showing hydrophobic (\(\theta_{\text{No. 2}}=144.2^{\circ}\); \(\theta_{\text{No. 3}}=135.8^{\circ}\); \(\theta_{\text{No. 5}}=143.4^{\circ}\); \(\theta_{\text{No. 6}}=138.7^{\circ}\)) and close to superhydrophobic properties (\(\theta_{\text{No. 4}}=149.5^{\circ}\)), the speed increased significantly (the  
Figure 8: Time dependences of the liquid front speed when a 10-mL water droplet spreads over the surfaces of Cu-SiC composite materials processed by laser radiation when the surfaces demonstrate hydrophobic properties. Liquid flow rate: (a)—5 mL/min; (b)—100 mL/min. Figure 7: The values of quasi-static advancing and receding contact angles, contact angle hysteresis and static contact angles. Liquid flow rate: (a)—5 mL/min; (b)—100 mL/min. maximum increase by 37% on sample No. 2) compared to that obtained before stabilization of the wetting properties (Figure 8b). According to the analysis of the liquid front speed values (Figures 8 and 9) and dynamic contact angles (Figures 10 and 11), it was found that at low liquid flow rates (5 \(\upmu\)L/min), the degree of hydrophobization does not affect the liquid front movement. This is due to very low speeds at which the liquid freely penetrates into micro- and nano-sized cavities, displacing the air located there. Therefore, despite the hydrophobic and close to superhydrophobic properties of the samples, the dynamic contact angles recorded on their surfaces (No. 2-6) do not exceed 110\({}^{\circ}\) (Figure 10a) and 120\({}^{\circ}\) (Figure 11a) before and after stabilization of the SCAs, respectively. At high liquid flow rates (100 \(\upmu\)L/min), the flow rate is significant, the liquid does not have time to fill the cavities, so the liquid front moves along the \"air cushion\" formed in the irregularities. This explains the increase in the liquid movement speed after stabilization of the wetting properties, as well as the fact that on surfaces with worse wetting properties after stabilization (samples No. 2-6), the speeds are much higher than on surfaces of samples No. 7 and No. 8 (Figure 9b). It is concluded that the surface areas containing an \"air cushion\" in the irregularities of the surface texture reduce the resistance to the liquid  
Figure 10: Time dependences of the dynamic contact angles for a 10-\(\upmu\)L water droplet spreading over the surfaces of Cu-SiC composite materials processed by laser radiation when the surfaces demonstrate hydrophobic properties. Liquid flow rate: (**a**)—5 \(\upmu\)L/min; (**b**)—100 \(\upmu\)L/min. I is the liquid supplying (advancing DCAs); II is the liquid pumping out (reeding DCAs).