## 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
Figure 9: Time dependences of the liquid front speed when a 10-\(\upmu\)L water droplet spreads over the surfaces of Cu-SiC composite materials processed by laser radiation when the surface wetting properties are stabilized. Liquid flow rate: (**a**)—5 \(\upmu\)L/min; (**b**)—100 \(\upmu\)L/min. front movement. In addition, in this case (100 \(\mu\)L/min) the DCAs are significantly higher than the DCAs at 5 \(\mu\)L/min. Another distinctive feature of the dynamic properties of a liquid on the surfaces of Cu-SiC composite materials processed by abrasive materials and laser radiation is the nature of the change in dynamic contact angles for advancing and receding liquid front. In contrast to constant DCAs in the stage of liquid supplying by a pump (I, Figure 6), DCAs increase significantly on surfaces modified by laser radiation in Stage I (Figures 10 and 11). At the stage of pumping out the liquid from the droplet formed on the surface, the DCAs sharply decrease until the droplet completely disappears both on surfaces processed by abrasive materials (II, Figure 6), and on surfaces modified by laser radiation (II, Figures 10 and 11). The values of the quasi-static advancing and receding DCAs and the CAH determined by them for the Cu-SiC samples modified by laser radiation are shown in Figure 12.  
It was found that under the conditions of SCA growth, the CAH is greater (Figure 12a,b) on laser-modified samples than on mechanically processed Cu-SiC samples (Figure 7). After wetting stabilization, the CAH of the laser-modified Cu-SiC surfaces has decreased and does not exceed 7\({}^{\circ}\). It is concluded that laser modification of the nanosecond duration of the Cu-SiC composite materials due to the developed multimodal roughness makes it possible to change the wetting properties in a wide range from the extreme state (superhydrophilicity) to the hydrophobic state. The mechanism of wetting inversion is similar to the inversion mechanism that occurs on the surfaces of metals modified by laser radiation. After processing Cu-SiC surfaces with laser radiation, the elemental composition of the surfaces becomes uniform. This eliminates one of the main disadvantages of Cu-SiC surfaces processed by abrasive materials. This, in turn, makes it possible to change the dynamic characteristics of the droplet spreading (at a flow rate of 5 \(\mu\)L/min, the liquid front speed is more than three times (Figures 8 and 9), and the dynamic contact angles are in the range of 30\({}^{\circ}\) (Figures 10 and 11)). These advantages and the small contact angle hysteresis make Cu-SiC suitable for use as a structural material for microfluidic devices, systems for separating and mixing micro- and nanoliter liquids. To develop practical recommendations for the development of research topics, it is planned to study the dynamic characteristics of known industrial methods of hydrophobization of Cu-SiC based on the chemisorption of a hydrophobic agent [4] which is devoid of the spontaneous hydrophobization disadvantages. Figure 11: Time dependences of the dynamic contact angles for a 10-\(\mu\)L water droplet spreading over the surfaces of Cu-SiC composite materials processed by laser radiation when the surface wetting properties are stabilized. Liquid flow rate: (**a**)—5 \(\mu\)L/min; (**b**)—100 \(\mu\)L/min. I is the liquid supplying (advancing DCAs); II is the liquid pumping out (receding DCAs).