## 3 Results and Discussion  
### Experimental Study of Wetting of the Pristine PVC Tubing  
All of the investigated extruded PVC tubing, namely pristine and plasma treated by different discharges demonstrated the partial wetting regime [35; 36; 37; 38; 39]. This means that the spreading parameter \(S\),  
\[S~{}=~{}\gamma_{SA}-(\gamma+\gamma_{SL}) \tag{1}\]  
governing the wetting regime was negative [37; 38; 39]. In Equation (1), \(\gamma~{}=~{}71.9\frac{\text{mJ}}{\text{m}^{2}}\), \(\gamma_{SA}\) and \(\gamma_{SL}\) are interfacial tensions at water/air, PVC/air, and PVC/water interfaces, respectively. The equilibrium \"as placed\" equilibrium water contact angle was established at the non-treated PVC as \(\theta~{}=~{}93\pm 0.5^{\circ}\). This value implies the useful physico-chemical insight. Indeed, according to the Young equation, defining the equilibrium (or Young) contact angle is given by Equation (2):  
\[cos\theta_{Y}~{}=~{}\frac{\gamma_{SA}-\gamma_{SL}}{\gamma} \tag{2}\]  
where \(\gamma_{SA},\gamma_{SL}\), and \(\gamma\) are the interfacial tension at the solid/air, solid/liquid, and liquid/vapor interfaces correspondingly. It is recognized from the experimental data that the equilibrium water contact angle for PVC is close to \(\theta\cong\theta_{Y}\cong\frac{\pi}{2}\). This means that \(\gamma_{SA}\cong\gamma_{SL}\) takes place for PVC and this is an important conclusion because the interfacial tension at solid/liquid interfaces is not a well-known physical value. Assuming \(\gamma_{SA}\cong 35\)-\(39~{}\frac{\text{mJ}}{\text{m}^{2}}\)[41; 42], we conclude that for PVC \(\gamma_{SL}\cong 35\)-\(39~{}\frac{\text{mJ}}{\text{m}^{2}}\) takes place. The establishment of the equilibrium contact angle enables the calculation of the specific energy of adhesion for the water/PVC pair according to the Young-Dupre formula:  
\[W\cong\gamma(1+cos\theta_{Y})\cong 70\pm 1\frac{\text{mJ}}{\text{m}^{2}}. \tag{3}\]  
The as-placed equilibrium contact angle does not exhaust the characterization of a solid surface's wettability. The advancing (maximal) \(\theta_{adv}\) and receding (minimal) \(\theta_{rec}\) contact angles should be established experimentally [35; 36; 37; 38; 39; 40]. Regrettably, the values of the advancing and receding contact angles depend strongly on the experimental technique used for their establishment [43; 44]. Thus, we carried the measurement in parallel with the well-known \"tilted plane\" and \"needle-syringe\" methods, illustrated with Figure 2  
Figure 3: Establishment of the contact angle hysteresis on the pristine PVC catheter is demonstrated. (**A**) Measurement of the advancing contact angle is shown. (**B**) Measurement of the receding contact angle is depicted. and 3. The maximal and minimal contact angles emerging from both of the techniques were \(\theta_{adv}~{}=~{}95\pm 1^{\circ}\) and \(\theta_{rec}~{}=~{}74\pm 1^{\circ}\). Thus, the contact angle hysteresis established for pristine PVC as high as \(\Delta\theta~{}=~{}\theta_{adv}-~{}\theta_{rec}\cong 21^{\circ}\) was established, which is typical for the polymer surfaces [40]. Measurement of the contact angle hysteresis enabled the calculation of the solid/air surface tension (free surface energy) for PVC according to the Chibowski approach, expressed by Equation (4) and discussed in detail in [45]:  
\[\gamma_{SA}~{}=~{}\gamma(cos\theta_{rec}-cos\theta_{adv})\frac{(1+cos\theta_{ adv})^{2}}{(1+cos\theta_{rec})^{2}-(1+cos\theta_{adv})^{2}}. \tag{4}\]  
A substitution of the aforementioned experimental values of \(\theta_{adv}\) and \(\theta_{rec}\) yielded the estimation \(\gamma_{SA}\cong 27.5\pm 0.5\frac{\text{m}}{\text{m}^{2}}\), which is lower, however, comparable with the \(\gamma_{SA}\cong 35\)-\(39\)\(\frac{\text{m}}{\text{m}^{2}}\) reported in [41; 42]. Moreover, it practically coincides with the value of the surface energy of the extruded PVC reported recently in [46].