## 3 Results and Discussion
### Surface Free Energy and Adhesion Parameters  
Results of the surface free energy (SFE) calculations obtained from contact angle measurements (Table S1) are presented in Figure 3a,b. It is visible that both paper substrates have a dominant dispersive component of SFE, with CB paper having a more expressed polar component of SFE than UT paper. When TLC printing ink was printed on the substrates, the polarity of the new surface (and total SFE) increased. Compared to the TLC printing ink, the printed layer of UVF ink on both papers displayed the dominance of the dispersive component of SFE, with the polar component being negligible. Therefore, it can be concluded that the uniform layer of UVF ink with complete coverage was printed on the CB and UT substrates. Interesting results of SFE calculations can be observed when hybrid ink systems are printed on two different papers. UVFp in TLC printed on CB paper displays the highest SFE values among the hybrid ink surfaces (48.65 mJ/m\({}^{2}\)). This can be attributed to the highest measured roughness parameters (Table 4). The results of SFE calculations are in the accordance with SFE values of TLC ink printed on papers, as well as with the hydrophobicity of the UVF ink [26; 30]. In general, surfaces of hybrid ink systems printed on CB paper presented higher SFE than prints on UT paper. Therefore, adhesion parameters are of importance for the assessment of the interactions between different papers and between the printed ink layers used in this research. Results of the calculated adhesion parameters are presented in Table 5. When assessing the strength of the interaction, all three parameters should be considered--work of adhesion (\(W_{12}\)) should be as high as possible, interfacial tension (\(\gamma_{12}\)) should be close to zero, and wetting coefficient (\(S_{12}\)) should be positive or equal to zero. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Samples** & \(R_{a}\) (\(\upmu\)m) & **SD** & \(R_{2}\) (\(\upmu\)m) & **SD** & \(R_{max}\) (\(\upmu\)m) & **SD** \\ \hline TLC/UT & 2.19 & 0.12 & 12.31 & 1.12 & 15.54 & 2.12 \\ UVF/UT & 1.89 & 0.23 & 10.71 & 1.08 & 15.23 & 2.51 \\ UVF on TLC/UT & 2.24 & 0.19 & 12.90 & 1.56 & 19.21 & 5.03 \\ UVFp in TLC/UT & 1.98 & 0.16 & 11.37 & 1.07 & 15.05 & 5.79 \\ TLC/CB & 5.45 & 0.40 & 31.03 & 3.49 & 39.13 & 5.53 \\ UVF/CB & 3.94 & 0.64 & 21.64 & 4.05 & 28.03 & 4.48 \\ UVF on TLC/CB & 3.98 & 0.35 & 22.13 & 2.45 & 30.09 & 4.62 \\ UVFp in TLC/CB & 4.88 & 0.70 & 25.98 & 3.30 & 32.87 & 4.91 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Roughness parameters measured on prints. Observing Table 5, it can be concluded that the adhesion between the papers and the layer of TLC ink is not optimal because of the high interfacial tension and negative values of wetting coefficients. Higher work of adhesion--work that is necessary to separate two layers--was achieved between CB and TLC than between UT and TLC (93.96 vs. 77.72 mJ/m\({}^{2}\)). Furthermore, when observing all three adhesion parameters for the samples with printed UVF ink layer, it can be concluded that the adhesion of UVF ink on CB paper was better than on UT paper in terms of the higher W\({}_{12}\) (74.23 vs. 62.72 mJ/m\({}^{2}\)), and the wetting coefficient was lower than for UVF ink on UT (7.37 vs. 18.23 mJ/m\({}^{2}\)), pointing to the better wetting of UVF ink on CB substrate. Interfacial tension is lower between UT paper and UVF ink, than between CB paper and UVF ink. In hybrid ink systems (UVFp in TLC, and UVF on TLC), the highest work of adhesion was present between CB paper and UVFp in TLC (92.4 mJ/m\({}^{2}\)), while the lowest work of adhesion was displayed between UT paper and UVFp in TLC (55.87 mJ/m\({}^{2}\)). Interfacial tension was positive for all samples, but the values were not close to zero. This points to the conclusion that the surfaces in contact have a tendency to separate. Highest interfacial tension was measured between TLC and UVF on CB substrate (14.38 mJ/cm\({}^{2}\)), and lowest between UVF ink and UT substrate (4.52 mJ/cm\({}^{2}\)).