## 2 Materials and Methods
### Methods
#### 2.2.3 Characterization Methods  
Smoothness of both paper substrates was measured using PTI line Bekk tester (PTI Austria GmbH, Laakirchen, Austria) on 10 samples for each paper substrate (5 on felt and 5 on wire side of the paper), following ISO 5627 standard. Caliper was determined with a micrometre DGTB001 Thickness Gauge (Enrico Toniolo S.r.l., Milano, Italy) and was measured on 20 samples for each paper substrate according to ISO 534:2011. Surface roughness of substrates and applied printing inks were measured in order to define the influence of roughness on the characteristics of observed ink systems. According to [31], substrates with different surface roughness play a crucial role in determining the optical properties of deposited materials, especially in the area of functional coatings. Considering the fact that two types of substrates were used in this work, the assumption was that their surface structures might affect the physical-chemical interactions between the materials and the visual response of the observed hybrid systems. Research published previously [32; 33] report on the influence of rough surface on surface free energy (SFE) and the interactions between the substances in contact. The profiling methods and roughness parameters were defined by international standards (ISO 11562, DIN 4777, DIN 4762). Three basic roughness parameters were measured: \(R_{a}\)-the arithmetic mean deviation of the profile (ISO 4287); \(R_{z}\) (ISO)-mean height of unevenness in ten points, numerically the difference in mean height between the five highest peaks and the five lowest peaks within the reference length (ISO 4287); \(R_{max}\)-maximum roughness depth is the largest single roughness depth within the evaluation length (ISO 4287). The roughness instrument MarSurf PS 10 (Mahr GmbH, Gottingen, Germany) with the stylus method was used. The diameter of a stylus was 2 \(\upmu\)m and measuring force was 0.00075 N. Measurement was performed ten times in two directions (in the fibre direction and in the opposite direction), on each sample and the results of a mean value were presented. Printed ink thickness was measured by means of a SaluTron D4-Fe device (Frechen, Germany). The SaluTron D4-Fe works on the magnetic induction principle and measures all nonmagnetic surfaces. The results of the ink thickness were used to identify the possible influence of the thickness on the visual response of printed inks. Surface free energy (SFE) and contact angles on samples were analysed using the Data Physics OCA 30 goniometer (DataPhysics Instruments GmbH, Filderstadt, Germany). Three referent liquids of known surface tension (demineralized water, diiodomethane, and glycerol) were applied on the substrates without prints and on the printed ink layers. Contact angles were measured by Sessile drop method, the shape of the drop was a spherical cap, and the volume of the drop was 1 \(\mu\)L. Ten drops of each liquid were applied on each sample and the mean value was calculated. All measurements of the contact angles were performed at 0.4 s after the drop had touched the measured substrate. Surface tension of the referent liquids and their contact angles were used to calculate SFE (\(\gamma\)) and its dispersive (\(\gamma^{d}\)) and polar (\(\gamma^{p}\)) components on all samples by using the Owens, Wendt, Rabel, and Kaelble (OWRK) method [34; 35; 36; 37; 38]. Total, dispersive, and polar surface tension components of probe liquids expressed in mJ/m\({}^{2}\) were respectively: diiodomethane--50.8, 50.8, and 0; glycerol--64.0, 34.0, and 30.0; and water--72.8, 21.8, and 51.0. Calculation of SFE provides the insight into the important surface properties of the printed layers and substrate, and enable the characterization of the material interactions [39]. From the obtained SFE adhesion, the parameters (surface free energy of the interphase (\(\gamma_{12}\)), work of adhesion (\(W_{12}\)), and wetting coefficient (\(S_{12}\)) were calculated.