## 2 Materials and Methods
### Theoretical Aspects
\[\gamma_{SV}=\gamma_{SL}+\gamma_{LV}cos\theta \tag{5}\]  
\[cos\theta^{\prime}=vcos\theta \tag{6}\]\[cos\theta=f_{1}cos\theta_{1}+f_{2}cos\theta_{2} \tag{7}\]  
where: \(\gamma\)SV, \(\gamma\)SL, \(\gamma\)LV are the free energies at the interface per unit area, at solid-gas, solid-liquid and liquid-gas contact, respectively; \(\theta\) is the contact angle; \(v\) represents the roughness as ratio between the real surface and the apparent surface; f\({}_{1}\) and f\({}_{2}\) are the Phase 1 and Phase 2 fractions in the heterogeneous surface; and \(\theta_{1}\) and \(\theta_{2}\) are the contact angles that would be recorded for homogeneous surfaces in Phase 1 and Phase 2, respectively. A hydrophilic surface that is more water-friendly has the following characteristics:  
* Water droplets spread over the surface and so, instead of being water-free zones and areas in contact with water, a continuous film of water is formed. In normal conditions under the action of gravity, some drops fall, roll or drip onto the surface, removing dirt particles; other drops, smaller and lighter, do not end up falling, remaining on the surface and eventually evaporating. A continuous film of water is heavier, so it will certainly slide on the surface under the action of gravitational force, taking over all the dirt particles encountered. Therefore, it is advantageous to determine the formation of a continuous film of water on the surface, the case of hydrophilic surfaces, because there is an obvious increase in the ability to clean them. * In contact with a hydrophilic surface, the water drop flattens (decreases the contact angle), thus the water-surface interface area is larger. By default, the potential for water absorption in the pores of the surface is higher; therefore, the absorption of water to the surface is an indirect indicator of the hydrophilicity degree. * On the other hand, and depending on the roughness of the surface, the formation of the water film differs greatly. On one surface there may be simultaneously several phenomena occurring: filling the open pores with water, increasing the thickness of the water film, and water film sliding under the influence of the resultant gravitational forces and friction. Although initially a matching among different other materials (brick, ceramic, natural stone, marble) was intended [42; 43], the literature shows that these situations are not comparable [1] because it is virtually impossible to compare the phenomena occurring on the surfaces of different materials. Consequently, cementitious composite material surfaces with TiO\({}_{2}\) nanoparticles were proven to have the ability to reduce the angle of contact from the usual 20-30\({}^{\circ}\) to almost 0\({}^{\circ}\), which is considered as representative of an ideal, superhydrophilic surface [38]. According to the literature [38; 44], an angle of contact that tends to 0\({}^{\circ}\) indicates a superhydrophilic surface. Figure 2: Schematic representation of the surface photoactivation with the change in the number of hydroxyl groups under the action of UV rays and, consequently, the induction of the two mechanisms that contribute to the self-cleaning capacity. Whether it is the assessment of the hydrophilicity or hydrophobicity (as an opposite phenomenon) of the surfaces, the point-shift of the literature indicates that, in principle, the two methods of measurement are: the direct method of measuring the contact angle of the droplet of liquid to the surface [1; 43; 45], and the indirect method for the determination of water absorption on the surface [1; 37; 43]. The specific cause of the high wettability (photochemical induced) will lead to a high potential of water adsorption on the surface. Some references [46] also indicate a method that describes the dynamic wettability of surfaces by analyzing and graphically representing the variation of the contact angle of the comparatively analyzed control sample.