## 2 Results and Discussion
### Functionalities of the thin hydrogel layer  
The polarity and the homogeneity of cross-linked copolymer coatings motivated us to further probe the ability of such layers to maintain the hydrophilic properties of SCG in presence of hydrophobic siloxane residues. Therefore solution of 2 wt % solid content were formulated and sprayed on activated SCG. To ensure that a water insoluble cross-linked polymer layer is formed the coated SCG was exposed for 2 hours to UV irradiation. Such treatment yielded in an optically homogenous hydrophilic surface with a water contact angle less than 20\({}^{\circ}\). Subsequently two types of sealants were applied on the edge of the coated surface and the contact angle of a water droplet was measured as function of time. The resulting contact angles are plotted _versus_ time (Figure 3; the origin denotes the time at which the silicon resin was applied). While the uncoated SCG becomes hydrophobic within a few hours after deposition of the sealant, the coated SCG preserves the initial hydrophilicity of the surface. Ageing of the sealant under ambient light and room temperature does not influence the results; the low contact angle of the coating on SCG persists over 20 days, whereas, the water contact angle of the uncoated SCG was constantly increasing with time which is consistent with our previous observations. Predominantly near the sealant within the first hour, the hydrophobic residues spread very rapidly to reach up to 10 cm after 20 hours. The performance of the protective copolymer layer in preventing the contamination of SCG surface by the hydrophobic siloxane residues has been demonstrated to last over a month with contact angle values that do not exceed 20\({}^{\circ}\). It should be noted that this duration is longer compared to the curing time of the silicon resin. To understand this result we considered the spreading parameter of siloxane residues on the two types of interface: Titanium dioxide/air and copolymer/air. Figure 2: Film thickness as a function of polymer concentration used and processing conditions applied. **(a)** Film prepared by spin coating; film thickness measured by ellipsometry (1). **(b)** Film prepared by spraying and thickness estimated from the sprayed volume assuming that the polymer density is 1 g/cm\({}^{3}\) (2). It is well know that when a liquid drop is deposited on a solid, three interfaces come into play, and the three corresponding interfacial tensions: the solid-air interfacial tensions (\(\gamma_{\rm SA}\)), the solid-liquid interfacial tensions (\(\gamma_{\rm SL}\)), and the liquid-air interfacial tensions (\(\gamma_{\rm LA}\)), respectively. These interfacial tensions measure the free energy (per unit area) associated with an increase in the corresponding interface [24]. The spreading parameter \(\mathbf{S}=\gamma_{\rm SA^{-}}\gamma_{\rm SL^{-}}\gamma_{\rm LA}\) is the energy gained when covering one unit area of the dry solid with a flat liquid film of macroscopic thickness. If \(\mathbf{S}\) is negative, the situation where the solid is covered by a liquid film is not favorable. The equilibrium shape of the drop is a spherical cap, characterized by its equilibrium contact angle \(\theta\), the solution of [PERSON]'s equation \(\cos\theta=\left(\gamma_{\rm SA^{-}}\gamma_{\rm SL}\right)/\gamma_{\rm LA}\)[25]. If S is positive, the liquid spreads and tends to cover the maximum solid surface. Thus, S = 0 appears as criterion to distinguish between partial and total wetting [26]. Yet, if we consider the surface energy of the titanium dioxide as \(\gamma_{\rm TiO_{2}air}\sim\) 280-380 mN/m [27-29] and assuming that the surface tension of the siloxane residue to be similar to polydimethylsiloxane PDMS \(\gamma_{\rm Siloxane/air}\sim\) 18-20 mN/m, we conclude that the spreading parameter must be positive since the interfacial energy \(\gamma_{\rm TiO_{2}Siloxane}\) cannot be larger than few tens of mN/m [29].