## 4 Discussion  
To achieve a strong and durable adhesion between the old and new composite surfaces, many factors play important roles, like the chemical composition of the interface, surface morphology, and, in association with these parameters, the wettability of polymerized composite surfaces by dental adhesive. The latter is influenced by the surface roughness, surface chemistry, and surface free energy of the composite. In this study, the surface free energy of one nanohybrid (TEC) and two bulk-fill resin-based composites (SDR and TECBF) was measured after the different surface treatments (polishing and sandblasting). The surface roughness measurements of all composite surfaces revealed that the sandblasting significantly increased the roughness (Ra). Among the polished composites, the SDR presented significantly higher Ra data compared to TECBF and TEC (Figure 2). The explanation is the different filler size and their distribution in the SDR composite, which was confirmed by SEM micrographs (Figure 3). The larger filler particles in resin composite increased the inherent heterogeneity that resulted in increased roughness [25; 26]. Large interfaces between particles enhance the tendency of debonding of the filler particles, and the resin matrix is exposed to a higher level of abrasion [27]. The TECBF and TEC contain closely packed smaller-sized (at nanometer scale) filler particles that prevent the resin matrix from abrasion, resulting in lower Ra data on polished surfaces [19; 27]. On the sandblasted surface, the three composites did not show significant differences in terms of Ra. During the sandblasting, the high kinetic energy aluminum oxide particles hit the surface and create multiple valleys according to the resistance of the material surface. The water and DIM contact angles displayed significant differences on polished and sandblasted composite surfaces. The water as polar liquid presented higher contact angles on composite surfaces than DIM apolar liquid. The relatively high water contact angle and low DIM contact angle indicated that the resin composites had a hydrophobic nature. According to the previous publication, materials with contact angles above 65\({}^{\circ}\) can be considered highly hydrophobic, reflected in surface energy values above 30 mJ/m\({}^{-2}\)[28]. The wettability of water and DIM displayed a change in the opposite direction as a result of surface roughening. The inherently bad wetting water did not spread well on rough surfaces until the good wetting DIM exhibited lower contact angles. The changes in the water contact angle are partially compatible with previous findings, which stated that the water contact angle increased as the surface became rougher in the case of a hydrophobic surface [14]. The water contact angles were significantly lower on the SDR surface than on TECBF or TEC. In the aspect of water contact angle, the three composites behaved differently due to their different chemical composition. The small surface fraction that was contaminated by 5 \(\upmu\)L the water drop contained a hydrophobic resin matrix and hydrophilic filler particles. The filler meant a better wettability for a water drop because of their higher surface free energy, which is the result of strong bonds in the bulk material. Until the resin was less wet by water because the polymer represents a small surface free energy material [27; 29]. In the case of SDR, larger resin interfaces were found between the large filler particles. These two factors strongly influence the wetting ability of water on the SDR surface [27]. Large filler particles with high surface free energy and hydrophilic character were more wettable by water, resulting in a lower contact angle on SDR than TECBF and TEC. In the case of good wetting liquid, such as apolar DIM, the surface roughness decreased its contact angle. The rough surface has a larger total surface area to offer a larger wetting area, and the wettability of the surface will increase. The SFE is important in that high SFE is desirable when adhesion is required.