## 5 Mechanism
### Wettability Alteration  
Wettability is defined as the tendency of a certain fluid to spread on the solid surface in the presence of other immiscible fluids in the same system [90]. It is directly related to the fluid-fluid and fluid-solid interactions, which indicate the interfacial energy between two immiscible phases [45]. In general, wettability is a rock property that depends on the type of minerals, pore distribution, and surface area. It is also a function of fluid composition and temperature [90].  
Several studies have revealed that wettability is an important factor for understanding the multiphase flow during hydrocarbon accumulation until production and achieving the highest oil recovery [159; 163]. Since wettability significantly affects capillary pressure and relative permeability, the oil flow inside the porous media can be improved considerably [45]. It also leads to the spontaneous imbibition of water that helps to push the hydrocarbon out of the matrix blocks during water flooding [123]. Surfactant flooding is a well-known method for altering wettability: However, using surfactants on a field scale is not very economical [164]. Therefore, many researchers have proposed nanoparticles as an alternative to wettability alteration agents.  
Nanoparticles and surfactants work by similar mechanisms on wettability alteration [123]. [PERSON] and [PERSON] [165] proposed the two most possible mechanisms: adsorption on the solid surface (coating mechanism) and removal of the absorbed molecule from the rock surface (cleaning mechanism). The adsorption process of NPs on the grain surface results in the formation of a composite nanostructure-surface which improves the water-wetting behavior [136]. SEM images show the adsorption of silica NPs on a calcite surface, which forms a layer of the nanostructure with some agglomerated particles after one hour of nanofluid treatment [166]. The surface modification by nanoparticles also increases the roughness of the surface, as can be seen in Figure 3[166], [PERSON] et al. [136] added that the main factors in wettability alteration are the area fractions of the nanostructure, partition coefficient, and surface roughness.  
Moreover, [PERSON] et al. [152] investigated the effect of LHP silica NPs on the wettability alteration of a polished-synthetic-silica surface in different concentrations. As shown in Figure 4, increasing concentrations of NPs will reduce the contact angle of crude oil and change the wettability  
Figure 3: SEM and atomic force microscopy image of calcite surface. Calcite surface before (**A**) and after (**B**) nanofluid treatment; (**C**) High resolution; (**D**) Max resolution; Topography picture before (**E**) and after (**F**) nanofluid treatment. Reproduced with permission from reference [167]. Copyright 2016 Elsevier.  
toward more water-wet. They concluded that a smaller size of NPs tends to decrease the contact angle more than larger size particles due to the higher electrostatic repulsion of smaller sizes. Moreover, wettability alteration is a function of salinity, ionic composition, initial wettability, a solid system, and exposure time [167]. There is also evidence of NPs altering wettability in both carbonate [168] and sandstone rocks [4].