# Plenty of Room at the Bottom: Nanotechnology as Solution to an Old Issue in Enhanced Oil Recovery
## 2 Nanofluids
### Effect of Nanoparticles on Reservoir and Fluid Properties
#### 2.2.4 Effect on the Rock Wettability and Surface Wetting  
In order to change the wettability, nanoparticles need to be adsorbed onto the rock surface [32]. It has been suggested that the improvement in contact line motion may have two potential underlying mechanisms, affected by the nanoparticles' presence [54; 81; 82]. These could be either the pressure gradient created due to the solid-like ordering fluid \"wedge\" in the vicinity of the three-phase contact line (Figure 8, left), or the nanoparticle adsorption onto the solid surface, resulting in a reduction of the friction (Figure 8, right). Pioneering work on this topic was presented by [PERSON] [83; 84]. The wetting and spreading behavior of liquids over solid surfaces changes if these contain nanoparticles, surfactant micelles, globular proteins, or macromolecules. The process of spreading thin nanofluid films is driven by the structural disjoining pressure gradient. The disjoining pressure can be defined as the pressure originating when two surface layers reciprocally overlap [32; 82]. The film tension is high near the vertex, caused by the particles ordering in the wedge confinement. As the tension in the film increases towards the top of the wedge, it causes the nanofluid to spread at the wedge tip. This results in an enhanced dynamic spreading behavior (Figure 9). Figure 8: Schemes of the nanoparticle ordering in the wedge of a spreading meniscus (**left**), and the lubrication effect provoked by nanoparticle adsorption onto the solid (**right**). Figure adapted from Ref. [81]. The result of this is that nanoparticles exert a large pressure throughout the wedge film relative to the bulk solution, eventually separating both phases [32]. An analytical expression based on the Ornstein-Zernike equation is as follows [84]:  
\[\Pi_{st}(h)= \begin{cases}\Pi_{1}\cos\left(\omega h+\phi_{2}\right)e^{-\kappa h}+ \Pi_{2}e^{-\delta(h-d)}&for\,h\geq d,\\ -P&for\,0\leq h<d,\end{cases}  \tag{1}\]  
where \(d\) is the nanoparticle diameter and the other parameters (\(\Pi_{1},\phi_{2},\omega,\kappa\)) are fitted as cubic polynomials, functions of the nanoparticle's volume fraction. The term \(P\) is the nanofluid bulk osmotic pressure. The film-meniscus microscopic contact angle, \(\theta_{e}\), is related to the disjoining pressure by means of the Frumkin-Derjaguin equation [83; 84]:  
\[\Pi_{0}\left(h_{e}\right)h_{e}+\int_{h_{e}}^{\infty}\Pi(h)dh=\sigma_{o/nf}(\cos \theta_{e}-1)=S, \tag{2}\]  
where \(\sigma_{o/nf}\) is the IFT between oil and nanofluid, \(S\) is the spreading coefficient, \(h_{e}\) is the equilibrium thickness of a thin film, \(\Pi_{0}\) is represented by the sum of the droplet capillary and hydrostatic pressures, and \(\Pi\) is the disjoining pressure represented by the terms, \(\Pi=\Pi_{vw}\) (van der Waals) \(+\Pi_{d}\) (electrostatic or steric) \(+\Pi_{st}\) (long-range structural forces). In different experiments using reflected-light digital video microscopy techniques, the spreading dynamics (and its underlying mechanism) in liquids containing different latex and silicon nanoparticles was studied (Figures 9 and 10) [83; 85; 86; 87]. When an oil drop is surrounded by the nanofluid, the nanoparticles will concentrate and reorder around the drop, creating a wedge-like region between the surface and the oil drop (Figure 11). Figure 9: Nanoparticles in a wedge film. Scheme of the experimental set-up (**top**) and the nanoparticle distribution in a wedge film (**bottom**). Latex particles had diameter 1 \(\upmu\)m. Figure adapted (top) and reproduced (bottom) from Ref. [83]. This might have significant applications in EOR. The positioning of the nanoparticles inside the wedge is a consequence of the increase in the entropy of the overall dispersion caused by allowing a higher degree of freedom for the nanoparticles in the bulk liquid [82; 83].