# Progress on the Application of Nanomaterial Expansion in Oil Displacement
## 3 Nanomaterial Swelling Oil Displacement Mechanism
### Pressure Oil Displacement by Structural Separation  
The research on wetting phenomena is generally based on the famous Young equation [81] and its modified model for rough surfaces [82]. However, the Young equation is based on the following assumptions: the shape of the droplet is spherical, the thickness of the liquid at the contact line between the droplet and the solid phase is 0, and the spherical contour of the droplet is always not deformed. However, experiments show that the wetting phenomenon of nanofluids on solid surfaces contradicts this assumption, making the Young equation no longer applicable to describe the wetting phenomenon of nanofluids on solid surfaces [83]. The concept of disjoining pressure was first proposed by [PERSON] in 1936 [84]. However, [PERSON] pointed out in a 2010 study that the naming of the term is somewhat misleading because it not only contains the separation effect but also involves the binding effect. In view of this, [PERSON] suggested the use of the term \"Derjaguin's pressure\", which can not only reflect the original contribution of the concept but also avoid the misunderstanding caused by the literal meaning of the term. However, considering the historical background of the term's long-term use in the academic field and the continuity of the literature, this paper still adopts the traditional name of \"separation pressure\". It is worth noting that a large number of scholars' studies have further confirmed the key role of separation pressure in the nanofluid displacement mechanism [85]. [PERSON] et al. [86] studied the effect of structural separation pressure on the diffusion rate of nanofluids by regulating the size of oil droplets and the content of nanofluids. It can be seen from the observations during the experiment that when the oil droplets on the surface of the glass sheet started to form, two obvious contact lines could be observed [87],  
Figure 10: Preparation process of amphiphilic Janus nanosheets [76]. as shown in Figure 11. One of them is defined as the outer contact line, and the other is defined as the outer contact line. This contact mainly refers to the contact between oil droplets, solids, and oil-water interfaces. The thickness of the interface film of the inner contact line will increase with the increase in time, which leads to an increase in the shrinkage rate of the inner contact line, which will eventually substantially increase the stripping efficiency of the oil droplets from the solid surface so as to improve the oil recovery efficiency. [PERSON] et al. [88] clarified the oil-washing efficiency of modified sheet nanofluids through oil sand-washing experiments. The experimental results show that under the condition of simulating the actual engineering environment, the oil film shrinkage rate of the solid interface is relatively slow, and there is no wedge area at this time. Under the same experimental conditions, when the modified nanofluids are added, the oil film at the solid interface will have a more obvious wedge-shaped area during the shrinkage process, and two more obvious contact lines will appear. At this time, the shrinkage rate of the outer contact line is 8.5817 \(\times\) 10\({}^{-5}\) cm/s, and the shrinkage rate of the inner contact line is 0.6617 \(\times\) 10\({}^{-5}\) cm/s. Through experimental verification, it can be concluded that the oil washing efficiency of the oil sands can be as high as 95.7% after adding the modified nanosheet fluid. [PERSON] et al. [89] observed the displacement of oil droplets by nanofluids through a reflection interference microscope and studied the diffusion law of hydrophilic nano-silica particles on a solid surface. The experimental results show that the displacement of oil droplets by ordinary salt water did not change the oil droplets, that is, the contact line did not shrink obviously. However, when nanofluids were used to displace oil droplets, the contact line contracted and the contraction rate was faster. That is to say, the wedge-film diffused under the action of structural pressure.