# Progress on the Application of Nanomaterial Expansion in Oil Displacement
## 3 Nanomaterial Swelling Oil Displacement Mechanism
### The \"Rolling Up\" and \"Diffusion\" Phenomena  
The early nanofluids used for oil displacement were mainly surfactant solutions with nano-scale micelles [92; 93]. In 1988, [PERSON] et al. [94] first revealed the oil displacement mechanism of a surfactant micellar solution. They used a reflected light differential interference microscope to observe in detail the process of oil droplet separation from the solid surface under the action of a surfactant micelle solution. For the first time, they found two obvious contact lines in the three-phase contact zone formed on an oil droplet-surfactant micelle solution-solid surface during oil droplet separation. One of them was a conventional contact line, called the outer line; the other was the contact line formed by the diffusion of water and surfactant molecules between the oil phase and the solid phase, which was located in front of the conventional contact line, called the inner line. They believed that the surfactant inhibits the surface tension to cause the oil droplets to roll up and the water and the surfactant micelle solution to diffuse between the oil and solid phases in front of the contact line. \"Rolling-up\" and \"diffusion\" occur together in the dual oil displacement mechanism. At the same time, the decrease in the contact angle during the oil displacement process is due to the existence of the roll-up mechanism. The second contact line exists because of the role of the diffusion mechanism. The principle is shown in Figure 13, below. In the process of crude oil exploitation, the study of the oil droplet migration law has always been the core scientific problem in this field. At present, the two main displacement mechanisms have been identified as interfacial stripping migration and the diffusion mass transfer process. In the aqueous-phase displacement environment, the process of oil droplet detachment and migration is mainly controlled by the interfacial wetting characteristics [96]. When the oil droplets contact with the surface of the hydrophilic rock, the higher interface energy of the oil-water interface will hinder the oil droplets from leaving the rock surface. It is worth noting that the introduction of a nanofluid system can significantly improve the interfacial wetting characteristics: by regulating the concentration of nanoparticles, the contact angle of oil droplets can be effectively reduced (as shown in Figure 14), thereby promoting the detachment and migration of oil droplets from the rock surface and ultimately improving oil recovery [97]. Research on the diffusion mass transfer mechanism mainly focuses on the diffusion behavior of fluid in porous media during gas flooding. In the aqueous-phase environment, the mass transfer efficiency of gas in crude oil can be enhanced by diffusion. In particular, when there is initial water saturation in porous media, this mechanism will exhibit nonlinear characteristics due to the complex interaction between  
Figure 13: Schematic diagram of oil droplets under the action of the rolling mechanism [95]. multi-component displacement agents and crude oil. Studies have shown that nanofluids can further enhance the diffusion and mass transfer process by improving the stability and fluidity of crude oil systems [98]. There is a significant synergistic effect between the two displacement mechanisms [99]. For example, in a nanofluid system containing surfactants, the decrease in the contact angle can promote the detachment and migration of oil droplets, and the addition of nanofluids can accelerate the diffusion rate of the displacement fluid. The coupling effect of the two can significantly improve the overall displacement efficiency. According to the discussion in the Principle of Enhanced Oil Recovery by Nanomaterials section, we know that nanoparticles will have a directional arrangement at the oil-water interface due to the differences in the physical and chemical properties of the different parts.