# Progress on the Application of Nanomaterial Expansion in Oil Displacement
## 2 Principle of Enhanced Oil Recovery by Nanomaterials
### Reduce Oil-Water Interfacial Tension
Studies demonstrate that silica nanoparticles (ATNPs) and the nonionic surfactant lauroyl monoethanolamide (LMEA) co-assemble at the oil-water interface via hydrogen bonding to construct an elastic adsorbed layer. This stabilizes the formation of a thermodynamically unstable but kinetically stable Pickering emulsion, achieving in situ mobility control over a wide water-oil ratio (WOR) range. The surfactant LMEA preferentially adsorbs at the oil-water interface, reducing interfacial tension to the magnitude of \(10^{-2}\) mN/m. ATNPs enhance the strength of the interfacial film through a physical barrier, inhibiting emulsion coalescence and Ostwald ripening. Their synergy enables the emulsion viscosity to self-adjust with WOR, maintaining a value higher than the crude oil viscosity. Furthermore, ATNPs and LMEA synergistically  
Figure 3: Schematic of the high-pressure–high-temperature pendant drop interfacial tension measurement apparatus [40]. modify the rock surface through physical adsorption, transforming oil-wet rock to weakly water-wet. This reduces oil film adsorption energy, enhances the capillary number, and synergistically displaces residual oil via the \"Jamin effect\" and interfacial effects. Essentially, nanoparticles form a nanolayer by adsorbing at the oil-water interface. This nanolayer replaces the pure oil-water interface, alters the properties of the oil-water interface, and thus reduces the interfacial tension between oil and water [41]. When surfactants are added to the nanofluid, the ability of the nanofluid to reduce the interfacial tension between oil and water is further enhanced. [PERSON] et al. [42] focused on the performance of silica nanofluid with and without sodium dodecyl sulfate (SDS). By measuring the interfacial tension (IFT) between the fluid and paraffin-based crude oil, it was found that the nanofluid containing SDS had a stronger ability to reduce IFT compared with the single nanofluid, with a reduction of 43.02%. This study shows that nanofluids containing surfactants have more promising application potential in the field of enhanced oil recovery. [PERSON] et al. [42] formed water-soluble active silica nanoparticles by condensation of adipic acid with hydroxyl groups of silica particles, the introduction of carboxyl groups on the surface of the particles, and neutralization of carboxylates by NaOH. The authors studied the physical and chemical properties of active silica nanoparticles by transmission electron microscopy and Fourier transform infrared spectroscopy. The interfacial activity was confirmed by the determination of interfacial tension and interfacial expansion moduli. As shown in Figure 4 below, the authors demonstrated the influence of silica nanofluids and active silica nanofluids at different concentrations on the oil-water interfacial tension. The oil-water interfacial tension was measured to be 26.8 mN/m at 60 \({}^{\circ}\)C. When 1 wt% silica nanofluid was added, the interfacial tension decreased to 20.6 mN/m, with a decrease of about 23.13%. When 1 wt% active silica nanofluid was added, the interfacial tension was greatly reduced to 7.0 mN/m, a decrease of 73.8%. This shows that the surface modification of active nanoparticles gives them higher interfacial adsorption efficiency, and the interfacial tension can be reduced more effectively at the same concentration. It can be seen that nanofluids can reduce interfacial tension, and active nanofluids have a more significant effect in terms of reducing interfacial tension. [PERSON] et al. [44] proposed an ultralow-concentration nanofluid based on graphene-based amphiphilic Janus nanosheets to improve the efficiency of secondary oil recovery through the interface regulation mechanism. Experiments show that at a concentration of 0.005 wt%,  
Figure 4: Effects of different nanofluids at different concentrations on oil–water interfacial tension at 60 \({}^{\circ}\)C [43].