Nanomechanical behaviors of nanoparticles at oil/water/rock interfaces and their roles in macroscopic interfacial regulation

Nanofluid has emerged as an effective enhanced oil recovery method through its capacity to regulate interfacial properties among rock and pore fluids. Microscopic interactions between nanoparticles and the ambient environment determine the adsorption behavior of nanoparticles, which, in turn, influences their ability to regulate rock wettability and oil–water interfacial tension (IFT). This study uses atomic force microscopy (AFM) to quantify nanomechanical forces between SiO2 nanoparticles and quartz substrates, and optical tweezers (mmi CellManipulator, MM1) are employed to probe interparticle forces at oil–water interfaces. AFM data are fitted using a mathematical model integrating steric repulsion, hydrophobic attraction, and Derjaguin–Landau–Verwey–Overbeek theory. The results show that as NaCl concentration increases from 0.01 to 1.0 mol L−1, the electrostatic repulsion decreases from 24.3 to 13.1 nN, and its range shrinks from 40 nm to approximately 10 nm. Consistent with this reduction in repulsion, increasing salinity shifts SiO2-quartz interactions toward stronger attraction, with a log –log linear correlation linking nanoparticle adhesion energy to contact angle changes. For oil–water interfaces, a mathematical model innovatively considering asymmetric electrical double-layer and dipole–dipole interactions is proposed to describe the contribution of different microscopic forces on the variation of interfacial properties. The model demonstrates that dipole–dipole forces generate stronger and longer-range repulsion (up to 200 nm) than classical double-layer effects (<15 nm). A log –log linear relationship is also observed between nanoparticle coverage and IFT changes. These results elucidate the microscopic mechanisms governing nanoparticle behaviors at multiphase interfaces, providing theoretical guidance for tailoring nanofluid formulations to specific reservoir conditions.