Green-Synthesized Heterointerfacial ZnO–CuO Nanofluids for Regulating Oil–Water–Solid Interfacial Interactions in Low-Permeability Porous Media

Abstract Colloidal nanoparticles provide a platform for regulating multiphase transport in confined porous media, where interfacial forces become dominant as pore dimensions decrease. Herein, ZnO–CuO composite nanoparticles (ZC NPs) were synthesized via a green coprecipitation route mediated by Cinnamomum camphora root extract. This design integrates green synthesis, ZnO–CuO heterointerface engineering, and multiscale evaluation in low-permeability porous media. HRTEM and FFT analyses revealed direct contact between CuO (111) and ZnO (101) lattice domains, while XRD and opposite shifts in the Zn 2p and Cu 2p binding energies confirmed the coexistence of the two crystalline phases and interfacial charge redistribution, supporting the formation of a closely coupled heterointerface. The resulting ZC nanofluid exhibited a ζ-potential of −36.3 mV and favorable dispersion stability. At 0.05 wt %, ZC reduced the contact angle of oil-aged quartz from 123.96° to 37.67°, compared with 59.69° and 82.33° for ZnO and CuO, respectively. At 0.1 wt %, ZC reduced the oil–water interfacial tension from 84.36 to 21.16 mN/m. Although the minimum interfacial tension occurred at the higher concentration, 0.05 wt % produced the greatest spontaneous imbibition enhancement. Spontaneous imbibition with 0.05 wt % ZC increased oil recovery by 13.08 and 15.19 percentage points in 4 and 30 mD cores, respectively, exceeding the corresponding improvements obtained with the single oxides. Microfluidic visualization revealed broader pore-space access and reduced clustered residual oil. These results establish a structure–interface–performance relationship for green-synthesized ZnO–CuO heterointerfacial nanofluids and demonstrate their potential for interfacial regulation and oil mobilization in low-permeability porous media.