Coal Interface Modified by the Nanofluid: Insights from Dynamic Adsorption Wetting to Structural Weakening.

SiO2 nanofluid has significant potential for weakening coal mechanical properties and improving coal seam water injection efficiency, while prewetting time critically governs its modification outcomes. Accordingly, this study systematically investigated the effects of nanofluid prewetting time on coal mechanical performance, energy dissipation, and fracture propagation. Furthermore, Langmuir adsorption theory and Young's equation were employed to elucidate the fluid dynamic mechanism underlying time-dependent structural weakening of the modified coal. Results demonstrate that nanofluid prewetting treatment significantly reduces dynamic contact angles on coal surfaces and effectively enhances hydrophilicity. Nanofluid prewetting diminishes coal resistance to deformation, with the maximum reduction rate occurring at 2 h prewetting, corresponding to the lowest elastic modulus and peak stress alongside intensified lateral expansion. Post-treatment cumulative and maximum ring-down counts decrease substantially, indicating that internal cracks slide and propagate more readily during loading, thereby alleviating instantaneous elastic energy release upon failure. Moreover, fracture development reaches its maximum at 2 h prewetting, with failure mode transitioning from tensile-dominated to shear-dominated behavior. Nanoparticle adsorption on coal surfaces initially increases and subsequently stabilizes with prolonged prewetting time. However, prolonged prewetting time leads to aggregation and deposition of the nanofluid. The channels for water infiltration become narrowed. The enhancement of wetting modification is thereby restricted. In summary, prewetting time modulates nanoparticle adsorption and aggregation behavior, thereby influencing coal water absorption and structural weakening degree, ultimately determining the failure mode transition of the modified coal. The research findings provide foundations for optimizing prewetting time in coal seam water injection.