Microscopic interactions and wetting behavior at the micro/nano-bubble water-coal interface: experiments and molecular simulations

Coal seam water injection is a key technology for pre-wetting and dust suppression prior to mining, with its effectiveness largely dependent on the wetting capability of water on coal. As a novel wetting medium, micro-nano bubble water (MNBW) has drawn increasing attention due to the close relationship between its wettability and preparation parameters, though the underlying mechanisms remain unclear. To address this, physicochemical characterization, contact experiments, and molecular simulations were carried out. Results showed that increasing the gas flow rate during MNBW preparation enhanced its properties: scattered light intensity, interfacial potential, and dissolved oxygen content increased, while surface tension and bubble size decreased, indicating improved system stability. The contact angle on the coal surface decreased accordingly and was positively correlated with dissolved oxygen and interfacial potential but negatively correlated with bubble size. This suggests that higher gas flow rates promote wetting by increasing bubble concentration, lowering surface tension, and enhancing stability. After MNBW treatment, the oxygen content on the coal surface increased by 10.1%, accompanied by significant enrichment of oxygen-containing functional groups such as –OH and –COOH. Molecular simulations revealed stronger adsorption of water molecules in hydroxyl-rich regions, with a 45% increase in the relative concentration of water molecules at the coal–water interface, enhanced interaction energy, and a markedly reduced contact angle. These findings provide a theoretical basis for optimizing MNBW-assisted coal seam water injection technology.