Multi-scale interaction characteristics and microscopic mechanisms of micro-nano CO₂ bubble water on coal

Coal dust posed serious threats to mine safety and the health of miners. Coal seam water injection is a commonly used technique for pre-wetting and dust reduction. However, due to the low surface energy of coal, ordinary water could not effectively wet the coal. Therefore, surfactants were often employed to enhance wetting performance. Nevertheless, surfactants were associated with high costs and poor biodegradability. Micro-nano bubbles water, which exhibited low surface tension, supplied promising applications in this field. In particular, CO₂ was a non-toxic and readily available gas that showed great potential for practical use. The wetting effect of micro-nano CO₂ bubble water on coal was essentially a multi-scale process. However, its characteristics and underlying microscopic mechanisms at different scales remained unclear, limiting its application in engineering practices. To address this issue, the multi-scale interactions between micro-nano CO₂ bubble water and coal, as well as the associated microscopic mechanisms, were systematically investigated. The results showed that the light intensity and zeta potential of micro-nano CO₂ bubble water increased initially and then decreased with increasing circulation preparation time, reaching maximum values at 10 minutes. In contrast, the surface tension and pH value first decreased and then increased, with the surface tension being reduced by up to 47.86% compared to water. At the macroscopic scale, the contact angle between micro-nano CO₂ bubble water and coal decreased initially and then increased with prolonged preparation time. At the mesoscopic scale, the treated coal samples exhibited an increase in inorganic pores and a decrease in methane adsorption capacity. At the microscopic scale, the oxygen content in the treated coal increased significantly, particularly in hydrophilic functional groups such as carbonyl and carboxyl groups. Based on proximate analysis, ultimate analysis, and micro-spectroscopic characterization of coal, a molecular structure model of coal after micro-nano CO₂ bubble water treatment was established (C₁₁₀H₆₈N₂O₇). Combined with the coal molecular model, Hirshfeld surface analysis, and electrostatic potential analysis, it was found that the ability of pyrrolic nitrogen and hydroxyl regions on the coal surface to adsorb water molecules was enhanced after treatment. In the microscale coal-water wetting system, the relative concentration of water molecules below the coal-water interface was higher after treatment, while the peak concentration above the interface was lower. Furthermore, the number of methane molecules adsorbed by treated coal was smaller than that before treatment. The absolute value of interaction energy between coal and methane before treatment was greater than that after treatment, indicating that micro-nano CO₂ bubble water inhibited methane adsorption and enhanced coal wettability. These findings provided theoretical support for the application of micro-nano CO₂ bubble water in coal seam water injection.