Study of mechanism of adhesion between bubble and coal particle surface using coal models prepared with different hydrophobicities

The conventional bubble probing technique using mica or gold sheet as coal models cannot accurately reflect the coal specimen′s surface micronano structure and chemical property, thus making it difficult to bring to light the bubble-coal particle surface adhesion mechanism. To address this issue, the coal models are prepared with SiO₂ which is coated with asphalt through rotodip process and oxidization treatment of different time periods. The models are similar to coal in chemical property and have different hydrophobic surfaces. Through testing on contact angles, roughness, high-speed dynamic test on collisional adhesion behavior, and AFM bubble probing, the mechanism of adhesion between bubble and coal particle with different hydrophobic surfaces is brought to light. As revealed by test results, the measured contact angles of the highly, moderately and weakly hydrophobic models′ surfaces are 95.19°, 75.24°, 55.23°, respectively, with their respective arithmetic square root roughness being 0.29, 0.46, 0.43 nm. Viewed from the macro adhesion behaviors, the bubble-particle surface interaction process is jointly governed by hydrodynamic force and surface force; as shown by result of high-speed dynamic test, there shows no significant difference in the number of times of collisions between bubble and different coal surfaces as the surface force is shielded by hydrodynamic force; after the turn of environment to a quasi-static state, the liquid film between bubble and coal surface ruptures under the effect of driving of surface force at 345 ms for highly hydrophobic surface and at 845 ms for moderately hydrophobic surface, and no rupture of film is observed for the weakly hydrophobic surface. In the bubble probe test, at a driving speed of 1 μm/s, the bubbles start to adhere to the highly hydrophobic surface at a repulsive force of 23.08 ± 3.93 nN during the probe insertion process, but the adhesion process tends to lag behind with the increase of the driving speed to 10 μm/s, and the adhesion behavior is seen to be completely suppressed at 30 μm/s; as the hydrophobicity of coal surface decreases, no adhesion of bubbles are observed at different driving speeds, and only the attractive force measured during the probe retraction process is positively correlated with the driving speed; for highly hydrophobic coal surface, decreasing the hydrodynamic force facilitates the thinning and rupture of liquid film to promote adhesion while for the moderately and weakly hydrophobic coal surface, increasing hydrodynamic force can boost the liquid suck-back attractive during the particle-bubble moving away process, which is helpful to the increase of particle-bubble adhesion probability.