Effect of in-situ microbubbles precipitated at the solid–liquid interface on the bubble-coal dynamic collision and adhesion processes

The presence of interfacial microbubbles (IMBs) transforms the particle-bubble interaction process into a more complex particle-microbubble-floatation bubble system. Additionally, the high-speed collisions between deformable gas–liquid interfaces of different scales led to a higher level of physical complexity. However, there has been limited research on the particle-microbubble-floatation bubble interaction process. A high-speed camera was used in this study to investigate the precipitation and growth of IMBs on the coal surface in situ and to explore the impact of IMBs on the dynamic collision and adhesion processes. Observations of IMBs precipitation revealed that it was related to the air saturation in water. The precipitation sites were selective, mainly occurring from the pores and cracks on coal surface. As the precipitation time increased (from 2 to 60 min), the quantity of IMBs remained nearly unchanged, but their diameter and coverage rate increased, and the contact angle (${\theta }_m$) decreased. Based on the frame-by-frame analysis of the dynamic collisions and adhesion processes between bubbles and coal surface, it was found that the bubble rebound number, rebound time, induction time and adhesion time all decreased in the presence of IMBs. Meanwhile, the kinetic energy ($E_K$) of the bubble that the coal surface can capture, adhesion diameter, and adhesion contact angle increased. The presence of IMBs can enhance the foam film drainage rate and form gas capillary bridges once the foam film ruptures, which helps the coal surface capture bubbles with greater $E_K$ and allows the bubble to enter the spreading stage more quickly. Additionally, the curved foam film formed between bubble and IMBs provides extra surface tension for the spreading of the three-phase contact (TPC) line. IMBs increase the probability and stability of bubble-coal adhesion. Furthermore, it was revealed that the microbubble morphology (diameter and contact angle) could affect the bubble-coal dynamic collision and adhesion processes. IMBs with a precipitation time of 5 min (diameter of 134 μm and ${\theta }_m$ of 47.1°) exhibited the best performance. This is mainly determined by the difficulty of bubble-IMBs coalescence, the increase in diameter after coalescence, and the possibility of consecutive coalescence. The findings of this study can provide new insights into using interface microbubbles to enhance the flotation yield and rate of coal particles, as well as to innovate flotation processes.