Advances in stability of nano-bubble on solid-liquid interface and mechanism for enhancing flotation adhesion

The long-range hydrophobic attraction plays an important role in bubble-particle attachment,while the latter is the key sub-process in flotation.As the main origin of the long-range hydrophobic attraction,nano-bubble is significant for interface control in flotation.The basic properties,stability mechanism,and flotation strengthening mechanism of nano-bubbles were systematically discussed.The abnormal stability and contact angle of nano-bubbles have been research hotspots in the past two decades.Classical physics theory predicts the lifetime of nano-bubbles on the microsecond scale,but experiments have found that the life of nano-bubbles usually reaches more than a few days.To explain this,several hypotheses such as contamination theory,dynamic equilibrium theory,and contact line pinning theory have been proposed.However,none of these stability theories can explain all the experimental phenomena,and the stability mechanism still needs to be further studied.The contact angle of nano-bubble is much lower than that of its macroscopic counterpart,and the decrease of solid-gas interface energy caused by high-density gas inside of the nano-bubble may be the main reason for the abnormal contact angle.Next,the enhancement mechanism of nano-bubbles in flotation adhesion was discussed.On the one hand,nano-bubble promotes the drainage of intervening liquid film during particle-bubble collisions through boundary slip,on the other hand,it provides long-range attraction and attractive DLVO force between particle and bubble to promote the particle-bubble adhesion.At present,the experiments have shown that nano-bubbles have a significant improvement effect on the flotation of various minerals such as coal,phosphate,scheelite and iron ore,etc.In the context of flotation becoming increasingly refined,the introduction of nano-bubble enhanced technology can provide a new theoretical perspective and technical means for flotation interface regulation,and is one of the key research directions in the field of flotation in the future.