Abstract: Flotation results showed that the ability of oily bubbles to collect low-rank coal particles is much stronger than that of air bubble in traditional flotation processes. It is because the surface of the oily bubble is covered by the collector,and its surface is much more hydrophobic than the surface of the air bubble. Therefore,in the interaction process of low-rank coal particle-oily bubble,the thinning speed of hydration film is much faster than that of low-rank coal particle-air bubble. The results of induction time test indicated that the induction time between low-rank coal par- ticles and air bubbles decreased from 93 ms to 12 ms as the concentration of DAH solution increased from 10-7 mol /L to 5×10-5 mol / L. As the concentration of DAH solution increased from 5×10-5 mol / L to 10-3 mol / L,the induction time between low-rank coal particles and air bubbles increased from 12 ms to 35 ms. The induction time between low- rank coal particles and oily bubbles decreased from 35 ms to 10 ms while the DAH concentration was increased from 10-7 mol / L (pure deionized water solution) to 5×10-5 mol / L. As the DAH concentration increased to 10-3 mol / L,the induction time between low-rank coal particles and oily bubbles increased from 10 ms to 25 ms. In order to compare the strong surface hydrophobicity of the oily bubble with the surface hydrophobicity of the bubble from the microscopic scale,the non-DLVO theory and the Stefan-Reynolds hydration film thinning model were adopted. Based on the induc- tion time results of low-rank coal particles-oily / bubble,the relationship between the initial hydration film thickness (h) and the hydrophobicity constant (K132 ) was fitted. Moreover,the relationship between the hydrophobic force con- stant (K132 ) and the concentrations of the DAH solution was obtained. The fitting results of hydrophobic force constant (K132 ) showed that the hydrophobic force constant (K132 ) between low-rank coal particles and oily bubbles was three times than that between low-rank coal particles and air bubbles while the concentration of DAH solution was 5×10-5 mol / L. The hydrophobic force constant ( K132 ) between low-rank coal particles and oily bubbles was in the order of 10-16 while the concentration of DAH solution is 10-6 mol / L. The hydrophobic force constant (K132 ) between low-rank coal particles and oily bubbles was 15 times than that between low-rank coal particles and air bubbles. Therefore,the surface hydrophobicity of the oily bubble was stronger than that of the air bubble. Thus,it concluded that the oily bub- ble flotation process is superior to the traditional flotation process.