It is critical for flotation to accomplish the bubble-mineral particle attachment as well as mineralization process. The surface free energy of mineral particle and liquid is a key parameter which affects the wettability and wetting process of mineral in mineralization. In this paper, the surface free energy components of coal sample were calculated by Washburn equation and Oss Chaudhury-Good-Van theory. Additionally, the wetting ability of modified diesel oil for coal sample was studied by Washburn dynamic capillary method while Lipophilic Hydrophilic Ratio and relative contact angle were calculated. Furthermore, the effect of both surfactants, 2-ethylhexanol and didodecyldimethylammonium bromide (DDAB), on the induction time between oily bubbles and coal particles was investigated. Results indicated that wetting ability of modified diesel oil for coal sample did not entirely increase with the decrease of surface tension, and liquid viscosity was also a relevant factor. The adhesion strength enhanced whereas the detachment probability reduced with the increase of collector viscosity. But too large viscosity was bad for fluidity of liquid, which therefore decreased its wettability for coal sample. The induction time between coal particles and reactive oily bubbles modified by 2-ethylhexanol and DDAB decreased by 74.35% and 86.45% respectively. As a result, the maximum combustible matter recovery of 72.20% (increasing by 22.23%) and highest flotation efficiency index of 56.50% were obtained when the weight ratio of 2-ethylhexanol to diesel oil was 0.04, while that of using reactive oily bubbles modified by DDAB showed an apparent decrease. It indicated that it was crucial to the flotation of low rank coal that proper surfactant was used to generate reactive oily bubbles. (C) 2017 Elsevier Ltd. All rights reserved.