Wettability analysis method for assessing the effect of chemical pretreatment on brown coal biosolubilization by Gordonia alkanivorans S7

•An alternative method of contact angle measurement was developed using OCT.•A relationship between the wettability and the pre-treatment of coal was observed.•Gordonia alkanivorans can liquefy coal with higher contact angle more effective.•HNO3 treatment introduce O2 and NO3 groups into a coal, that increase liquefaction.