Coal water slurry (CWS) with high solid content and low viscosity can allow for efficient and environmentally friendly gasification. In the process of CWS preparation, water will enter pores of coal particle to form inner water, resulting in a decrease in free water among particles and an increase in viscosity. However, pre-absorption of moisture from air was found to result in a sharp decrease of suspension viscosity, since small amount of air were sealed in particle pores. Coal particles have many polar functional groups, and pre-absorbed water sealed the entrance of pores through hydrogen bonds like a "bottle stopper", which effectively inhibited the diffusion of water from slurry into the pores of coal particles. Pre-absorption of water from air was also found to improve the efficiency of the second fluid (immiscible with water), and cause a further decrease in viscosity and yield stress. (C) 2016 Elsevier Ltd. All rights reserved.