Numerical simulations using volume of fluid (VOF) method are performed to investigate the effects of the surface microstructures of gas diffusion layer (GDL) on the dynamic behaviors of a water droplet in a micro gas channel (GC) of proton exchange membrane fuel cells (PEMFCs). The detachment size of a droplet under different air velocity and GDL wettability is investigated. Simulation results show that the microstructures of the GDL surface indeed affect the dynamics of water droplets. The directional distributions of carbon fibers in the flow direction are desirable for detaching droplets from the GDL surface and reducing flooding in GC. In addition, a force analytical model is developed, which takes into account the effects of microstructures of GDL surface as well as the surface tension force due to droplet deformation with some simplified assumptions. It is found that surface tension force due to the droplet deformation cannot be simply ignored when analyzing forces acting on the droplet, as the contact angle hysteresis is usually relatively large. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.