Water drops emerge from large pores of the hydrophobic Gas Diffusion Layers (GDL) into the cathode gas flow channel of Polymer Electrolyte Membrane (PEM) Fuel Cells. The drops grow into slugs that span the cross-section of the flow channels. The slugs detach and are forced out the gas flow channel by the air flow. An acrylic micro-fluidic flow cell with a 1.6 mm gas flow channel and a 100 mu m liquid pore through a carbon paper GDL has been used to quantitatively determine slug volumes, velocity of slug motion, and the force required to move slugs as functions of the gas and liquid flow rates. In a channel with 4 acrylic walls, slugs detach immediately upon formation. A porous GDL wall allows gas flow to bypass the slugs, thus allowing slugs to continue to grow after spanning the open area of the channel. The differential pressure to detach and move slugs is equal to the dynamic interfacial force on a slug normalized by the cross-sectional area of the channel. The dynamic interfacial force is equal to the difference between the downstream (advancing) and upstream (receding) contact lines of the water with the channel walls. Slugs will stop moving if the differential pressure drop for gas flow to bypass the slug and flow through the GDL under the rib separating the channels is less than the differential pressure required to move the slug. The results improve our physical insight into the state of water hold up in PEM fuel cells. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.