Droplet formation and detachment in the gas flow channel are important steps in the water management in polymer electrolyte fuel cells. Formation and detachment of the droplet are expected to cause changes in the local saturation in the GDL, which are important to understand the relevant forces determining formation and detachment.In order to control the two important boundary conditions of water feed rate and gas dynamics in the channel accurately, a non-operating set-up was developed. Water is injected through a 0.5 mm hole from the bottom into a SGL 25 BA gas diffusion layer (GDL) facing a gas flow channel on its top. The liquid water distribution in the GDL and the behaviour of droplet detachment at the GDL-gas channel interface are monitored using sub-second X-ray tomographic microscopy (XTM) with 0.75 s scan time and 2.55 s repeating frequency and fast X-ray 2D radiography (3 ms exposure time).

With the constant water injection rates, the droplet formation was found to be very periodic with droplet growth showing a repeating pattern. The droplet diameter evolution for two different injection rates is shown in Figure 1 (a). The final droplet volumes were found to be 17.6 nL at an injection rate of 350 nL/min and 10.3 nL at 1000 nL/min.Figure 1 (b) shows the water volume in the GDL. The liquid volume in near the inlet was found to be high and decreasing towards the gas channel, corresponding to a high saturation near the inlet and a low saturation near the gas channel. In the bulk of the GDL, no changes occurred at droplet detachment. But in the upper layers of the GDL, for the injection rate of 350 nL/min, a fluctuation of water volume caused by droplet detachment can be observed.