The characteristics of the bubble formation in the process of powder discharge were investigated in a pseudo-2D aerated hopper by high-speed camera. The effects of particle size and gas velocity on the minimum bubble velocity, bubble growth, detachment and bubble formation frequency were systematically studied. Experimental results show that aeration can dramatically increase the discharge rate by up to about double, while the fluctuations of the powder discharge rate are also strengthened by bubbling as its standard deviation increases to more than 3 times that obtained without aeration. It appears that particle size and aeration gas velocity have significant effects in the aerated flowing hopper on the bubble formation process. With the increase of particle size, both the minimum bubble velocity and the formation frequency increase while the detach surface area, the maximum surface area and its lifespan decrease obviously, just like in a standard fluidized bed. Furthermore, comparison of the minimum bubble velocity with that in fluidized bed shows that it is easier to form bubble in aerated hopper and the velocity can be predicted by a revised model of the minimum fluidization velocity. (C) 2018 Elsevier B.V. All rights reserved.