We present an analytical and experimental study of the formation of bubbles from a submerged vertical needle under mixed injection conditions, where neither the gas flow rate nor the feeding pressure remain constant during the process. In particular, we focus on the temporal evolution of the pressure inside the gas injection chamber during the bubble formation process, p(t), modeling it and analyzing the bubble size and shape as functions of the volume of the chamber, V-c, and the mean gas flow rate, Q(i), for a given needle radius, a. Under this configuration, it has been shown that the water column penetrating inside the injection needle plays an important role on the bubble formation process. Consequently, an analytical model that includes the description of the time evolution of the liquid column penetrating inside the needle after the separation of a bubble, and the subsequent pressurization of the gas chamber, has been proposed. The results given by the model agree well with the experimental measurements. (C) 2017 Elsevier Ltd. All rights reserved.