We report experiments on bubble formation from needles with and without liquid co-flow, carried out with needles in the range of 0.79 < d(n) < 2.06 mm, for gas flow rates up to 4.5 cm(3)/s per needle, and with liquid co-flow velocities up to 0.4 m/s. Bubble sizes and frequencies were obtained by means measuring an acoustic signal in the pressurized chamber upstream, which is validated by high-speed imaging analysis. Bubble contours, bubble growth curves and time return plots were obtained to analyse the bubble formation process. Different bubbling regimes are distinguished and a novel dimensionless pressure ratio is proposed to forecast the emergence of weeping and the transition from constant flow rate bubbling to constant chamber pressure bubbling. A single correlation for the non-dimensional bubble size with and without liquid co-flow was developed and validated with the experimental data obtained in the present study. (C) 2019 Elsevier Ltd. All rights reserved.