We investigate the bubble dynamics and subsequent changes in the liquid-phase flow characteristics of upward bubbly flows in a square pipe with a sudden expansion (expansion ratio of 4.0). The experiments are conducted under three liquid-phase Reynolds numbers of 0 (stationary), 420 (laminar) and 6000 (turbulent). The inlet volume void fraction ranges about 0 - 1.0%, and we use a high-speed two-phase particle image velocimetry and shadowgraphy to measure two phases simultaneously. It is observed that after the expansion, smaller bubbles tend to migrate toward the wall and larger bubbles rise in a core region, which is more encouraged by the steeper velocity gradient with increasing the Reynolds number. This bubble distribution is further analyzed by estimating the interfacial forces acting on rising bubbles. Affected by this bubble distribution, the enhanced turbulence in the inlet flow energizes the separating shear layer, resulting in the reduction of reattachment length behind the edge. Finally, we suggest an empirical relation for a two-phase flow reattachment length in terms of Reynolds number and mean void fraction, which explains the contribution of added dispersed phase on the mixing enhancement in the backstep flows. (C) 2019 Elsevier Ltd. All rights reserved.