Bubble characteristics downstream of a 150 mu m micro-pillar embedded inside a 225 mu m deep microchannel were experimentally studied. Nitrogen was issued through a slit along the pillar's depth into liquid water flow. The gas flow was imaged and recorded by a high speed camera and the liquid flow field was obtained through a micro particle image velocimetry (mu-PIV) system. Gas flows through different slit angles were examined, and it was shown that the gas flow characteristics were significantly different for different slit angles. Three distinct bubble flow patterns were observed, which were dependent on the slit angle and liquid/gas flow rates. These patterns include discrete bubbles, attached ligament, and transitional flow. The liquid flow measurements were used to reveal the mechanisms controlling the bubble formation and flow patterns. (C) 2013 Elsevier Ltd. All rights reserved.