This communication presents experimentally the bubble coalescence in a microfluidic expansion device using a high-speed digital camera and a micro-particle image velocimetry (micro-Ply) system. Experiments were conducted in a PMMA microchannel of 400 mu m deep, composed of an expansion with an angle of 60 degrees. N-2 bubbles were generated in 0.5 wt% polyacrylamide (PAAm) non-Newtonian fluids by hydrodynamic flow-focusing method and flowed past the expansion section. Two different mechanisms for bubble coalescence were observed by changing the gas and liquid flow rates. Three various processes of bubble coalescence were presented and analyzed. The probability, time and position for bubble coalescence were also calculated. The velocity fields in the liquid phase around the aligned bubbles were determined by micro-Ply measurements to gain insight into the mechanism for bubble coalescence. (C) 2015 Elsevier B.V. All rights reserved.