Effects of T-junction size on bubble generation and flow instability for two-phase flows in circular microchannels

► Bubble generation at four different T-junctions was visualized. ► Validity of a predictive model derived from a scaling analysis was confirmed. ► Air-water flow was unstable when Qair/Qwater < 0.2 and Qair/Qwater > 2.0 in all the channels. ► With smallest air injector, quasi-stable flows were observed when 0.2 < Qair/Qwater < 0.7. ► Transient mechanism to unstable or quasi-stable flow was investigated through image analyses and a pressure analysis.