The study focuses on the transition mechanisms between four various How patterns in asymmetric microfluidic T-junctions, including breakup with permanent obstruction (B1), breakup with part obstruction (B2), breakup with permanent tunnel (B3) and non-breakup (NB). Two critical dimensionless bubble lengths are introduced to describe the transitions between B1 and B2 and between B2 and B3. The existence of these two critical dimensionless bubble lengths is explained in terms of the evolutions of forefront and rear positions of the breaking bubbles and their geometric configuration in the T-junction. The asymmetric microfluidic T-junction used in the present work could more easily break bubbles than a symmetric T-junction. The results show that the breakup and non-breakup behaviors of bubbles in the asymmetric T-junction used in the present work are similar to those of bubbles in the symmetric T-junction and to those of droplets in the asymmetric I-junction. (C) 2014 Elsevier Ltd. All rights reserved.