General rules of bubble formation in viscous liquids in a modified step T-junction microdevice

[Display omitted] •The general rules of bubble formation in viscous liquids are obtained.•The rules are confirmed in a very wide range of bubble formation frequency.•The maximal bubble formation frequency is determined.•A scaling law is developed to predicted the bubble diameter. The understanding of the microbubble formation characteristics in viscous solutions remains confused in terms of the evolution of bubble neck and formation frequency. In this work, the general rules of bubble formation in viscous liquids were systematically investigated in a modified step T-junction. Different viscous glycerol-water solutions and nitrogen were chosen as the continuous and dispersed phase, respectively. A very wide range of bubble formation frequency was determined, and the formation frequencies could be divided into four periods: slightly increased, dramatically increased, approximately linearly increased to the maximum, and gradually decreased as the continuous phase flow rate increased, and the maximum was close to the transition line of bubbly and Taylor bubble flow. To the best of our knowledge, it is the first time to report the general bubble formation frequency range. Finally, the bubble diameter was perfectly scaled with the gas-liquid flow rate ratio and the capillary number.