Bubble formation in a step-emulsification microdevice: hydrodynamic effects in the cavity

[Display omitted] •The three-dimensional characteristics of bubble formation in step emulsification microdevices were observed.•The influence mechanism of bubble swarm in cavity is revealed.•A semi-empirical law for predicting bubble volume with more variables is proposed. This study focuses on the generation of bubbles in a step-emulsification microdevice via a two-angle photography method. It's found that the bubble generation mechanism is controlled by the interfacial tension, below a critical capillary number; while controlled by viscous force, inertial force and the disturbance induced by the bubble swarm, above the critical capillary number. From the two-angle photography method, a model is established for predicting the bubble size, by taking into account of the dynamic contact angle between gas-liquid interface and wall, and the hydrodynamic feedback of the cavity on bubble formation via the quantification of resistance by the volume fraction of gas in the cavity.