Formation and deformation of double emulsions in a cross-shaped microchannel: From Janus to core–shell structure

•Two formation mechanisms of double emulsion droplet were studied.•The transformation between the Janus and core–shell mechanisms were observed.•Three morphologies of droplet under flowing conditions were examined.•The flow behavior of double emulsion droplets was analyzed. The formation and deformation of double emulsion droplets in a cross-shaped microchannel are investigated. The transformation of the formation mechanisms with constant physical properties is observed. During droplet formation, the competition between hydrodynamic and thermodynamic processes leads to the mutual transformation between the Janus mechanism and the core–shell mechanism. Furthermore, three morphologies of the double emulsion droplets are observed under the combined action of shear stress and interfacial tension. A new characteristic capillary number is proposed to indicate the critical condition of each morphology based on the dynamic analysis of droplet flow behavior. This paper provides an experimental basis for the application of multi-emulsion droplets in microfluidic equipment.