Formation of Dispersed Small Bubbles in Flow-Focusing Microchannels

The formation of dispersed small bubbles in microfluidic flow-focusing devices was investigated experimentally by means of using a high speed camera.The dimensions of the experimental square microchannels are 400 m×400 m and 600 m×600 m,respectively.N2 was used as the gas phase,and water-glycerol mixtures(25%(wt),50%(wt) and 62%(wt)) with different concentrations(0～0.5%(wt)) of surfactant SDS(sodium dodecyl sulfate) were used as liquid phase.The parameters,such as liquid flow rate,the viscosity and the surface tension of the liquid phase,which influence the size of formed small bubbles,were studied.It was found that the bubble size in bubbly regime decreases with the increase of the flow rate and the viscosity of the liquid phase,while it increases with the increase of the surface tension of the liquid phase.The bubble formation mechanism was discussed,and it was conclude that the small bubbles are formed due to the viscosity force and interfacial forces.A new power law model was proposed to describe the relationship between the size of formed bubbles and the local capillary number.