Mechanism And Mesoscale Effect Of Bubble Formation In Microchannels

In the micro chemical industry,it is very important to study the mechanism of bubble formation.The factors that affect the bubble formation include the type of operating conditions,physical parameters and equipment size,but in the new stepemulsification devices in which the step-emulsification principle is important,how these factors affect bubble formation is not clear.In this paper,the formation mechanism of bubbles in T-junction and step-emulsification devices was studied systematically by using a high-speed camera with nitrogen as the dispersed phase and glycerin aqueous solution with different concentration as the continuous phase.Firstly,the influence of the type of operating conditions on bubble formation in the T-junction is studied.The operating conditions are divided into constant pressure injection and constant flow rate injection.It is found that the characteristic parameters of bubble formation depend more on the change of pressure ratio at the entrance than the change of flow rate ratio.Finally,the resistance model of fluid flow at T-junction was established to explain the different effects of constant pressure injection and constant flow rate injection on bubble formation.Secondly,the mechanism of bubble formation in step-emulsification devices is studied.The research is divided into the evolution of head distance and neck width with time at the microscale level,and the variation of bubble volume and formation frequency with operating conditions at the macroscale level.Finally,the basic guideline to keep the excellent monodispersity of bubbles is put forward.Thirdly,the formation mechanism of bubbles in step-emulsification devices in high-viscosity fluids is explored.In high-viscosity fluids,the number of factors controlling bubble formation is more than that in low-viscosity fluids.The change of liquid flow rate affects bubble formation by applying shear effect and changing the pressure in the chamber.At the same time,the influence of contact angle between continuous phase and wall begins to appear.Finally,from a more general point of view,the reasons for the transition of bubble flow pattern and the basic criteria for maintaining the monodispersity of bubble volume in high viscosity fluids are proposed.Finally,the mechanism of bubble formation in parallel step-emulsification devices is explored.The hydrodynamic interaction of bubble formation is explained by the view of compromise in competition in the concept of mesoscale.It is found that the increase of compromise effect is beneficial to the double-microchannel formation of bubbles and the uniformity of characteristic parameters,while the competition effect will weaken the uniformity of characteristic parameters.The relative magnitude of compromise and competition effect can be changed by adjusting the variation of pressure drop or resistance distribution in the chamber.