Theoretical Analysis On Micro Bubble/Droplet Formation In Microchannels

Owing to their special hydrodynamic characteristics and scale effect,micro bubbles/droplets are having wider and wider applications in many fields such as medicine,chemistry,power engineering, environmental science,and,even our daily life.By using microfluidic devices,micro bubbles/droplets with finer monodispersity and more controllable size can be formed compared with the traditional formation methods.Microchannel emulsification,Capillary flow focusing and T-junction microchannel formation method are all good examples of the microfluidic-device-based formation methods.However,few investigations have been done on these methods up to now, which limits people from fully understanding the mechanisms,results and certain peculiar phenomena of the formation processes.On the other hand,although there have been a certain amount of researches on the two phase flow characteristics in microchannels,analysis on the transition boundaries among different flow regimes is still confined to superficial empirical experimental results.This paper will concentrate on these problems based on theoretical analysis.For the Microchannel emulsification formation method,work this paper has done is as follows. By reasonably expressing the external energy input and the increasing of the bubble/droplet surface free energy during the formation process,it is confirmed that for the reversible inflation process of bubbles/droplets on the terrace,increasing of the surface free energy equals the total external work, which is believed to also apply to other systems in which surface tension dominants.Then,by analyzing the increasing of surface area per unit volume of bubble/droplet fluid,it is found that if the initial volume of the bubble/droplet is zero,energy consumption of the initial expansion will be infinite,which is,however,very difficult to realize in natural world.Thus we can obtain the reason why there is always remaining dispersed phase fluid on the terrace during each formation process. Lastly,available experimental data relating bubble/drop diameter d to terrace length L and microchannel depth h in the literature are carefully analyzed.Through using nondimensional numbers d/h and L/h,and re-expressing these data in logarithm form,a scaling relation predicting the sizes of the formed bubbles/droplets without any assumptions is proposed which agrees well with experimental data.Then a comparison between this scaling relation and that deduced by other authors was made.For the T-junction microchannel formation method,we compared the magnitudes of different kinds of forces in the formation process and then found that drag force of the continuous phase fluid and surface tension force dominant the detachment of micro bubbles/droplets.By expressing these two forces reasonably in micro scale,we then obtained a scaling relation for the sizes of bubbles/droplets which are not big enough to occupy the entire microchannel cross section based on force balance,which agrees well with experimental results.Afterward,a discussion was conducted,which found that the final sizes of the micro bubbles/droplets increase while the Ca number decrease and increase while the flow rate ratio between the dispersed phase fluid and the continuous phase fluid increase.On the basis of transition mechanisms of different two phase flow regimes in macrochannels, we found that it is also applicable in microchannels that only when the length of the fluid bulk is long enough so that the deformed continuous phase velocity distribution can be fully reestablished, can the velocity of the bubbles/droplets be the same and the length of the fluid bulk remain constant with time and position in the direction of flow so as to remain a stable slug flow.Accordingly,we explained the transition mechanism from slug flow pattern to transition flow pattern in microchannels and then proposed a theoretical transition boundary scaling relation.