Branch Selection Behavior Of Droplets/bubbles In Asymmetric Microchannels

Microdroplets/bubbles have been widely used in chemical,pharmaceutical,and biological fields due to their advantages of good monodispersity,stability of closed system,and small size.The controllable droplet branch selection process is the basis for the realization of application manipulations such as capture,fusion,and sorting,which is conducive to the stable control of droplets/bubbles.In this thesis,aiming at the branch selection behavior of droplets/bubbles in asymmetric channels,experiments and numerical calculations are combined to record the branch selection behavior of droplets/bubbles and to study the flow characteristics.It focuses on the flow resistance characteristics of droplets entering the intersection,the classification of different branching modes,the effect of droplet length in branch selection mode,and the influence of pressure and viscosity on the critical splitting formula of droplets.Specific content includes:（1）Droplet branch selection mode and its deformation analysis.The regulation of different flow modes is achieved through polydimethylsilane（PDMS）microchannels,and the numerical values of pressure and flow ratio in the channels are extracted,to quantify the effect of droplets on the continuous phase flow in the microchannel system.The path selection presents three modes:flow into the main channel,flow into the side branch and splitting at the junction.The deformation process in the three modes is basically the same before the dimensionless time t=0.5,while it is completely opposite after 0.5.（2）The influence of droplet length on the modes of droplet branch selection.It is found that the droplet deformation coefficient changes with time when l>2w_m.The flow distribution in branch selection is related to the flow rates of the inlets.When the flow rates are relatively small,vortexes are easily generated inside the droplets,which prompt the droplets to enter the branch channels.The flow process of droplets with different lengths is similar before t=0.5,but long droplets block all branches in the junction,while short droplets are not long enough to block any branch.The main difference between droplets of different length is whether the continuous phase flow in the branch and downstream channel undergoes a"rebound"process.When the droplets flow into the side branch and splitting at the intersection in turn,the critical capillary number decreases,with the increase of the dimensionless length l₀/w_m of the droplet,The smaller the capillary number,the harder it is for the droplet to split.（3）The influence of flow conditions on the critical capillary number of droplets splitting in droplet branch selection.It is found that with the increase of the outlet pressure,three different flow behaviors including flow into the side branch,splitting at the junction and flow into the main channel appear in turn and the critical splitting curve of the droplet behavior shifts to the right.The critical capillary number change little in the flow of bubbles and droplets,implying that the split boundary is similar for the bubbles and droplets.However,when the discrete phases are three different concentrations of glycerol in water,a larger difference of coefficient can be observed.The influence of viscosity is not only reflected in the critical splitting condition,but also in the deformation of the droplet.Under the same flow conditions and initial droplet length,the branch selection mode of droplets varies with the increasing viscosity,while the deformation rules of the curvature radius were similar in the branch direction and showed different changes in the downstream direction.