Investigation Of Two-phase Flow Characteristics In The Double Branch Microchannel Trapping Network

Droplet-based microfluidics has gained considerable attentions in academic and practical fields,due to its unique advantages of small volume,high throughput and controllable compositions.Trapping a train of moving microdroplets in the preset positions of microfluidic device facilitates a long-term observation of biochemical reactions inside the droplets.In this paper,the hydrodynamic behaviors of microdroplets in microfluidic trapping network(MTN),which has the function of sequentially uniform trapping,are studied experimentally and numerically,specially concentrating on the internal flow field of droplets moving in the front of MTN,path selection modes of droplets and two-phase flow characteristics in a trapping unit,and the internal flow field of droplets selecting different branches in the MTN.Based on the above investigations,a new bubble-guided trapping method is proposed.The highlighted results provide a technical guidance for the application of microdroplet trapping technique.(1)The internal flow field of droplet moving in the front of MTN is studied experimentally.Based on the absolute velocity field of droplet obtained by means of Micro-PIV,a relative velocity field in the reference of moving droplet is obtained by the post-proccesing.In this way,the dependence of capillary number,viscosity ratio,droplet size,and interfacial tension on the relative velocity field is studied.Two evident flow field transitions,during which two critical capillary numbers exist,are observed as the capillary number increases.These two critical capillary numbers increase with viscosity ratio in the evolution of flow features.Droplet size only influences x and y velocity component values other than flow structure within intervals separated by these two critical values.It is also indicated that two transitions exist in a surfactant added system in which interfacial tension is reduced.(2)The hydrodynamic behaviors of droplet and two-phase flow characteristics are studied experimentally and numerically in a trapping unit extracted from MTN.Flow behavior of droplet is recorded by using high speed camera at varied two-phase flow rates.Three path selection modes of stable trapping,only flowing through the trapping channel,and disordered dynamics are shown.To further study the two-phase flow characteristics,which are not easy to be measured by experimental means,flow condition of stable trapping in experiments is chosen as boundary condition of numerical simulation.And then the effects of flow resistance ratio R_B/R_T on path selection mode,the flow rate and pressure drop of trapping channel are investigated.It is shown that stable trapping for R_B/R_T varying from 2.98 to 5.15 and squeezing out of the trapping chamber for R_B/R_T varying from 6.18 to 8.90 are indicated.For all R_B/R_T,the flow rate and pressure drop of the trapping channel show a complex nonlinear evolution in the process of droplet selecting the trapping branch and bypass successively.(3)The evolution of internal velocity field of droplet is investigated experimentally in the process of droplet selecting two branches of direct and indirect MTN.Internal velocity field of droplet train selecting trapping channel and bypass is acquired by means of Micro-PIV.Combined with the morphology and location of droplet in bright field image,the forces are analyzed to explain the evolution of flow field.Complex change of flow field,in which internal fluids in the bifurcation flow into inlets of two branches and the fluids of droplet tail recover rapidly at the inlet of the trapping channel,is presented in the process of droplet selecting the trapping branch of direct MTN.However,the behaviors of internal fluid flowing into inlets of two branches are not shown when subsequent droplets selecting bypass.In the process of droplet selecting the bypass of indirect MTN,internal fluids in the bifurcation flow into inlets of two branches and swing at the inlet of the trapping channel.When the subsequent droplet chooses to trapping branch,the evolution of flow field is the same as that of the direct MTN.The experimental results also show that the flow feature of trapped droplets changes two times with the increase of two-phase flow rate.(4)A new bubble-guided trapping method is proposed.The effects of two-phase flow rate on the flow behavior of droplet train are studied experimentally in both direct and indirect MTN.The experimental results show that the droplets exhibit four trapping modes of sequentially uniform trapping,non-uniform trapping induced by coalescence and breakup,etc.The causes of non-uniform trapping are analyzed from the point of view of droplet velocity and spacing.Comparing with the trapping performance of two types of channels,it is found that the two-phase flow rate range of uniform trapping in indirect MTN is larger than that in the direct MTN.Aiming at the shortage that the two-phase flow rate range of uniform trapping is still not high enough in indirect MTN,an improved method of bubble guided droplet trapping is proposed.The flow behaviors of bubble-free and bubble-guided droplet train are compared under the same two-phase flow rates,it is shown that the range of two-phase flow rate of uniform trapping is increased significantly and the uniformity of the trapped droplet array is improved.