Investigations Of Droplet Flow Characteristics At The Microchannel Intersection And Bifurcation

Owing to the advantages of uniform and stable size,high throughput,low consumption,less cross contamination and convenient manipulation,droplet-based microfluidics has been widely appiled in the fields of biologic pharmacy,medical-mediated therapy,cosmetic emulsification,micro-scale phase change and heat transfer,material synthesis.At the microscale flow,the interface effect shows an obvious impact on the droplet flow behaviors.Especially when the droplet size and the characteristic length of the microchannel are on the same length scale,the channel wall would largely restrict the droplet flow.Therefore,the droplet flow behaviors could be tuned precisely with the aid of different channel structures.Droplet coalescence and breakup are essential functions in order to realize the accurate quantification of the droplet size and improve on-line operations of microfluidic chips.Systematical investigations of the working forces of the droplet interface would help to deepen the understanding the mechanisms that dominate droplet coalescence and breakup as well as to find the effective methods to enlarge the domain of stable functioning.This dissertation mainly focuses on two kinds of droplet behaviors,namely droplet coalescence and droplet breakup,by experiments and theoretical analyses.Influences of channel local structures,flow conditions and working liquids on droplet behaviors are studied and the evolution rules of the droplet interface are analyzed.Critical conditions of various droplet behaviors are quantified.Theoretical models are constructed to predict the droplet flow bahaviors.The specific study contents include:?1?The influence of the microchannel intersections on collision behaviors of synchronous droplets is investigated.It is found that two types of coalescence regimes are presented,namely compressing coalescence and decompressing coalescence.As the droplet velocity increases,the moment imbalance caused by the difference between droplets is strengthened and the coalescence regime transfers from compressing coalescence to decompressing coalescence.The critical capillary number for coalescence varies within the range of 0.0021⁰.01 and changes with different intersection junctions or viscosity ratios.A smaller channel space could limit the rotation of droplets and favor the drainage process,effectively increasing the critical capillary number.A power law relationship,Ca*^?-0.75,is found between the critical capillary number and the viscosity ratio.While colliding with each other,the liquid between droplets is accelerated by compression and drained out along the direction that is tangential to the droplet interface.During the decompressing coalescence process,a vortex,which quickens the drainage of the continuous phase,shows up within the latter droplet with the restriction of the channel wall and the earlier droplet.?2?Collision behaviors of droplet pairs with the presence of arriving distance differences are studied and four behavior regimes are shown,that is,no-collision,coalescence,slipping and splitting,which distributed regularly within the phase regimes with the droplet length and the arriving distance difference being the two axes.The decompressing coalescence is induced by the local deformation of the contacting interface caused by the low pressure when droplets are pulled apart.It is confirmed that relatively larger distance differences favor the decompressing coalescence.The relationship between the coalescence distance and the drainage time could be divided into two stages,and the drainage time of the turning point is close to the residence time of the droplet at the intersection,namely tdr*?tre.After the turning point,the coalescence distance increases linearly with the drainage time and the slope is equal to the average velocity of the joint channel.The droplet recovers its shape within the joint channel and the behavior regime transfer from coalescence to slipping.?3?Behaviors of droplets flowing through an asymmetric bifurcation and the critical conditions between different behavior regimes are investigated.It is found that the critical condition of droplet breakup still agrees with the relationship equation proposed from symmetric junctions,which is L₀/W=aCa^-0.21,while the fitting parameters are influenced by the viscosity ratio of liquids.For breakup with obstruction,the critical condition stays at L₀/W?2.5 when Ca<⁰.01,otherwise the critical droplet length increases with increasing capillary number,which is different from the symmetric junctions.By taking into consideration the resistances of the droplet and the continuous liquid,a prediction model that reflects the average flow conditions within branches is constructed and confirmed to be accurate in comparison with experimental results.It is proven by the resistance model that the breakup of droplets is determined by both the channel structure and the flow of the droplet interface.The total resistance of branches is tuned by the presence of daughter droplets,which further affects the flow of droplet interfaces and the breakup outcomes.?4?Evolutions of the droplet interface shape and working forces are researched for different breakup regimes at the bifurcation.For breakup with obstruction,the upstream pressure of the droplet is accumulated as the flow of the continuous liquid is blocked and the force related to the pressure drop across the droplet is the dominating driving force for droplet deformations.The theoretical model that predicts the droplet interface at small capillary numbers is built based on the mass conservation law,showing that evolutions of the droplet interface is shaped by the geometric parameters of the channel for breakup with obstruction,irrespective of the capillary number and the droplet length.The accuracy of the predicted results is confirmed by comparing with experimental results and the governing equations are suitable for channel structures with arbitrary angles.For breakup with gaps,the maximal velocity at the squeezing stage is distributed at the position around the gap that is affected by the viscous shear force of the flow through the gap,while the largest velocity at the pinch-off stage moves to the rapid thinning droplet neck when the interfacial tension dominates.The critical neck thickness that determines whether droplets break or not is quantified to be?*/W?0.4.