Research On Characteristics Of Droplet Formation And Droplet Coalescence Of Liquid-Liquid Two-phase Flow In Microchannels

As the most important flow pattern of liquid-liquid two-phase flow in microchannel,droplets are widely used in biomedical engineering,materials industry and chemical engineering which provide stable templates for the screening and detection of biomacromolecules,the control of material structure and preparation of microparticles.Although great progress has been made in manipulation of droplet,there are challenges in precise control of droplet characteristics,highthroughput preparation of droplet and control flexibility.Therefore,the precise control of droplet flow,especially the studies on dynamic characteristics in droplet such as concentration field,velocity field and temperature field are of great significance for promoting application of the microfluidic systems in all areas.Three main subjects are investigated by experimental and numerical tools.First,effects of different junction configurations,channel geometries,viscosities of two phases and flow rates of two phases on droplet formation were clarified.Then mixing performance in droplets were investigated in microchannels with different configurations.Finally,passive droplet coalescence based on variation of two-phase velocity was carried out experimentally.The major research contents and results are as follows:(1)Microfluidic test bench was designed.Experiments were carried out to study the droplet formation processes in cross-shaped microchannels and different droplet length was obtained via changing the velocity of two phases.The variations of the droplet length and space between the droplets were measured in different velocity of two phases.Simulations which consistent with the experimental conditions were performed by the VOF method.The variations of the droplet length and space between the droplets were measured in different velocity of two phases and the results were compared with the experimental one.In addition,the experiments existed in literatures were chose and conducted simulations using the VOF method.The flow patterns,droplet formation process,mixing in droplet and velocity field were compared between simulations and experiments.The simulation results were highly consistent with the experimental results which indicated that the VOF method is accurate enough for analysis of droplet formation in micochannels.(2)Models of microchannels with cross-shaped junction,T junction,and co-flow T junction were built.The effects of junction configurations,channel geometries,viscosities of two phases and flow rates of two phases on droplet characteristics was investigated comprehensively.Dimensionless numbers were used to analyze the droplet length and droplet formation frequency in different conditions.The correlations based on Capaliary number and Weber number were proposed for flow regime transition.The correlation based on Capaliary number was proposed for predication of droplet length.(3)Mixing efficiency in different junction configurations and dispersed phase fractions were studied by the VOF method coupled with UDS(user defined scalar).The initial distributions of the mixing components in the droplet in lower/upper parts and front/back parts were both considered.The mixing efficiency and energy dissipation were analyzed based on the scalar concentration in the droplet and the pressure drop,respectively.Results shown that:When the mixing components were distributed in the upper and lower parts of the droplet,the serpentine microchannels had better performance for the enhancement of the mixing process in the droplet.The mixing efficiency achieved a higher degree in the microchannel with a small bend radius.However,the small bend radius also led to the higher energy dissipation.When the mixing components were distributed in the front and back parts of the droplet,the mixing efficiency achieved 55% at the moment of the droplet formed due to the twilling effect in the droplet.The scalar distribution in the droplet was determined by the junction configuration of the microchannel.At the moment of droplet formed,the scalar distribution was symmetrical in the cross-shaped junction.Thus,the mixing process can be greatly enhanced by the inner circulation in the droplet.However,in the T junction and the co-flow T junction,the scalar concentration in the lower part of the droplet is much higher than that in the upper part of the droplet.Thus,during the droplet moving along the microchannel,the scalar distribution became uniformity in the lower and upper parts of the droplet,respectively.And the scalar concentration in the lower part was always higher than in the upper parts.Among three junction configurations,the energy dissipation in the cross-shaped junction was always higher than in others.In addition,when the droplet moving in the microchannel with a constant velocity,the mixing efficiency decreased with an increase of the dispersed phase fraction.Because the increase of the dispersed phase fraction caused the increase of the droplet length.The inner circulation path was longer in the large droplet than in the small droplet.Thus,when the droplet moved through the turns,the symmetrical inner circulation in the small droplet was more easily broke up and formed asymmetrical inner circulation.The asymmetrical inner circulation facilitated the mixing greatly.In the meantime,the longer inner circulation path the longer mixing time,this also hampered the fast mixing in the droplet.(4)Passive droplet coalescence based on variation of two-phase velocity was proposed.Effects of two-phase velocity on droplet coalescence were studied experimentally.Results indicated that for achieving droplet formation alternatively,the ratio of two dispersed phases should be higher than 0.3.Meanwhile,for droplet coalescence effectively,the ratio of dispersed phase velocity and continuous phase velocity were recommended larger than 0.8.For investigating the flow field in droplet coalescence process,numerical simulations were conducted via VOF method combing with UDF.Velocity along the radial direction and axial direction was compared in different droplet length and droplet coalescence process.Results showed that the velocity gradient in droplet was independent of the droplet length when the droplet length was far larger than the wide of microchannel.With decreasing droplet length,velocity distribution in droplet was dominated by the shear force exerted by the continuous phase.The maximum velocity in droplet whether in radial direction or axial direction was about 2 times higher than the droplet superficial velocity.However,in droplet coalescence process,the maximum velocity in coalescing droplet was almost 3.5 times and 7.5 times higher than the droplet superficial velocity which was facilitated the redistribution of scalar in droplet.The inner circulation path in droplet can be illustrated by the ratio between radial(axial)velocity and droplet superficial velocity.When the ratio was less than 1,it indicated that the appearance of inner circulation.And the closer to the center of the droplet the larger of the circulation path.