Investigation On Fabrication And Influence Factor Of Monodisperse Emulsions In Novel Tree-like Parallel Microchannel Contactor With Mass Production

Monodisperse emulsions have been extensively studied and found a wide range of applica-tion in various engineering applications including pharmaceuticals,chemical engineering,food science,etc.The production of emulsions having uniform-sized droplets and plugs are mainly determined by the microfluidic devices and the physical properties of continuous phase flow and disperse phase flow,and the monodisperse of emulsion droplets and plugs has a huge influence on emulsion behaviour such as appearance,stability,rheology,reaction kinetics.To actively control the size of droplets/plugs production via parallel microchannel devices,it is of significance to fully explore the application of monodisperse emulsions in industrial field-s.Nowadays,the mechanisms of droplets/plugs formation and the inherent law of multiphase flow fluid effects in tree-like parallel Y-junction microchannels contactor are still not completely investigated.Especially,the droplet/plug formation and movement,the flow patterns,the ex-tent of monodisperse emulsions impact of tree-like parallel microchannels based on dichotomic ramification,and the interaction of two phase flows are still need to be revealed.In this the-sis,we focused on the development of novel tree-like parallel microchannels contactor,and explored its utility and for the formation of monodisperse emulsions with high throughout.To further investigate the local flow patterns,the dynamic behaviors of droplet/plug formation and the frequency,a single Y-junction was first fabricated and tested numerically and experimental-ly.Then,based CFD simulation approach,the actual water and oil flow distribution properties among the bifurcated tree-like structure and the impact of distribution non-uniformity on the emulsion polydispersity were investigated and discussed completely and deeply.Additional-ly,the experimental set-up were presented with the tree-like contactor.Stable mass production of monodisperse emulsion have been observed by high-speed CCD.The influence of paral-lelization,flow maldistribution factors and fluids physical properties on the polydispersity and concentration were examined deeply.To sum up,a series of important results and conclusions can be drawn:The hydrodynamic behavior of the droplet/plug formation process was studied through ex-perimental and VOF methods.The influence mechanism of the two-phase flow pattern and fluid properties on the water-in-oil emulsification process was analyzed,and the quantitative relation-ship between the droplet/plug length and volume and flow ratio,surface tension,viscosity ratio were also discussed.The experimental and numerical results showed that two typical emulsifi-cation modes were prepared under the interaction of the surface tension and the cohesive force:the droplet flow and the plug flow,and the continuous phase capillary number Ca_c=0.01 was the critical value of the transition of the droplet flow to the plug flow.The length and volume of the droplet/liquid column increase with the increased of the surface tension,but decreased with the increase of the viscosity.When the capillary number is less than the critical value,the surface tension is stronger than the viscous shear force and dominates in the plug formation.The length and volume of droplets/plugs increased linearly with increasing water-to-pure oil flow ratio.In order to have more suitable length and volume of droplets/plugs predicted correlation with liq-uids under various physical properties,correction factors(?_o,modify/?_o,pureand ?_w/o,modify/?_w/o,pure)is added to the equation of the length and volume of the plug based on the flow ratio.The new length and volume value calculated by the prediction correlation after adding the correction factor had a good fit with the experimental results.A three-dimensional CFD was used to establish a theoretical analysis model for the fluid distribution in bifurcated tree-like distributor-collector.The inhomogeneity of fluid distribution under different flow rates and physical properties was investigated.The relative flow deviation??_i?and maldistribution factor?MF?were used to quantify the degree of fluid distribution inho-mogeneity.The numerical calculation results showed that the fluid distribution inhomogeneity in the tree-like distributor-collector was determined only by the Reynolds number,but having no relationship with the physical properties of the fluid.A transitional Reynolds number exist-ed in the tree-like structure.When the Reynolds number was less than the transition Reynolds number,the maximum of relative flow deviations and the maldistribution factors were small and remained constant,because the inertia of the fluid and bifurcation and elbow of the tree structure under the transitional Reynolds number had no impact on the fluid distribution.However,the Reynolds number is higher than the transitional Reynolds number,the inertia of the fluid itself and the elbows and bifurcations of the tree structure will caused a significant vortex flow.As each branch generation were strengthened,the fluid distribution in the parallel microchannels was no longer uniform and the?_iand MF increases with increasing Reynolds number.An experimental platform was set up to apply the novel tree-like contactor to the prepare monodispere emulsions with mass production,and based on the theoretical calculation model of droplet volume and CFD simulation results,the influence of the parallelization,single-phase distribution inhomogeneity,two-phase distribution inhomogeneity and inlet flow ratio on the emulsion polydispersity were studied and the predicted correlation were also developed on these influencing factors.The results showed that the tree-like contactor can prepare monodisperse emulsions with high throughtput.As the tree-like structure is optimized for scale size,the par-allel emulsion droplets will not break or focus in the collector.The polydispersity value of the emulsion?P_w?increased with the increase of the non-uniform factor of the single-phase flow distribution,and the single-phase non-uniformity of the water phase and the oil phase had the same influence.When the two-phase flow is not uniform,the value of P_wwill increase linearly with the value of two-plase maldistributor factor.The inlet water-oil flow ratio also had a pos-itive effect on P_w,but when the flow ratio was more than 1.0,the effect gradually decreased.Based on the heterogeneity of the two-phase fluid,the prediction correlation of the emulsion P_w was developed,and the calculated value is in a good agreement with the calculated results of the two-phase flow of the CFD simulation.An experimental test-rig was also set up to study the effects of surface tension,viscosity,number of capillaries,nanoparticles,and temperature on the polydispersity and concentration of the emulsion droplets.The frequency of emulsion droplet generation was qualitatively s-tudied by the VOF method.Then,the Onnezog number was used to quantitatively analyze the influence of the fluid properties on the polydispersity of the emulsion,and finally calculated the energy cost rate of the emulsification reaction under all experiments.The results showed that the emulsion P_wincreased as the interfacial tension between the fluids decreased,and the concentration of the emulsion droplet also increased with the decrease of the interfacial tension.This is because when the surface tension was reduced,undet the shear force,the required time of one droplet was broken would be reduced accordingly,so the frequency of droplet forma-tion increased,and the concentration of the emulsion prepared per unit of time would increase,too.The decrease of viscosity to some extent would favor the preparation of monodisperse emulsions.When the capillary in the contactor was less than 0.02,the prepared emulsion is a monodispersed emulsion.At high temperatures,P_walso increased as the temperature increased.The added nanoparticle with emulsions would cause the high polydispersity,and he concentra-tion increased slightly.For water-in-oil emulsification in the single-microchannel and tree-like parallel microchannels contactor,an Onnezog number of 0.4 can be used as the critical line be-tween emulsion monodispersity and polydispersity in the contactor.When the Ornezog number is less than 0.4,the emulsion is a monodisperse emulsion,whereas the emulsion is a polydisperse emulsion.Based on the key parameters in the emulsification reaction and the Nelder-Mead pure form equation,Oh_prewas developed to predict the polydispersity of the emulsion.And the pre-diction value Oh_preis a good predictor of the experimental value Oh_expts,the average absolute error is 8.7%.Finally,the maximum energy cost rate of the contactor emulsification under all operating conditions is 35J.kg^-1.