| Micro-droplets are widely used in the areas of pharmaceuticals,cosmetics,food,biology and chemistry,where droplet generation is the important premise for these applications.Because the droplet-based microfluidic technology can realize the effective organization and precise manipulation of multiphase fluid flow,it becomes an effective method for precise and controllable generation of mico-droplots.However,on the other hand,the characteristics of droplet generation in microfluidics,including the small channel size,high flow resistance and low flow rate,make its generation throughput still waiting to be improved.Thus,the throughput multiplication of droplet generation has brought widespread attention to the academic.Therefore,nowadays droplet generation and splitting in microfluidics have become hotspots in microfluidic multiphase flow fields.It is of great importance for scientific research and engineering application to deeply investigate the process of droplet generation and splitting in microfluidics.At present,due to the complicated hydrodynamics of liquid-liquid multiphase flow and their interface evolution,the hydrodynamic mechanisms of typical flow regimes in droplet generation and splitting processes,especially for the multiple emulsion droplet,are not fully revealed.The effects of channel geometry,operation conditions and fluid properties on the hydrodynamics of droplet generation and splitting and the quality of generated droplet need to be further elucidated.The prediction equation of droplet size and critical criterion for determining the droplet breakup,which offering important guide significance in engineering applications,are urgent to be established.In order to systematically and deeply solve these problems,the numerical simulation,experimental observation and theoretical analysis methods are used in this paper.The unsteady theoretical models of liquid-liquid multiphase flow and its interface evolution are developed.The generation of single emulsion droplet in microfluidic cross-junction and multiple emulsion droplet in flow-focusing device,the splitting of double emulsion droplet in microfluidic Y-junction are numerically investigated.Additionally,experiment system of droplet generation in microfluidic cross-junction is designed and built to visually observe the droplet generation process.The main research contents and conclusions are summarized as follows:(1)The droplet generation in microfluidic cross-junction with different junction angles is numerically studied.The hydrodynamic mechanisms of typical flow regimes and their interface evolution are emphatically explored.The influences of flow condition and junction angle on the quality of generated droplet are revealed.Furthermore,a prediction equation considering?is proposed to predict the droplet size under squeezing regime.The results indicate that,the squeezing,dripping,jetting and threading are observed in the microfluidic cross-junction,and these four regimes will appear sequentially with increasing Q~*under the same Ca_c.Under the large Ca_c,the viscous shear and drag will enhance by enlarging Ca_c,which causes the inertia of dispersed phase for flow regime transition to reduce.The change in?alters the interface evolutions occurring in the junction region and nearby under squeezing and dripping regimes,but has little influence on the jetting regime and is insufficient to fundamentally alter the flow regimes of droplet generation.In the transition from squeezing to dripping with increasing Q~*under the same Ca_c,the droplet generation frequency increases and the droplet size becomes smaller,which is particularly apparent when?deviates from 90°.In addition,the equation l~*=(?(exp|cos?|)~n+(?Q~*)/(2sin?)+??cot?)Ca_c~m is developed to predict the dimensionless length of generated droplet under squeezing regime.(2)The droplet generation in microfluidic cross-junction with different junction angles is experimental studied and the effects of junction angles on the typical flow regimes and quality of generated droplet are analyzed.Meanwhile,the reliability of prediction equation for droplet size is further verified.The results indicate that the shrinkage of neck at necking stage and elongation of dispersed thread at expansion stage are decelerated in the order of?=90°,30°,150°under squeezing regime.The junction angle?has greater influence on the droplet size under squeezing regime,while?has little effect on droplet size under dripping regime,because the interface evolution of the forepart and neck of dispersed thread mainly occurs at the outlet and even in the downstream region of the junction.In the transition from squeezing to dripping,the decline of droplet size decreases and critical Q~*increases in the order of?=150°,30°,90°,the data predicted by prediction equation are in reasonable agreement with the experimental results with the relative error less than±30%.(3)A numerical study is conducted to investigate the generation of triple emulsion in the flow-focusing device.The hydrodynamic mechanisms of typical flow regimes and their interface evolution are examined emphatically.The differences between the generations of single,double and triple emulsion droplets are discussed.The influences of operation conditions(flow rates,viscosities and surface tension coefficients)on the quality of generated droplet are quantitatively analyzed.Additionally,flow regime diagrams of single,double and triple emulsion are summarized in the coordinate system of the capillary number of the outer and middle phases.The results indicate that there are three typical flow regimes in the flow-focusing device namely dripping,jetting and threading regimes.Under dripping regime,the hydrodynamic focusing effect of the orifice facilitates the formation of the neck,which contributes to the elongation of dispersed phase.When entering the orifice,the droplet triggers the radial expanding of its external droplet leading to decrement in the neck shrinking rate.Under jetting regime,the detaching process of the droplet is dominated by the viscous force at downstream of the orifice,thus the evolutions of neck thickness of each droplet are similar.As the detachment of inner droplet is beneficial for the detachment of outer droplet,the generation cycle,droplet size and flow regime are different for single,double and triple emulsion droplets under the same operating condition.Increasing the flow rate of one phase results in larger external droplet and smaller internal droplet.Increasing surface tension coefficient or decreasing viscosity leads to the transition from jetting to dripping with a decrease of droplet size.With the increase of interfacial tension coefficient,the resistance against interface deformation from the dispersed phase is strengthened and bigger droplet is generated.On the other hand,higher viscosity of outer,middle and inner phases lead to smaller droplet size.In addition,since the detachment of inner droplet facilities the outer interface breakup,the critical viscous force from outer phase that causes the flow regime transition is reduced,and accordingly the critical capillary number of continuous phase for the flow regime transition decreases as the levels of hierarchy in emulsion droplets increase.(4)A numerical study on the splitting of double emulsion in microfluidic Y-junction is conducted.Detailed hydrodynamics information on double emulsion passing the junction is presented,together with the droplet deformation characteristics as well as the quantitative evolutions of driving and resistance forces,which reveal the hydrodynamic mechanism underlying the double emulsion breakup and non-breakup.The flow regime diagrams of both single and double emulsion droplets are summarized in the coordinate system of capillary number Ca and emulsion length l~*.The results indicate that the behavior of the double emulsion in microfluidic Y-junction can be classified into three classes namely obstructed breakup,tunnel breakup and non-breakup.The upstream pressure acts as the driven force of the deformation of the double emulsion while the Laplace pressure difference between the forefront and rear of the droplet interfaces acts as the resistance of the deformation.A positive correspondence relationship is observed between the two pressures.Under tunnel breakup regime,the appearance of tunnels affects the double emulsion deformation,resulting in the slower squeezing speed and elongation speed of outer droplet as well as the slower elongation speed of inner droplet,but has little effect on the squeezing speed of inner droplet.The critical threshold determining the droplet breakup is approximately expressed as a power-law between the capillary number and the emulsion length as l~*=?Ca~b,while the threshold between tunnel breakup and obstructed breakup is approximately expressed as a linear function as l~*=?.Compared with the flow regime diagram of single emulsion droplet,the coefficients?and?are larger in the flow regime diagram of double emulsion droplet.This study provides an in-depth understanding of the hydrodynamics of liquid-liquid multiphase flow and its interface evolution during the process of droplet generation and splitting,which enriches the theories of microfluidic multiphase flow and its manipulation.Moreover,the prediction equation for droplet size and the theoretical criterion for determining the droplet breakup are proposed in this study to provide essential theoretical and technical supports for the device design,the process control and the quality optimization of droplet generation and splitting in microfluidics. |
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