Numerical Simulation Of Droplet Formation Characteristics In Microchannel

Droplet microfluidic technology has been widely used in biochemistry,material synthesis,energy chemistry and other fields.Despite the simple structure,the involved multi-phase flow problem is complicated.Currently,most researches focus on its applications,the mechanisms of droplet formation and breakup in different channel configurations are still unclear or have no unified understanding,which will hinder the optimization and operation of microfluidic devices.In addition,the current research on droplet generation mostly uses experimental methods,but due to the many factors influencing droplet generation at the microscale,the existing experimental results have poor repeatability due to the slight differences in the selection of channel structure,fluid composition,channel wettability and other parameters,and usually have only consistent trend.At the same time,the experimental process also has its limitations,such as measurement errors,some experimental results are not easy to obtain,and detailed information of fluid flow cannot be obtained,etc.Therefore,in-depth research and understanding of droplet generation characteristics and internal mechanisms in microchannels will help to better understand droplet formation,breakup and two-phase flow pattern transformation,and provide important guidance for subsequent precise control of droplets and optimal design of channel structure.The main research work of this article is as follows:Firstly,the numerical simulation method was used to study the influence of the change of the continuous phase velocity,two-phase viscosity coefficient,interfacial tension and other parameters on droplet generation in a co-flowing microchannel.The internal mechanism of droplet formation and breakup was studied through the morphological changes,pressure field and velocity field of droplets during droplet generation.The results show that when the continuous phase capillary number Ca_c and the dispersed phase Weber number We_d are far less than 1,the droplets show a dripping mode,the main reason is that the two phases interfacial tension is relatively dominant in the competition with the continuous phase viscous stress and the dispersed phase inertial force.When the viscous stress of the continuous phase is relatively dominant in the competition with the interfacial tension of the two phases and the inertial force of the dispersed phase,the two-phase flow pattern of the droplet changes from the dripping mode to the jetting mode.Secondly,the numerical simulation method was used to study the effect of the change of the dispersed phase velocity on droplet generation in a co-flowing microchannel.The simulation results showed that with the increase of the dispersed phase velocity,the two-phase flow pattern changed successively from the narrowing dripping mode to the widening dripping mode and then to the widening double droplets jetting mode.Through the quantitative study of droplet formation form,pressure velocity field,droplet neck and other typical characteristics,the internal mechanism and difference under each mode were studied.The results show that with the increase of the momentum of the inlet of the dispersed phase,the internal inertia force of the neck can resist the extrusion effect of the external fluid for a long time when the neck is contracted,so that the upstream liquid column of the neck forms a bulge and the two-phase flow pattern changes.In the large droplet mode,the front point of the droplet is close to the inlet of the dispersed phase,and the droplet is greatly affected by the inertia force of the dispersed phase.There is a long-term mutual balance between the forces of the fluid inside and outside the neck.In the small droplet mode,the front point of the droplet is far from the inlet of the dispersed phase,and the droplet is less affected by the inertia force of the dispersed phase.The external fluid of the neck always maintains the dominant competitive advantage in the competition between internal and external forces.At last,the numerical simulation method was used to study the influence of the taper channel entrance,downstream orifice width and their coordinated changes on the droplet formation in flow-focusing microchannels.Through the droplet two-phase cloud image,pressure velocity field and the data quantification of droplet neck characteristics during the droplet generation process,the internal mechanism of the influence of the orifice width on the droplet generation was studied.By comparing the time of the droplet growth stage and squeezing stage under different channel configurations,the cooperative effect of the taper angle and the orifice width was studied.The results show that when the width of the orifice is small,the focusing effect of the continuous phase near the orifice plays a dominant role,and the droplet generation diameter and period are not sensitive to the change of the taper angle.As the orifice width increases,the focusing effect of the continuous phase near the orifice continues to weaken.Changing the horizontal taper angle can cause the droplet two-phase flow pattern to change from the dripping mode to the jetting mode.The horizontal taper angle begins to play a leading role in the droplet generation process.