Experimental Study On Droplet Formation Process And Droplet Length In Convection T-junction Microchannel

As technology began to move toward miniaturization and refinement,microfluidic technology was proposed and developed rapidly.The micro-droplets formed using microfluidic technology are widely used in pharmaceutical preparation,power,crude oil mining,metallurgy and other industrial fields.Because the micro-scale liquid-liquid multiphase hydrodynamic characteristics are very different from the macro scale,achieving precise control of the size and uniformity of micro-droplets has become the focus of current research.The research on the influencing factors of the droplet formation process and droplet length in the liquid-liquid multiphase systems is conducive to the precise control of the micro-droplet size and uniformity.In this paper,the liquid-liquid two-phase system in 400 ?m×400?m microchannels was studied.The silicone oil with viscosity of 5cs,20cs and 60cs was selected as the dispersed phase,distilled water with different surfactant concentrations(0.1 wt%,0.3wt%,0.5wt%and 1.0wt%SDS)was used as the continuous phase.The high-speed camera was used to collect images of the experimental results of liquid-liquid two-phase system in the microchannels with standard T-junction,convection T-junction and cross-junction.The image post-processing was carried out through software Matlab.The effects of each phase force,two-phase flow rate,two-phase physical parameters,microchannel inlet construction and surfactant concentration on the formation process and droplet length of the micro-droplets were investigated.In order to predict and control the generation time of micro-droplets well,to achieve the stability and periodicity of micro-droplet generation,a study of the generation time of micro-droplets was also conducted.In the study of the formation process of micro-droplets,it was found that with the decrease of the flow ratio,four flow patterns of annular flow,slug flow,droplet flow and jet flow were observed in turn.Different flow patterns have different formation mechanisms.The inertial force dominated the formation of annular flow,Interfacial tension affect the formation of slug flow,and the formation of droplet flow and jet flow were mainly dominated by shear force.Keep the flow rate of one phase stable and change the flow rate of the other phase.It was found that when the flow ratio reduced,the more favorable the next flow pattern formed.When the viscosity of the dispersed phase increased,the coverage of the annular flow area gradually expanded,and the coverage of the slug flow area gradually decreased;before the critical micelle concentration was reached,as the surfactant concentration increased,the more favorable the micro-droplets formed;standard T-junction microchannels produce micro-droplets with the highest quality,and cross-junction microchannels produce micro-droplets with the largest two-phase flow range.In this paper,the flow pattern transition model of micro-droplets from annular-slug and slug-droplet processes were established.The predicted results were compared with the experimental results of the flow pattern transition model in other people's literature.The predicted results show a good agreement.The study found that flow parameters were the most important influencing factor of droplet length.When the continuous phase flow rate,capillary number and surfactant concentration increased,the droplet length decreased;when the dispersed phase flow rate and flow rate ratio increased,the droplet length increased;the change of the viscosity of the dispersed phase only had a weak effect on the droplet length in the slug flow pattern,and had almost no effect in most cases;the droplet length formed in the standard T-junction microchannel was the smallest.This paper proposed scaling laws for the droplet lengths of the slug flow and the droplet flow.Compared with the experimental data of this paper and the experimental data of others,the predicted results were good.Using the two-phase flow ratio as a parameter,the relative regularity of the droplet length formed under the standard T-junction and convective T-junction structures was determined.In the study of the droplet generation time,it was found that the droplet generation time was controlled by the droplet length,liquid column length and two-phase flow rate.The droplet generation time gradually decreased with the increase of the liquid column length,the capillary number of the continuous phase and the flow rate of the dispersed phase and the continuous phase,and decreased with the decrease of the droplet length.Based on the existing scaling laws for droplet length,the scaling laws for the liquid column length and droplet generation time were proposed,and the predicted results were good.