Study On Breakup And Coalescence Dynamics Of Droplet In Microchannels

In recent years,microfluidics has drawn extensive attention due to the advantages of high safety and ease of control.In the present study,the breakup and volume distribution of droplets,the dynamics of droplet breakup with permanent tunnel and the droplet coalescence were investigated experimentally by a high-speed camera.The main contents are as follows:The breakup and distribution of droplets at a branching T-junction were investigated experimentally.The effects of mother droplet volume,two-phase Reynolds number and capillary number of the dispersed phase on droplet volume distribution were explored.The results indicated that the volume distribution ratio decreases firstly and then increases with the increase of dimensionless mother droplet volume V/W_c³.As the Reynolds number Re_c of the continuous phase was increased,the volume distribution ratio decreased at first and then increased.When continuous phase Reynolds number Re_c was fixed,the increase of Reynolds number Re_d of the dispersed phase would lead to a reduction in the volume distribution ratio.Moreover,the increase of the capillary number Ca_d of dispersed phase could result in an increase in the volume distribution ratio.Furthermore,the correlations for predicting the volume distribution ratio were proposed.The droplet breakup with permanent tunnel in an asymmetric microfluidic T-junction was investigated experimentally.The breakup process of droplet could be divided into three stages:entering stage,deformation stage and breakup stage.The breakup stage could be further divided into two sub-stages:fast breakup stage and thread breakup stage.The results indicated that the fast breakup stage was a self-similar process,the evolution of the dimensionless minimum width of the droplet neck with the dimensionless remaining time could be scaled by a power-law relationship,and the value of power law index was about 1.35.In the thread breakup stage,the dimensionless minimum width of the droplet neck varied linearly with the dimensionless remaining time.The coalescence of droplets in the shrinking convergence of symmetrical T-junction was investigated.Five types of droplet coalescence regimes were observed:non-contact,contact non-coalescence,Asynchronous coalescence,synchronous coalescence and synchronous non-coalescence.The existence of shrinking convergence allowed a certain difference between the arriving times of two droplets for coalescence.The maximum arriving time difference of the two droplets for coalescence was analyzed.The critical capillary number(Ca_cr),which determines whether synchronous colliding droplets could coalesce,decreased with the increase of the viscosity of the dispersed phase.The result indicated that the increase of the viscosity of the dispersed phase was not conducive to coalescence.