Dynamics And Manipulation Of Droplet Formation And Breakup In Y-junction Microchannels

Microfluidic technology is the chemical production process that is carried out in a micron or sub-millimeter restricted space.In this study,the formation dynamics of the ferrofluid droplets in different Y-shaped flow-focusing microchannels and the breakup dynamics of droplets in a Y-junction were studied by using a high-speed camera.In addition,the velocity distribution in the breakup process of droplets was investigated by using the micro particle image velocimetry（μ-PIV）.The main contents include:The formation process and dynamics of ferrofluid droplets in Y-shaped flow-focusing microchannels was studied.The formation process of droplet could be divided into two stages:squeezing stage and pinch-off stage.The result shows that the power-law indexαof the squeezing stage is nearly unaffected by continuous phase flow rate,magnetic flux density and channel angle.The power-law indexβof the pinch-off stage is also independent of continuous phase flow rate and channel angle,but it changes with the variation of magnetic flux density.The length of the dispersed phase head firstly decreases and then increases during droplet formation.The moving speed of the dispersed phase head decreases with the increase of continuous phase flow,while increases with the increase of magnetic flux density.In addition,a prediction correlation of droplet size was proposed and validated.The breakup process and dynamics of breakup of droplets in Y-junction microchannel were studied.The breakup process of droplet could be divided into three stages:squeezing stage,transition stage and rapid pinch-off stage.In the squeezing stage,the power rate indexαdecreases with the increase of the viscosity ratio of the two phases,while effects of the capillary number of continuous phase and the length of the mother droplet on the power law indexαare almost ignorable.In the transition stage,the slope k decreases with increasing the two-phase viscosity ratio,the number of capillaries or the length of the mother droplet.In the rapid pinch-off stage,the power law indexβdecreases with the increase of the viscosity ratio and the decrease of the dimensionless mother droplet length l₀/w_c.However,it is hardly affected by the capillary number Ca_c.Moreover,the internal flow field of droplet in breakup process was determined by micro particle image velocimetry.The position of the maximum velocity moves from the tail of the droplet towards the walls of the two downstream subchannels,and finally appears at the neck of the droplet at the sharp corner.