Study On Breakup Mechanism And Interfacial Dynamics Of Droplet Of Unconventional Fluid In Microchannel

Droplet-based microfluidic technique could produce and precisely manipulate droplets in microchannel,it thereby attracts extensive attention in many applications such as chemical synthesis and biomedicine,etc.,owing to the advantages of process high efficiency and controllability,technology greenness and safety.Unconventional fluids with some special properties,such as high viscosity,viscoelastic properties,or shear thinning,etc.have wide application in chemical production and daily life.Therefore,the interfacial evolution and breakup dynamic of high-viscosity Newtonian droplets and viscoelastic non-Newtonian droplets in a microfluidic T-junction were investigated experimentally by use of a high-speed digital camera.Dynamics for high viscosity droplet breakup with tunnels and formation of satellite droplets in a symmetric microfluidic T-junction were investigated.The breakup process of droplet could be divided into four sequential stages:squeezing,transition,pinch-off,and thread rupture stages.The effects of the viscosity ratio of both phasesλ,the capillary number of the continuous phase Ca_c,and the dimensionless droplet length l₀/w_c on the breakup process of droplet were analyzed.In the squeezing stage controlled by squeezing pressure,the variation of the minimum width of the droplet neck with time could be scaled as a power-law relationship with exponent related toλand l₀/w_c.The transition stage is dominated by the velocity of the fluid,and the pinch-off stage is mainly driven by the capillary force.For these two stages,the evolution of the minimum width of the droplet neck with the remaining time could be described as various power-law relationship with exponent dependent onλ.In the thread rupture stage controlled by viscous stresses of both phases and the surface tension,the minimum width of droplet neck decreases linearly with time,and the coefficient of the linear function depends onλ.Additionally,for the formation of satellite droplets,the size of satellite droplet increases with the increase of superficial velocity of the fluid and droplet viscosity.Furthermore,a critical capillary number0.03 is observed in droplet with low viscosity,which divides the variation of the size of the main satellite droplet with the capillary number into constant area and growth zone.The dynamics of elastic droplet breakup and stretching of polymeric filaments in a microfluidic T-junction were investigated experimentally.Through comparisons between cases of Newtonian droplet and a series of viscoelastic droplets（drop of PEO aqueous solution）with similar shear viscosity,the effect of elasticity on the droplet breakup dynamic was especially focused.Four sequential stages of droplets breakup in a microfluidic T-junction were observed:squeezing,transition,pinch-off and filament rupture stages.For former three stages,the dynamic behaviors of viscoelastic droplets with lower molecular weight（M_w=1×10⁵ g/mol and 3×10⁵ g/mol）and Newtonian droplets are similar,while for viscoelastic droplets with higher molecular weight（M_w=1×10⁶ g/mol）,the spatial obstruction of previous filaments accelerates the shrinkage of droplet neck.Moreover,the transformation of dominant force from inertial to capillary force leads to the variation of scaling laws describing the neck evolution.For filament rupture stages,with the extension of polymer molecules,the elastocapillary balance contributes to the exponential thinning of the polymeric filament.Differently,the width of thread neck decreases linearly with the time for Newtonian droplets.Furthermore,the stretching rate of viscoelastic filament is proportional to two-phase flow rate but independent of elasticity.