Study On Droplet Formation And Interfacial Dynamic In Unconventional Fluids In Microchannels

Microchemical technology,as a kind of sustainable development of chemical process and intensification technology,has attracted widespread attention in current two decades.The droplet formation and dynamics in microchannel is one of the most important issues for this technology.The droplet formation in unconventional fluids(high-viscosity,viscoelasticity,shear-thinning)in microchannels was investigated experimentally using a high-speed digital camera in the present study.The main contents are as follows:The breakup dynamics of high-viscosity thread for droplet formation in other immiscible viscous fluids in a flow-focusing device was studied systematically.The breakup process of dispersed thread presented two types: symmetrical rupture and asymmetrical rupture.Furthermore,the rupture behaviors could be divided into two stages: the squeezing stage controlled by the squeezing pressure and the pinch-off stage controlled by viscous stresses of both phases and surface tension.Specifically,it suggested that the differences in the shape of the liquid-liquid interface and the dynamics in the two breakup processes were caused by the disparity of the strain field at the point of detachment.Moreover,the thinning rate and the dynamics of the dispersed thread change with the viscosity of the continuous phase,but less dependent on the flow rate of the continuous phase.The breakup dynamics of the viscoelastic dispersed thread for droplet formation in a flow-focusing device was investigated.Four breakup regimes of the viscoelastic thread were observed: besides the thread rupturing at both ends or at the downstream end likewise in Newtonian fluid,the breakup could also occur at the upstream end,displaying two additional patterns.For each breakup type,the breakup of the viscoelastic thread could be divided into a ?flow-driven? stage controlled by the inertia of the dispersed phase and the dynamical radial pressure of the continuous phase,and an ?elastocapillary? stage controlled by the elastic and capillary forces.The ?elastocapillary? stage included the stretching process and the fluid drainage process,depending on whether the balance between the elastic forces and the capillary forces reaches or not.The differences in breakup dynamics and liquid-liquid interface shape for the four breakup types depend exclusively on the elasticity of the viscoelastic thread.The self-similarity of the breakup of viscoelastic dispersed thread for droplet formation in flow-focusing devices was investigated.The self-similar profiles for the liquid-liquid interface were obtained by normalizing the interface with the minimum width of the dispersed thread.The breakup dynamics of the dispersed thread transfered from a self-similar power law scaling stage with an exponent of 0.36 to a self-similar exponential scaling stage.The asymptotic cone angles prior to final breakup were consistent with the value of 125.5° and 151°,respectively.The viscoelasticity inhibits the development of finite-time singularity for the breakup of the liquid-liquid interface at microscale,similar to the capillary breakup at macroscale.The results demonstrated that the breakup of the viscoelastic dispersed thread for droplet formation exhibits self-similarity at microscale.The breakup dynamics of the shear-thinning dispersed thread for droplet formation in a flow-focusing device was investigated.The breakup process of shear-thinning dispersed thread could be divided into four stages: the droplet growth stage and the squeezing stage controlled by the squeezing pressure,the stretching stage controlled by the viscous stresses of dispersed phase and surface tension,and the pinch-off stage controlled by viscous stresses of both phases and surface tension.It suggested that the breakup dynamics of the dispersed thread for the shear-thinning fluid is very similar to that of the constant viscosity Newtonian liquids,signifying that the breakup is unaffected by the rheological properties in each stage.