Modeling And Measurements Of Droplet Formation In Flow-Focusing Devices With Newtonian And Non-Newtonian Fluids As Continuous Phase

Microfluidic droplets have been commonly applied in industry,like biomedicine,aerospace,energy utilization,chemical engineering,due to the advantages of high specific surface area,uniform size and better flow control.Precise and effective control over droplet length and formation frequency is critical for all those applications.Liquid-liquid two phase flows confined in micro-scale show many different properties that cannot be observed at macro-level.For example,interfacial tension force and viscous shear force dominate the interface breakup.Besides,most fluids in nature and industry show complex non-Newtonian behaviors,like blood,protein solutions,suspensions,which are more obvious for flows at micro-scale.Therefore,the investigations of droplet formation in non-Newtonian fluids are necessary but systematic research is still lacking.To fill this knowledge gap,the present work set up an experimental microfluidic platform based on the micro-imaging technology and observed the n-Decane droplet formation in flow-focusing devices with Newtonian and non-Newtonian fluids as the continuous phase.Two different flow-focusing droplet formation devices(Type ?and Type ?)were fabricated to study the effects of channel structure.Furthermore,an open source CFD platform,OpenFOAM,has also been used with the interFoam solver to model the effects of rheological parameters on droplet formationExperimental measurements show that the whole formation process is separated into three phases.However,the formation status in each stage differs according to the microchannel structure and the non-Newtonian behaviors of the continuous phase When Newtonian fluids are used as the continuous phase,droplet size,formation frequency and droplet velocity increase with the rising flow rate ratio.Specifically,the droplet length can be scaled with the flow rate ratio as a power-law relationship However,both the formation frequency and droplet velocity vary linearly with the flow rate ratio.For the Type ? channel,the relationship of droplet length with flow rate ratio divides into two parts,since the breakup position changes influenced by the channel structure.The phenomenon has also been observed in formation frequency.However,the relationship for droplet velocity has not been affected obviously.When non-Newtonian fluids are used as the continuous phase,some generally accepted correlations for Newtonian continuous phase are still available.For example,the droplet length still has a power-law relationship with the flow rate ratio,and the formation frequency also increases linearly as the flow rate ratio increases.However,some new characteristics have been identified.For the Type I microchannel.the shape of droplets differs from that in Newtonian liquids.Influenced by the non-Newtonian behaviors of the continuous phase,the droplets are elongated and the deformation index shows a power-law relationship with the flow rate ratio.For the Type II microchannel,the droplet breaks up before the nozzle for all the flow rates.Thus,the segmentation for Newtonian continuous phase does not appear as well.When the non-Newtonian behavior is strong,a unique phenomenon was observed that the distance between two droplets disappears and the droplets are squeezed into a "flat shape" when the deformation index is less than zero.The results show that the deformation has not been observed for any continuous phase flow rates at low dispersed phase flow rates and the values of the deformation index are always positive.However,for high dispersed phase flow rates,the values of the deformation index decrease to a negative number and then increase as the flow rate ratio increases.Besides,the droplet velocity can also be affected,and the relationship between droplet velocity and flow rate ratio becomes quadratic rather than linear.The present work also investigated the effects of the power-law index,n,and consistency coefficient,K,of the power-law continuous phase on droplet formation using CFD method.The results show that the droplet length and distance between two droplets decreases with the increasing n and K,which is caused by the increase in the apparent viscosity of the continuous phase.The decrease is much more obvious in the transition stage between dripping and jetting and near zero in jetting.In contrast,the formation frequency and droplet velocity increase with the increasing n and K.The results also show that the effects of n are much greater than that of K.Besides,considering the effects of continuous phase rheological parameters,this work included a method to calculate the averaged shear rate of power-law non-Newtonian flows in microchannels,which was used to obtain the continuous phase Capillary number and develop prediction models for droplet length and formation frequency.These models can predict the properties well for droplet formation in squeezing and dripping.The present work is useful for controlling oil-in-water droplet formation and designing microfluidic devices in the areas where non-Newtonian fluids are used as the continuous phases.