Study On Microfluidic Generation Of Compound Droplets And Its Mass Transfer Characteristics

Due to the distinctive properties,compound droplets are widely used in many fields,such as energy utilization,chemical analysis and biomedicine.Preparing droplets is the necessary prerequisite for its basic and applied researches.Relying on good ability of manuplating multi-phase fluid,microfluidic technology shows unique advantages in the generation of compound droplets with good controllability and high monodispersity.Moreover,based on the generated compound droplets,functional microsphere materials with a core-shell structure can be obtained through further physical and chemical treatments.Thus,it can enhance the practicability of compound droplets and expand the application fields.However,the multi-scale and multi-phase flow,phase-to-phase mass transfer,and phase interface evolution processes in the compound droplet systems involved in microfluidics are quite complex and coupled with each other,and their respective formation and development laws,especially the coupling mechanism between the processes,are far from being revealed.The reasons that affect the geometric sizes and mass transfer characteristics of compound droplets during curing still need to be clarified.Furthermore,the generation technology of compound droplets containing solid cores has always been an important branch in the research of microfluidic technology.At the same time,the current methods for generating solid-water-oil compound droplets have many problems,such as low control accuracy,poor uniformity of the thickness of liquid film,poor monodispersity of the geometric size of compound droplets.In this context,an optimized system for the generation and curing of compound droplets is established.Through experimental methods,the microfluidic generation process of compound droplet in the optimized system,the size changes and mass transfer behavior during curing are studied.A generation and curing system of compound droplets is designed and built to study the size changes during generation process,and modify the size measurement of compound droplet.The effects of osmotic pressure and oil phase concentration on the size changes and mass transfer characteristics of droplets during curing are studied by experiments.Double T-junction microchannel and the generation and curing system of compound droplets are deasigned and bulit to study the dynamic characteristics and mass transfer behavior of solid-water-oil compound droplets during the generation and curing process experimentally.The major work and conclusions of this article are summarized as follows:(1)The studies on the microfluidic generation of compound droplets are performed.The co-flowing droplets generator is developed,and the visual observation system for the generation of compound droplets is designed and built.Water-oil-water compound droplets are prepared with “two-step” methods.The effects of the flow of inner,oil and outer phase on the formation of compound droplets and the changes in geometric sizes are investigated.And the actual inner diameter of compound droplet is measured and corrected.Preliminary studies of the curing process of compound droplets are performed.The results indicate that water-oil-water compound droplets with good monodispersity can be generated stably and continuously by using the co-flowing droplets generator.The structure of compound droplets mainly depends on the relative time for two close W1 phase cores moving to the end of the oil phase pipeline and the breakup time of the O phase.And the inner and outer diameter of compound droplets is affected by the combination of the flow of inner,oil and outer phase.The actual inner diameter of compound droplets is larger than the measured value,and some correction is required.Based on the prepared compound droplets,the core-shell microspheres with better monodispersity can be prepared by rotary evaporation method.(2)The studies on the control of mass transfer characteristics of water-oil-water compound droplets during curing are performed.The effects of osmotic pressure and oil phase concentration on the geometric size changes and mass transfer characteristics of compound droplets during curing are explored.The osmotic pressure in the inner and outer aqueous phase is changed by adding anhydrous calcium chloride in the aqueous solution,and the concentration of oil phase is changed by altering the mass fraction of polystyrene.Control groups are set up for experimental researches.The results indicate that when the osmotic pressure of the W2 phase is larger than that of the W1 phase,the inner diameter of the compound droplet decreases,the effect of osmatic pressure on the inner diameter change is linear.When the osmotic pressure of the W1 phase is larger than that of the W2 phase,the inner diameter of the compound droplet increases,but the increase of the inner diameter becomes insensitive to the increasing absolute value of the osmotic pressure difference.The outer diameter of the compound droplet decreases as the osmotic pressure difference increases.As the mass fraction of polystyrene in oil phase increases,the inner diameter of the compound droplet increases after curing,but the effect of the concentration of oil phase on the inner diameter change is nolinear,and the outer diameter of compound droplet gradually increases after curing.With other conditions unchanged,the change rate of the inner diameter of compound droplet before and after curing is related to the W1 phase in the compound droplet core.However,the difference of the W1 phase in the core of compound droplet has almost no effect on the change rate of the outer diameter of compound droplet before and after curing.(3)The studies on the generation and curing of solid-water-oil compound droplets in vertical double T-junction microchannel are carried out.Double T-junction microchannel,the generation and curing system of compound droplets are designed and bulit.The formation process of solid-water-oil compound droplets in the double T-junction microchannel,the effects of surfactants on the stability of compound droplets,the change of flow pattern during the generation process,the effects of changing the flow of each phase to the geometric sizes and monodispersion,and the process of curing are mainly investigated.The results indicate that after adding surfactants,the stability of the movement of solid-water-oil compound droplets in the main channel is significantly improved,and there is an optimal value for the mass fraction of surfactant.There are three flow patterns during the formation of solid-water-oil compound droplets: plug flow,dripping flow and laminar flow.The flow pattern diagram dependent on dispersed and continuous phase is obtained.The outer diameter of solid-water-oil compound droplet decreases with increasing the flow of continuous phase,and increases with increasing the flow of dispersed phase.The structure of the double T-junction microchannel disperses the solid core and increases the flow of dispersed phase into the main channel,which makes the outer diameter of the solid-water-oil compound droplet increases,but it will also increase the number of water-oil single droplet.The monodispersity of compound droplets prepared by using double T-junction microchannels is relatively good,and the coefficient of variation of solid-water-oil compound droplets decreases as the flow of continuous phase increases.The uniformity of the distribution of the outer diamater is also improved.The research work not only provides experimental and theoretical materials for revealing the size changes and mass transfer laws during the generation and curing of compound droplets,but also provides important support for the development and improvement of microfludic generation technology of compound droplets.