Study On Generation And Fluid Dynamic Behaviors Of Multiple Emulsions In Three-dimensional Microfluidic Devices

Multiple emulsions,which possess a distinctive multi-interface coupling structure and inner droplet designability,are widely used in many hot-point and forward position fields,such as fusion energy utilization,biomedicine,and advanced materials.Efficient and controllable preparation is a primary and necessary prerequisite for the successful application of multiple emulsions.The traditional multi-step emulsification methods(such as the mechanical stirring method,membrane emulsification method)involve incompact process control and large material consumption.Compared with traditional emulsification methods with low yield and poor monodispersity,microfluidic technologies can be used to realize the effective organization and precise manipulation of multiphase fluid flow and have thus become a preferred solution to solve bottleneck problems.Moreover,microfluidic chips have many advantages,such as small size,integration,and high degree of automation.Therefore,this method has become a frontier research hotspot in the field of microscale multiphase flow research.However,the traditional pump-driven source in an emulsion microfluidic system has inherent fluctuations,and the raw materials are suggested to be intermittently replenished during the preparation process,which means that the quality and preparation continuity of the emulsions still needs to be improved.At the same time,traditional two-dimensional soft lithography microfluidic chips still struggle with problems in the swelling by reagents and difficult surface modification.In addition,although the microfluidic control of liquid-core emulsions has become increasingly mature,the application of liquid-core emulsions are limited by the incorporation of cores and functional failures.Therefore,the method used for the preparation of double emulsions containing solid cores,which can effectively avoid the incorporation of liquid cores,is also in urgent need of development.Additionally,the emulsion yield in a singlestage microfluidic generation device is still low,and the preparation efficiency needs to be greatly improved.Furthermore,the frontier application of emulsion templates based on microfluidic generation technologies is also an important research topic.In this context,a variety of new microfluidic methods and technologies are developed for the generation of multiple emulsions.Through the combination of experimental observation,theoretical analysis and numerical simulation,multiphase flow dynamic behavior in the microfluidic preparation process of multiple emulsions is studied in depth.A gravity-driven overflow microfluidic system is designed and built to experimentally study the size parameters and monodispersity characteristics of single and double emulsions under different operational conditions.Three-dimensional flow-focusing microfluidic devices are developed to prepare solid-oil-water/solid-water-oil emulsions and explore the microfluidic generation process for double emulsions containing solid cores.Three-dimensional splitting microfluidic devices are developed to experimentally study the splitting behavior of single and double emulsions in the microfluidic splitting microchannels.Meanwhile,a multiphase flow model for the splitting process is established to numerically simulate the multiphase fluid dynamics in the emulsion splitting process.In addition,a new single-step microfluidic method for preparing multicomponent self-healing microcapsules is proposed,the self-healing microcapsules are successfully prepared and the effect of microcapsules on matrixes are tested.The main research content and conclusions are as follows:(1)The development and performance experiment for a gravity-driven overflow microfluidic system are performed.Two-unit and three-unit gravity-driven overflow microfluidic systems are designed and built,and a comparative verification experiment for the interface stability under an ultra-low interfacial fluid system is performed.Meanwhile,the formation process for single emulsions and double emulsions in the microfluidic chip are visually observed,and the size and monodispersity of single and double emulsions under different operating conditions are studied.The results indicate that the overflow design in the gravity-driven overflow microfluidic system can provide stable liquid injection pressure in the microchannel,while the semi-open liquid replenishing device can continuously replenish the fluid with minimal influence on the fluid flow.In ultra-low interfacial tension two-phase flow,the interface disturbance generated by the gravity-driven overflow microfluidic system is smaller than that in the conventional syringe-pump driven microfluidic system,thereby contributing to an improvement in the emulsion quality.In addition,the droplet sizes for both single emulsions and double emulsions can be adjusted by varying the overflow height,with the droplet sizes of double emulsions being significantly affected by the interaction between adjacent two phases.(2)The microfluidic generation of multiple emulsions encapsulating solid cores is carried out.Three-dimensional flow-focusing microfluidic devices for encapsulation of solid cores into a solid-oil-water/solid-water-oil double emulsion are developed and used to experimentally study the microencapsulation process of solid cores.The typical microencapsulation physical processes for solid cores in flow-focusing microchannels are revealed.Moreover,the effect of different flow parameters on multiple emulsion formation is discussed.Furthermore,a multichannel microencapsulation technology is proposed and constructed to encapsulate solid cores.The results indicate that the three-dimensional flow focusing microfluidic device can be used to realize three-dimensional microencapsulation of solid cores with a success rate of up to 100% and that the prepared multiple emulsions have high monodispersity.The solid-core encapsulation behavior of double emulsions in a flow-focusing microchannel can be either in the stable microencapsulation state(i.e.,stable single-core,double-core and triple-core microencapsulation states)or in the transition state(i.e.,single-double-core and double-triplecore transition encapsulation states),depending on the flow rates of the inner and outer phase fluids.Moreover,the number of solid cores encapsulated in the double emulsion for a flow-focusing microchannel is determined by the competition among the inertial force of the inner phase,the shear force of the outer phase fluid,and the interfacial tension.The microencapsulation of single,double and triple cores appears in sequence when the flow rate of the inner phase increases or the flow rate of the outer phase decreases.Regardless of encapsulation states,the solid-core encapsulation process in a flow-focusing microchannel can be divided into entering,neck stretch,neck shrinking and breakup stages.(3)Experiments to study emulsion splitting behavior in three-dimensional microfluidic channels are performed.Three-dimensional split microfluidic devices for emulsions are developed and the multifold splitting behavior of both single emulsions and double emulsions in the microfluidic devices is studied.The splitting parameters,monodispersity characteristics and splitting mechanism in microchannels under different working conditions are analyzed.The results indicate that the three-dimensional microfluidic devices can split the single emulsion and the double emulsion into double or triple parts and that the single emulsions and the double emulsions obtained by the splitting have better monodispersity.Influenced by the symmetry of the split microchannel,the daughter droplets generated in the different microchannels show a difference in monodispersity.After triple splitting,the coefficients of variation of the size of single and double emulsions are larger than that of the double splitting,which is caused by the capillary instability of the fluid flow in the three-bore capillary being stronger than that of the ?-shaped capillary,resulting in deterioration of the coefficient of variation.Additionally,the inner droplet for the double emulsions is less restricted than the outer droplet during the splitting process,which causes the size-distribution coefficient for the inner droplets to be always worse than the outer droplet after splitting.(4)The dynamic behavior of emulsion breakup in the microchannels is studied based on a VOF liquid/liquid phase interface tracking method.The multiphase flow model for the emulsion breakup process is established,and the feasibility and applicability of the theoretical model are verified.A corresponding numerical simulation is carried out to study the multiphase flow dynamics for the emulsion breakup process,revealing the evolution process for the pressure field and velocity field during droplet breakup.The influence mechanism in different flow regimes is discussed and the droplet breakup pattern is concluded.The results indicate that the droplet breakup process in the split microchannel has four flow regimes,including constant blocking breakup,slit blocking breakup,tunnel blocking breakup and non-breakup.During the breakup process,the outlet pressure of the split microchannels is uniform;during the process of non-breakup,the droplets selectively enter the split microchannel,causing significant fluctuations in the pressure of the outlet.Moreover,increasing the capillary number(Ca)of the outer phase fluid will provide greater upstream pressure and interfacial shear force,which will promote the deformation of the droplets and facilitate the emulsion breakup.In addition,a decrease in the droplet size will lead to a change in the shape of the droplets from a plug shape to a spherical shape,weakening contact with the wall surface,resulting in a decrease in the rate of droplet deformation and promoting the generation of a non-breakup regime.(5)Experiments to study the microfluidic generation of multicomponent microcapsules for self-healing materials are performed.A new method of multicomponent self-contained microcapsules based on microfluidic technology is proposed,and the multi-chamber structure of the as-prepared self-healing microcapsules is characterized by a scanning electron microscope.The regulation law for the microfluidic generation and the self-healing effect of the microcapsules on the matrixes is discussed in detail.The results indicate that the selfcontained multicomponent microcapsules prepared by the two-channel co-flow capillary microfluidic device have good monodispersity and that the coefficient of variation of the healant droplets,curing agent droplets and shell layer is less than 5%.The size of the microcapsules can be dynamically adjusted by varying the flow rate of each phase.Increasing the flow rate of the inner phase can increase the growth rate of the inner droplets,promoting the detachment of the inner droplets from the outlet of the inner capillary,and facilitating the generation of more inner droplets.Decreasing the outer phase flow rate can prolong the detachment time for the outer droplets from the middle capillary,providing more growth time for the inner droplets,facilitating the production of double emulsions with multiple inner droplets.In addition,the stress-strain characteristics of the matrix filled with microcapsules is slightly worse than those of the matrix without microcapsules,and compared with the matrix before and after healing,it is still considered that the self-healing microcapsules have a reliable repairing effect.In summary,the above investigations systematically study microfluidic methods and technologies for multiple emulsion generation and clarify the hydrodynamic behavior of multiple emulsions in microfluidic devices,the inherent law for the influence parameters and the regulation mechanism in the process of emulsion generation.The relevant research results not only provide strong support for the innovative development and design optimization of microfluidic systems but also provide important materials for the improvement of micro-scale liquid-liquid/solid-liquid multiphase flow dynamics theory.