Study On Fabrication And Fluid Dynamic Behaviors Of Double Emulsions In Microfluidic Devices

Highly monodispersed double emulsions have significant potential in a wide range of applications, such as fusion energy utilization, chemical engineering, pharmaceutical, etc. However, the conventional emulsion fabrication techniques are often complicated and ill-controlled which result in polydispersed drops and limit the applicability of double emulsions in processes requiring precise control and release. Alternatively, microfluidics which circumvent the vagaries of the bulk emulsification process provide a promising route for producing uniform double emulsions in a reproducible and controllable way. Axisymmetric microchannels, in which the double emulsion drops can be manipulated individually, have been widely used due to the unprecedented monodispersity, flexibility and efficiency. Various kinds of fluids can be emulsified into monodispersed double emulsions with high degree of sphericity. Hence, multiphase flow in axisymmetric microfluidics has incurred major international research interest.To actively control double emulsions production via microfluidics, it is of significance to fully explore the fluid dynamic behaviors of the drop formation processes. Nowadays, the mechanisms of drop formation and the inherent law of influence effects on multiphase fluid flow in microscale are still not completely understood. Especially, the interaction between each fluids, the movement and deformation of the interfaces and the distinct flow patterns during emulsification are still waiting to be revealed. In this context, both numerical simuations and experimental studies are conducted to investigate the technological potential of microfluidcs thoroughly and probe the fundamental fluid dynamics behind the devices. Based on the volume of fluid (VOF) method, axisymmetric two-dimensional unsteady models of three incompressible and Newtonian fluids flow in flow-focusing and co-flowing microchannels are developed and numerically analyzed, in an effort to elucidate the dynamic behaviors of double emulsion formation. In this way, the detailed hydrodynamic information involved is provided, including the velocity field, pressure distribution, streams lines in the flow field during the evolution of the interfaces. The underlying physics of drop formation under both dripping and jetting modes are clarified to provide guidelines for better control over double emulsion morphology. By exploring the processes over wide ranges of the materials and operating parameters, the effects of physical properties and flow conditions on drop formation are discussed. The similarities and differences of drop formation processes between co-flowing system and flow-focusing system are compared as an important addition to explore the influence of the geometry on producing double emulsions in microfludics. Additionally, the design of simple and robust co-flowing and flow-focusing microfluidic devices is presented with ease of alignment. Stable formation of both monodispersed and polydispersed double emulsions is achieved in a variety of flow condtions, and typical flow patterns as well as transitional behaviors have been observed using high-speed CCD. The effects of flow rates of each fluid on drop sizes and drop formation frequencies as well as polydispersities are examined experimentally. A series of important results and conclusions are obtained as follows:(1) The double emulsion drop formation in an axisymmetric flow-focusing microfluidic device is investigated numerically by computational fluid dynamics simulation. A three-phase incompressible and Newtonian fluid system with each component is immiscible with each other is considered. The simulation produces the two typical drop formation modes, dripping and jetting with comparison to the single emulsion formation processes. Besides, the effects of flow rate ratio, viscosity ratio, and interfacial tension ratio on the double emulsion formation are clarified quantitatively. The results indicate that, the interfacial tension force dominates the drop formation processes in dripping mode whereas the viscous force from the outer fluid dominates the drop formation processes in jetting mode causing distinct fluid dynamic behaviors. Attributed to the interaction between the inner interface and the outer interface, the formation of double emulsions presents more complex characteristics than that of single emulsions. The deformation and breakup of the inner interface facilitates the deformation of the outer interface. Increasing the the flow rate of the outer fluid induces the transition from dripping to jetting and results in smaller double emulsion drops with thinner shell thickness. An abrupt increment in the radii of the outer drop and inner drop is observed during the transition due to the alteration in the dominating force. While the flow rate of the middle fluid contributes only to the drop size of the outer drop and does not bring obvious transition on drop formation regime in a wide range of flow rate ratio. Varying the viscosity of the middle fluid can also trigger the transition from dripping to jetting. In the dripping regime, the viscosity ratio of the middle fluid to the inner one has little influence on the produced drop size, while in the jetting regime, the size of the generated drop increase with the rise in this viscosity ratio. The interfacial tension force has remarkable influence on the interface shapes but does not change the drop formation regime in a wide range of interfacial tension coefficient ratios.(2) The comparisons of double emulsion formation in co-flowing and flow-focusing microfluidic devices with similar configurations are studied numerically. The similarity and differences of double emulsions formed under typical dripping and jetting mode in both systems are discussed. The transitions of drop formation modes from dripping to jetting induced by the capillary number of the outer fluid are compared. In addition, the effects of the geometry in flow-focusing microfluidic device are discussed. The results indicate that, the transition from dripping to jetting in co-flowing system is observed with increasing capillary number of the outer fluid. Yet the transition in the flow-focusing system happens at much lower capillary number of the outer fluid than that in the co-flowing system, which indicates that due to the hydrodynamic-focusing effect, the jetting in flow-focusing system requires less viscous force from the outer fluid. Both sizes of the inner drops and outer drops are more sensitive to the variation of viscous force from the outer fluid in co-flowing system. Under the same flow conditions, the double emulsion drops formed in co-flowing stream have bigger diameter and thicker shell than those formed in flow-focusing device and show slightly better monodispersity. But the differences are weakened with the increasing capillary number of the outer fluid. Compared to the co-flowing microfludic system, the orifice in the flow-focusing microfluidic device enhances the deformation and break up of the necks under dripping regimes and facilitates the formation of the coaxial jets under jetting regimes. The robust jetting occurs in flow-focusing system with small orifice radius even at low flow rate of the outer fluid producing smaller double emulsion drops with higher polydispersity. Enlarging the radius of the orifice in flow-focusing system weakens the hydrodynamic-focusing effect and causes the upstream movement of the detaching point of the drops. As a result, a significant increment in the drop size with better monodispersity is realized. On the other hand, the length of the orifice does not have obvious influence on double emulsion formation process. The drop size as well as the polydispersity is insensitive to the variation of the orifice length.(3) An axisymmetric co-flowing microfludic device comprised of three coaxial capillary tubes is designed and assembled for double emulsion production. Various emulsions are produced including the single-core double emulsions, multi-core double emulsions and binary emulsions of single and double emulsions. The behaviors of the fluids during the evolution of interfaces are recorded and discussed. The effects of flow rates of each fluid on drop sizes, drop formation frequency and polydispersity are studied. The experimental results show that, the number of inner drops, both sizes of the inner and outer drops and the drop formation modes can be actively controlled by the flow rate of the outer fluid. The thickness of the shell of the double emulsion is determined by the flow rate of the middle fluid and the inner fluid. The single core double emulsions and binary emulsions generated under dripping regimes are highly uniformed with polydispersity below 3%. While only the outer drops of the multi-core double emulsions produced under dripping regime are monodisped with polydispersity below 8%. The inner drops of multi-core double emulsions are polydispersed since the detaching process of each inner drop is affected by the last inner drop that is still inside the outer drop.(4) An axisymmetric flow-focusing microfludic device is also designed and assembled to produce double emulsions. Likewise, the emulsions with various morphologies are produced and the influence factors are discussed quantitatively. The drop formation processes in co-flowing and flow-focusing system are compared based on the recording data. In the experiment, the orifice in flow-focusing system enhance the deformation of the outer interface and facilitate the break up, hence, monodispersed single core emulsions are produced under low flow of the outer fluid and the formation of multi-core double emulsions is not observed. The distance between the adjacent double emulsion drops is much closer in flow-focusing system due to the deceleration of the fluids downstream of the orifice. Though the double emulsions generated in flow-focusing system show slightly worse monodispersity under most flow conditions than those in co-flowing system, the drop formation frequency is much higher. Moreover, the hydrodynamic-focusing effect enhanceing the deformation and breakup of the interfaces makes the flow-focusing microfludic device to be highly desired in applications that requires huge amount of smaller double emulsions.In summary, the above investigations systematically gain a further insight into the inherent law of fluid dynamic behaviors and influence factors of double emulsion formation in axisymmetric microfluidic devices, and provide an effective theoretical support for the design and optimization of microfluidic devices. The research also enriches fundermantal understandings on the emulsion dynamics and microscale multiphase fluid flow.