双乳液的制备及其流体动力学行为研究

发布时间：2018-05-10 16:01

  本文选题：双乳液 + 微流控　； 参考：《东南大学》2016年博士论文

【摘要】：单分散的高品质双乳液在聚变能源利用、化学化工、医药等行业中有着广泛的应用。传统的双乳液生产工艺通常伴随强烈的振荡过程,制备过程的重复性差,系统的可控性与原料消耗率等指标不尽人意,所生成的双液滴中内、外液滴的尺寸均一性都难以得到保证。近年来,微流控技术的蓬勃发展为单分散的高品质双乳液制备提供了一条有效途径。特别是,轴对称三维微流控装置作为被动式乳化系统的代表,具有高度的可控性和原料利用率,所制备的双乳液单分散性好、均一度高、球形度佳,表现出了优越的流变特性,可用于多种流体的乳化,相比传统工艺有了很大进步,该方向的研究现已成为工程热物理微尺度多相流学科的一个前沿研究热点。目前,双乳液的生成过程及其流体动力学行为仍然缺乏深入、系统的研究,尤其是各相流体之间的相互作用方式、界面的运动与破裂的规律、流动状态对乳化过程的影响机理等尚未得到完全揭示。为此,本文基于VOF相界面追踪方法,建立了流动聚焦式微通道和协流式微通道中乳化过程流体动力学行为特性的理论分析模型。数值研究了两种系统中双液滴的形成过程；给出了典型乳化模式的流型演化；分析了流体物性、流动状况对双液滴形成过程的影响；对比了协流结构与流动聚焦结构的中液滴形成过程的异同；探讨了微通道局部结构的变化对乳化模式及其转换的影响。同时,本文还设计并搭建了协流式与流动聚焦式两套微流控乳化装置实验台,制备了多种形态的双乳液,采用高速动态摄像装置观测并记录了两种系统中双液滴的生成过程,分析了各相流量对乳化过程的影响,对比了两种微流控装置所生成的双液滴尺寸、生成频率以及多分散度等特性。概括起来,本文的研究内容及获得的主要研究结论如下：开展了流动聚焦式微通道中双乳液生成过程的理论建模和数值模拟研究,复现了滴式与喷式两种典型乳化模式,对比了单、双乳液生成过程的异同,建立了流量比、粘度比、界面张力比与所生成的双液滴尺寸间的定量联系,并阐明了流动状况与流体物性对乳化过程的影响机理。数值研究表明：滴式模式下液滴脱离的驱动力是界面张力,而喷式模式下则是外流体粘性力,因此两种模式具有不同的界面形状与流体动力学行为。双乳液的生成过程相比单乳液更复杂,内界面的形变与破裂加速了外界面的形变过程。在滴式模式下,增大外流体的流量将引发乳化模式向喷式的转换,所生成的双液滴的尺寸与壁厚都随之减小,但流型转换时尺寸的变化有不连续的突增。中间流体流量在很大范围内只对双液滴的壁厚起到决定性的作用,对液滴生成模式和内液滴尺寸影响均不明显。滴式模式下,改变中间流体的粘度直至超过其他流体粘度时,液滴生成模式由滴式突然转变为喷式,所生成的液滴尺寸减小。界面张力系数的改变对相界面的形状有显著的影响,但对所形成的液滴尺寸几乎不起作用。基于轴对称微通道中的双乳液生成过程的数值研究,对比了协流系统与流动聚焦系统中典型乳化模式,分析了外流体毛细数、通道局部结构对乳液乳化过程的影响机理。数值研究结果表明：协流式微通道中,外流体毛细数的增大也会引起乳化模式从滴式向喷式的转换,但转换的临界毛细数要远大于流动聚焦式系统中的临界毛细数,并且双液滴尺寸与壁厚对外流体毛细数的变化更敏感。在相同物性与流动参数条件下,流动聚焦式微通道中双液滴的脱离位置总是比协流式微通道中更靠近下游,并且所生成的双液滴尺寸与壁厚更小,单分散性稍逊,但这一差异随着外流体毛细数的增大逐渐减弱。与协流式微通道相比,流动聚焦式微通道中聚焦孔的存在在滴式模式下起到了促进相界面变形、加速双液滴脱落的作用,在喷式模式下则起到了促进喷射段形成的作用。聚焦孔半径较小的情况下双液滴更容易在喷式模式下生成,所生成的液滴尺寸较小、多分散度较高；随着聚焦孔半径增大直至流动聚焦结构变成协流结构,水力聚焦作用逐渐减弱,双液滴的脱离位置向上游移动、所生成的液滴尺寸增大、多分散度降低。此外,聚焦孔深度的变化不对双乳液生成模式造成任何影响,并且对所生成的双液滴尺寸、壁厚以及单分散性的作用也不明显。设计搭建了基于协流结构的双乳液制备与可视化观测实验台,开展了油包水包油双乳液的制备实验,获得了单核双乳液、多核双乳液、单乳液和双乳液混合乳液等多种形态的乳液,分析了多个典型工况中界面的形变过程,获得了各相流量与双液滴尺寸、液滴生成频率、多分散度的定量关系。实验研究结果表明：内液滴的个数、双液滴的尺寸、液滴生成模式与生成频率等均可以通过外流体流量进行精确控制,双液滴壁厚则通过中间流体与内流体流量进行调整。低流速的滴式模式下所制备的单核双乳液和单、双乳液混合的二元乳液尺寸均一性好,多分散度能够控制在3%以内。而滴式模式下制备的多核双乳液外液滴的多分散度能够控制在8%以内,但每一个内液滴在脱离时均受到前-个内液滴的影响,因而内液滴尺寸是多分散的。设计搭建了基于流动聚焦结构的双乳液制备与可视化观测实验台,同样开展了油包水包油双乳液的制备实验,获得了多种形态的乳液以及流量与液滴尺寸、生成频率、多分散度的定量关系,并对比分析了协流系统与流动聚焦系统中乳化过程。实验研究结果表明：在与协流式系统相同的实验条件下,聚焦孔的存在促进了相界面的形变与破裂,因而低外流体流速下的流动聚焦系统中通常不会生成多核双乳液。流体在通过聚焦孔后的减速使得双液滴在外流体通道中的排列更紧密,合并的可能性较大。在同样的流量与物性参数条件下,相比协流式微通道,采用流动聚焦式微通道所制备的双乳液虽然单分散性稍逊,但是其中液滴生成频率相对较高,在高效制备小尺寸双乳液的场合具有更好的效果。本文工作较为系统地揭示了轴对称微流控装置中双乳液生成的流动形态演化、乳化模式转换机理以及通道结构的影响,相关研究成果可为乳液制备微流控装置的设计与优化提供有力的理论与关键技术支撑,同时,也是对微小结构内液液多相流动、界面相互作用及其流体动力学行为基础理论的重要补充和完善。
[Abstract]:The single dispersed high quality double emulsion is widely used in the field of fusion energy utilization, chemical industry, medicine and other industries. The traditional double emulsion production process is usually accompanied by strong oscillation process, poor reproducibility of the preparation process, unsatisfactory indexes of system controllability and raw material consumption, and the formation of the inner and outer drops in the double droplets. In recent years, the rapid development of microfluidic technology has provided an effective way for the preparation of monodisperse high quality double emulsion. In particular, the axisymmetric three-dimensional microfluidic device, represented by the passive emulsion system, has high controllability and raw material utilization, and the prepared double emulsion has good monodispersibility. It has a high degree of uniformity and good sphericity, showing a superior rheological property, which can be used in the emulsification of various fluids. Compared with the traditional technology, the research of this direction has become a frontier research hotspot in the subject of Engineering Thermo physics microscale multiphase flow. At present, the process of producing double emulsion and its hydrodynamics are still short of depth. The study of the system, especially the interaction mode of each phase fluid, the law of the movement and rupture of the interface, the influence mechanism of the flow state to the emulsification process, has not been fully revealed. Therefore, based on the VOF phase interface tracing method, the fluid dynamics of the flow focusing microchannel and the co flow microchannel are established. The theoretical analysis model of behavior characteristics is studied. The formation process of double droplets in the two systems is numerically studied. The flow pattern evolution of the typical emulsion mode is given. The effects of fluid physical properties and flow conditions on the formation of double droplets are analyzed, and the similarities and differences of the formation of the droplets in the flow focusing structure are compared with the flow focusing structure. The influence of the change of the local structure of the channel on the emulsification mode and its conversion. At the same time, this paper also designed and built a two set of micro fluidic emulsification device experiment platform, which is co flow and flow focusing. A variety of double emulsion forms are prepared. The formation process of double droplets in the two systems is observed and recorded by high speed dynamic camera. The effect of flow on the emulsification process is compared with the characteristics of the double droplet size, generation frequency and multi dispersion generated by two kinds of microfluidic devices. Two typical emulsification modes of drop type and spray type are presented, and the difference between single and double emulsion production processes is compared. The quantitative relation between flow ratio, viscosity ratio, interfacial tension ratio and the size of the produced double droplets is established. The mechanism of the flow situation and the effect of fluid property on the emulsification process is clarified. The driving force is the interfacial tension, while the spray mode is the viscous force of the external fluid, so the two modes have different interface shape and hydrodynamic behavior. The formation of double emulsion is more complex than the single emulsion. The deformation and fracture of the inner interface accelerates the deformation process of the external interface. In the drop mode, the flow rate of the external fluid is increased. The size and wall thickness of the two droplets will decrease with the conversion of the emulsification mode to the spray type, but the change of the size of the flow pattern is incontiguous. The flow rate of the middle fluid only plays a decisive role in the wall thickness of the two droplets in a large range, and the droplet generation mode and the droplet size are not obvious. In the mode, when the viscosity of the intermediate fluid is changed until the viscosity of the other fluid is exceeded, the droplet generation mode is suddenly transformed from the drop type to the spray type, and the droplet size decreases. The change of the interfacial tension coefficient has a significant influence on the shape of the phase interface, but it almost does not play a role in the formation of the droplet size inch. The numerical study of the production process of double emulsion is compared with the typical emulsification mode in the coflow system and the flow focusing system. The influence mechanism of the number of outer body hair and the local structure of the channel on the emulsion emulsification is analyzed. The numerical results show that in the coflux microchannel, the increase of the number of outer body hair will also cause the emulsification mode from the drop to spray. However, the critical capillary number of the conversion is far greater than the critical capillary number in the flow focusing system, and the size of the double droplet is more sensitive to the change of the capillary number of the external fluid. Under the same physical and flow parameters, the removal of the double droplets in the flow focused microchannel is always closer to the lower than the covariance microchannel. The size of the two droplets is smaller and the wall thickness is smaller and the monodisperse is slightly inferior, but this difference decreases with the increase of the number of outer body hair. Compared with the coflow microchannel, the existence of the focusing hole in the flow focused microchannel is the effect of accelerating the phase boundary surface deformation and accelerating the drop off of the double droplets in the drop mode. When the radius of the focusing hole is smaller, the two droplets are more easily generated in the spray mode, and the droplets are smaller and have a higher dispersion. With the increasing of the radius of the focusing hole until the flow focusing structure becomes a cocurrent structure, the hydraulic focusing function gradually decreases and the dislocations of the double droplets are displaced. In addition, the change of the depth of the focusing hole does not affect the pattern of the double emulsion generation, and the effect on the size of the droplets, the thickness of the wall and the monodisperse is not obvious. The preparation and visual observation of the double emulsion based on the coflow structure are designed and built. The experimental platform has carried out the preparation of oil package oil and oil double emulsion. A variety of emulsion forms, such as mononuclear double emulsion, multi core double emulsion, single emulsion and double emulsion mixed emulsion, have been obtained. The deformation process of interface in several typical working conditions is analyzed. The quantitative relation of phase flow and double droplet size, droplet generation frequency and polydispersity is obtained. The experimental results show that the number of internal droplets, the size of double droplets, the mode of droplet generation and the generation frequency can be accurately controlled by the flow rate of the external fluid. The thickness of the double droplets is adjusted by the intermediate fluid and the flow of the inner fluid. The mixture of mononuclear double emulsion and single, double emulsion prepared in the drop mode of low flow rate is two. The size uniformity of the emulsion is good, and the polydispersity can be controlled within 3%. The multi dispersion of the multi core double emulsion droplets prepared under the drop mode can be controlled within 8%, but the droplets in each of the internal droplets are affected by the droplets in the front and the inner droplets, so the internal droplet size is dispersed. The flow focusing structure is designed and built. The double emulsion preparation and visual observation test bench have also carried out the preparation experiments of oil package oil double emulsion. The quantitative relationship of various forms of emulsion, the flow rate and droplet size, the generation frequency and the dispersion degree are obtained, and the emulsification process in the coflow system and the flow focusing system is compared and analyzed. The experimental results show that: Under the same experimental conditions, the existence of the focusing hole promotes the deformation and fracture of the phase interface under the same experimental conditions. Therefore, the multi core double emulsion is not usually generated in the flow focusing system at the low flow velocity. The reduction of the fluid after the focusing hole makes the arrangement of the double droplets more closely arranged in the passage of the external flow body, and the possibility of merging is larger. Under the same flow and physical parameters, compared with the coflow microchannel, the double emulsion prepared by the flow focusing microchannel is slightly less monodisperse, but the droplet generation frequency is relatively high, and it has better effect in the high efficiency preparation of small size double emulsion. This paper has systematically revealed the axisymmetric microsphere. The flow morphology evolution, the mechanism of the emulsion mode conversion and the influence of the channel structure in the flow control device can provide a powerful theoretical and key technical support for the design and optimization of the emulsion preparation of microfluidic devices. At the same time, it is also the liquid and liquid multiphase flow, interface interaction and fluid dynamics in the micro structure. The important supplement and perfection of the basic theory of mechanical behavior.

【学位授予单位】：东南大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O35
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本文编号：1869954