剪切流中乳液碰撞动力学行为研究

发布时间：2018-06-26 02:02

本文选题：乳液液滴 + 液滴碰撞　；参考：《东南大学》2016年博士论文

【摘要】：乳液体系广泛应用于能源、化工、生医、材料等前沿科技领域。乳液微流控技术(droplet microfluidics)是一种借助微流控过程中互不相溶流体间相互作用来实现微乳液液滴制备及精确操控的技术,在微混合、微反应、生物封装、功能材料制备、药物运输与释放等领域中极具发展前景,己成为国际上微流控技术领域发展的新热点。乳液微流控过程中涉及系列复杂的界面现象及流体动力学问题,如乳液的形变、破碎及其间的相互作用(聚并、碰撞等)、多相离散与汇聚及其多界面演化等,直接影响到乳液微流控过程的操控精度与效率,从而决定了工程实际中乳液产品的外部形貌和内部微结构等重要品质指标。乳液体系的多相界面演化及流体动力学行为机理已成为乳液微流控技术基础研究的前沿热点。液滴碰撞是液滴群乳液体系中液滴间相互作用的基本形式,特别是在液滴密集度高的情况下,液滴碰撞行为直接影响乳液液滴在流场中的形态演变。因此,充分认识液滴碰撞动力学行为,不仅对多相流体动力学具有重要的学术价值,而且对乳液液滴精密调控也具有重要的现实意义。由于多界面间相互作用过程的复杂性,当前有关微流控过程外流场中乳液液滴间碰撞行为研究仍处于起步阶段,特别是具有多界面嵌套结构的双重乳液液滴间的相互碰撞特性及其内在机理仍未得到充分揭示。为此,本文将采用实验观测、理论分析、数值模拟相结合的研究方法,开展外流场内单(双)乳液液滴碰撞过程的动力学行为的研究。基于VOF相界面追踪方法,建立了外流场内不可压缩单乳液液滴和双乳液液滴碰撞过程的非稳态模型,并设计搭建了剪切流场中单(双)乳液液滴碰撞的高速可视化实验平台,结合数值模拟与可视化实验,研究了剪切流场内乳液液滴碰撞过程中液滴的形变及相互作用等动力学行为,探讨了单(双)乳液液滴不同碰撞行为的产生机理及其演化特性,揭示了流场结构、液滴尺寸、液滴结构、液滴初始位置、流速等参数对乳液液滴碰撞动力学行为的影响规律。概括起来,本文的主要研究内容及研究结论如下:设计搭建剪切流场中单(双)乳液液滴碰撞的高速可视化实验平台,观察到乳液液滴特别是双乳液液滴在剪切流场内碰撞过程中所出现的两类动力学行为:掠过式碰撞行为和回转式碰撞行为。掠过式碰撞行为的特点是随着剪切流场开始作用,乳液液滴先变形为椭圆状,随后在流场拖曳力的作用下,液滴相互靠近并碰撞,直至液滴最终相互绕过对方,碰撞完成。回转式碰撞行为的碰撞特点是随着剪切流场开始作用,液滴先变形为椭圆状,随后在流场拖曳力的作用下,液滴相互靠近一段距离后,液滴各自回转并沿与初始运动方向相反的方向相互脱离。基于VOF液/液相界面追踪方法的数值模拟复现了剪切流场中单(双)乳液液滴的碰撞过程中出现的掠过式碰撞行为和回转式碰撞行为。在乳液液滴碰撞的演化过程中明显地观察到了两种不同的流动区域,分别为横掠流区域和回转流区域。掠过式碰撞行为和回转式碰撞行为形成的原因主要取决于横掠流区域和回转流区域各自对碰撞液滴拖曳作用的竞争关系,即当横掠流区域的拖曳作用强于回转流的拖曳作用时,乳液液滴便出现掠过式碰撞行为,反之则出现回转式碰撞行为。对比剪切流场内单乳液液滴碰撞过程与双乳液液滴碰撞过程中液滴动力学行为之间的差异,揭示了内液滴在液滴碰撞过程中存在着的"促形变"及"抑形变"机制。由于双乳液液滴独特的结构特点,双乳液内液滴的存在对外液滴的形变存在两种影响机制:(a)内外液滴界面形成了两个"喉部","喉部"处的压力小,使得颈部外界面处的压力梯度减小,短轴方向上界面曲率半径增加,双乳液液滴往扁长方向发展,促进双乳液液滴的拉伸形变;(b)内液滴端部的高压区向着外液滴的短轴方向靠近,双乳液外液滴界面处压力梯度会相应增加,短轴方向上界面曲率半径减小,双乳液液滴向趋于球形趋势发展,减小了液滴形变的伸长量,抑制了双乳液外液滴的拉伸形变。基于数值模拟与实验研究相结合的方法,探索了流场受限程度、流场流速、乳液液滴的初始位置以及乳液液滴的结构尺寸等工况参数对单(双)乳液液滴动力学行为的定量影响规律。研究结果表明:(1)随着剪切流场受限程度的增加,回转流流动区域在外流场所占的比例增加,乳液液滴暴露于回转流流动区域的份额增加,受到卷吸作用增强,碰撞模式从掠过碰撞行为向回转碰撞行为转变;受限程度的增加,加速了乳液液滴的运动,缩短了液滴间相互作用的时间,增大了乳液液滴在碰撞过程中轨迹偏向流场中心的偏移量,增大了乳液液滴的形变量;(2)随着毛细数的增大,乳液液滴位于横掠流区域的体积逐渐增加,液滴受到的拖曳力逐渐增大,当液滴受到的拖曳力作用强于液滴所受到的卷吸作用时,液滴的碰撞行为从回转式向掠过式转变;毛细数的增大,加速了乳液液滴的运动,使得液滴碰撞过程所花费的时间减小,并且流场剪切强度增大,增大了乳液液滴所受到的粘性剪切力,使得液滴在相互作用过程中被拉伸的形变量增大;(3)随着两个乳液液滴间初始间距增大,乳液液滴远离流场中的回转流流动区域,使得乳液液滴受到的拖曳力部分有所增加,乳液液滴的碰撞模式从回转式碰撞行为向掠过式碰撞行为转变;(4)随着双乳液内外液滴半径比增大,内外液滴的形变趋势从过阻尼振荡向欠阻尼震荡趋势转变,并且增加内液滴尺寸会减小外液滴形变量而增大内液滴形变量。本文采用实验观测、理论分析、数值模拟相结合的研究方法,给出了剪切流场中两个单(双)乳液液滴在碰撞过程的不同动力学行为并揭示了各自的形成机理,阐明了碰撞过程中单乳液与双重乳液动力学行为的差异,探明了内液滴在乳液碰撞行为过程中的作用机制,掌握了流场受限程度、流场流速、乳液液滴的初始位置以及乳液液滴的结构尺寸等工况参数对乳液液滴动力学行为的定量影响规律。相关研究成果不仅对于深入揭示多相流体及其界面动力学行为具有重要的学术意义,而且将为液滴微流控技术工艺的设计和优化提供必要的理论及技术支撑。
[Abstract]:Emulsion system is widely used in the frontier science and technology fields such as energy, chemical, medical, and materials. Emulsion microfluidic technology (droplet microfluidics) is a technology for the preparation and precise manipulation of microemulsion droplets by the interaction of dissoluble fluids in the process of microfluidic control. Micromixing, microreaction, biological packaging, functional materials preparation, and drug preparation In the field of transportation and release, it has become a new hotspot in the field of international microfluidic technology. In the process of emulsion microfluidic, a series of complex interfacial phenomena and fluid dynamics are involved, such as the deformation, fragmentation and interaction (coalescence, collisions), multiphase dispersion and aggregation and the evolution of multi interface. And so on, it directly affects the control precision and efficiency of the emulsion microfluidic process, which determines the important quality indexes such as the external morphology and internal microstructure of the emulsion products in the engineering practice. The multi-phase interface evolution and the hydrodynamic behavior mechanism of the emulsion system have become the forefront of the research on the basic Research of the emulsion microfluidic technology. The basic form of droplet interaction in the droplet emulsion system, especially in the case of high droplet density, the droplet collision behavior directly affects the morphological evolution of the emulsion droplets in the flow field. Therefore, the full understanding of the dynamic behavior of droplets is not only of great academic value to the dynamic mechanics of multiphase fluid, but also to the emulsion liquid. Precision regulation also has important practical significance. Due to the complexity of the interaction process between multiple interfaces, the current research on the collision behavior of emulsion droplets in the flow field of the microfluidic process is still in the initial stage, especially the collision characteristics of the double emulsion droplets with multi interface nested structure and its inherent mechanism still have not yet been obtained. To this end, the dynamic behavior of single (double) emulsion droplet collision in the outflow field is studied by the method of experimental observation, theoretical analysis and numerical simulation. Based on the VOF phase interface tracing method, the instability of the collision process between the incompressible single emulsion droplets and the double emulsion droplets in the outflow field is established. A high speed visual experimental platform for single (double) emulsion droplet collision in the shear flow field is designed and built. The dynamic behavior of droplet deformation and interaction during the droplet collision process in the shear flow field is studied by combining numerical simulation and visualization experiments. The mechanism of the different collision behavior of single (double) emulsion droplets is discussed. The influence of flow structure, droplet size, droplet structure, droplet initial position, velocity and other parameters on the dynamic behavior of emulsion droplet collision is revealed. The main contents and conclusions of this paper are as follows: design and build a high-speed visual experimental platform for single (double) emulsion droplet collision in the shear flow field, Two kinds of dynamic behavior of the emulsion droplets, especially the double emulsion droplets, were observed during the collisions in the shear flow field: the passing type collision behavior and the rotary collision behavior. The characteristics of the skimming collision behavior are that the emulsion droplets first deform into ellipsoid with the beginning of the shear flow field, and then the droplets are carried out under the action of the drag force of the flow field. It is close and collided with each other until the drop of droplets eventually detours each other and collides with each other. The characteristic of the collision behavior is that with the beginning of the shear flow field, the droplets first deform into elliptical shape, and then the droplets turn each other near a distance under the action of the drag force of the flow field, and the droplets rotate each other along the opposite direction to the initial movement direction. The numerical simulation based on the VOF liquid / liquid phase interface tracing method is a numerical simulation of the swept over collision behavior and the rotary collision behavior during the collision process of the single (double) emulsion droplets in the shear flow field. In the evolution process of the emulsion droplet collision, two different flow regions are observed, respectively. The reasons for the formation of the swept over collision and the rotary collision mainly depend on the competitive relationship between the drag and drop of the droplets in the cross flow region and the rotational flow region, that is, when the drag effect of the cross flow region is stronger than the drag effect of the rotating flow, the drop of the liquid droplets will appear as a skimming collision, and vice versa. The difference between the single emulsion droplet collision process and the droplet dynamic behavior during the double emulsion droplet collision in the shear flow field is compared, and the mechanism of "deforming" and "deforming" in the process of droplet collision is revealed. There are two influence mechanisms in the external droplet deformation: (a) the internal and external droplet interface formed two "larynx", the pressure in the throat is small, the pressure gradient in the neck outside the interface is reduced, the radius of the curvature of the interface in the short axis increases, and the double emulsion droplets develop to the flat long square and promote the tensile deformation of the double emulsion droplets; (b) the end of the droplets inside (()) The pressure gradient in the high pressure area is close to the short axis of the external droplet, the pressure gradient at the interface of the double emulsion droplets will increase correspondingly, the radius of the curvature of the interface in the short axis decreases and the droplet direction of the double emulsion tends to be spherical, which reduces the elongation of the droplet deformation and inhibits the tensile deformation of the droplets outside the double emulsion. The quantitative influence of the flow field limitation, the flow velocity, the initial position of the emulsion droplet and the structure size of the emulsion droplets on the dynamic behavior of the single (double) emulsion droplets is explored. The results show that (1) the ratio of the flow area in the flow area is proportional to the increase of the limit of the shear flow field. In addition, the proportion of the emulsion droplets exposed to the flow area of the rotating flow is increased, and the volume of the emulsion droplets is enhanced. The collision mode changes from the collision behavior to the rotational collision behavior. The increase of the limitation degree accelerates the movement of the emulsion droplets, shortens the time between the droplet interaction and increases the trajectory bias of the emulsion droplet during the collision process. The displacement of the flow center increases the shape variable of the emulsion droplet; (2) with the increase of the capillary number, the volume of the droplet in the cross flow region increases gradually, the drag force of the droplet increases gradually. When the drag force of the droplet is stronger than that of the droplet, the collision behavior of the droplet is from the rotary to the sweep. The increase of the capillary number accelerates the movement of the emulsion droplets, reduces the time spent on the droplet collision process, and increases the shear strength of the flow field, increases the viscous shear force of the emulsion droplets, and increases the shape variables that are stretched during the interaction process; (3) as the initial spacing between the two emulsions increases, the droplets are increased. The droplet of the emulsion droplet is far away from the flow area in the flow field, which makes the drag force part of the emulsion droplet increase, and the collision mode of the emulsion droplet changes from the rotary collision to the swept over collision. (4) the deformation trend of the internal and external droplets from over damped oscillation to under damped oscillations with the increase of the radius ratio of the droplets inside and outside the double emulsion. In this paper, the different dynamic behavior of two single (double) emulsion droplets in the shear flow field is presented, and the formation mechanism of the droplets in the shear flow field is revealed, and the formation mechanism of the droplets in the shear flow field is revealed. The difference between the dynamic behavior of the single emulsion and the double emulsion during the collision was found. The mechanism of the internal droplet in the process of the emulsion collision was explored. The quantitative influence of the flow field limitation, the flow velocity, the initial position of the emulsion droplet and the structure size of the emulsion droplet on the dynamics of the emulsion droplet was grasped. The relevant research results not only have important academic significance to reveal the multi-phase fluid and its interfacial dynamic behavior, but also provide the necessary theoretical and technical support for the design and optimization of the liquid droplet microfluidic technology.
【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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