Study On Hydrodynamic Behaviors Of Emulsion Droplet Collision In Shear Flow

Emulsion system has been widely applied in various field including energy,chemical engineering,biomedical engineering,materials.Droplet microfluidics,which refers to the science and technology of the generation and manipulation of the discrete droplets by the virtue of the interaction between immiscible fluids inside micro devices,has significant potential in micro-mixing,micro-reaction,biological encapsulation and analysis,preparation of functional materials and targeted drug delivery and release and it has been a new hot spot in the development of microfluidics in the world.Droplet-based microfluidics involves complex free interface hydrodynamics and the basic scientific problem in fluidics,such as deformation and breakup of emulsion droplet,hydrodynamic interaction between droplets(e.g.,coalescence and collision)and so on.Such a complex problem directly affects the control precision and efficiency in the processes of the droplet microfluidics,which determines the important quality indicators of the external morphology and internal micro-structure of the emulsion products in the practical engineering.Therefore,the underlying mechanism of the above multi-interface and the underlying mechanism of the multiphase fluid dynamics have incurred major international research interest in the droplet microfluidics.Droplet collision is the basic behavior of the hydrodynamic interaction between droplets in the emulsion system of droplet cluster.Especially in the concentrated emulsion system,morphology development under external flow is deeply affected by the collision between droplets.In this context,a thorough understanding of the hydrodynamic behaviors of droplets during the collision is essential for not only the academic value in the research of droplet hydrodynamics,but also the practical significance of the the precise control of final product structures of the droplets.Nowadays,due to the complexity of the interaction between multi-interfaces,the research of the hydrodynamic behaviors(e.g.,coalescence and collision)between interactive emulsion droplets under external flow has been still in its infancy.Especially,the fundamental hydrodynamic underlying the interactive double emulsion droplets with nested structures has not been completely known.For these reasons,a combined experimental,theoretical and numerical study is undertaken to investigate the hydrodynamic characteristics of emulsion droplet collision in an external flow.Based on the volume of fluid(VOF)method,a mathematical model of the hydrodynamic behaviors of incompressible single-phase droplets and double emulsion droplets in the shear flow is developed and the experiment of the collision between single-phase droplets or double emulsion droplets under shear flow is designed and established,which is observed by high-speed CCD.A combined numerical and experimental study is undertaken to investigate the hydrodynamic behaviors of droplet collision in a shear flow.The hydrodynamics of collision behaviors between interactive droplets and the flow regimes are explored with a focus on elucidation of underlying physical mechanism.The effect of flow field configuration,droplet size,droplet structure,initial lateral separation and the fluid velocity on the hydrodynamic interaction between droplets are revealed.In summary,a series of main results and conclusions are obtained as follows:The visualized experiment using a high-speed CCD system is conducted to investigate single-phase droplet collision and double emulsion droplet collision under shear flow.Two types of droplet behaviors including passing-over motion and reversing motion are observed in the double emulsion droplet collision experiment under shear flow.The droplet firstly deforms to be elliptical shape with the shear flow applied and then two droplets approach and then pass over each other due to the drag force of the shear flow,which is defined as passing-over motion.Differing from the passing-over motion,another type of the droplet motion appears that the deformed droplets approach each other and then reverse their initial trajectory due to the entrainment,which is defined as reversing motion.Base on the VOF representing the interface,two types of droplet motion including passing-over motion and reversing motion during the droplet collision under shear are investigated.From the hydrodynamic information during the droplet collision,two distinct flow regions are observed in the matrix fluid under both passing-over and reversing motion,which are passing-flow region and reversing-flow region.The occurrence of the passing-over motion and reversing motion during the collision are governed by the competition between the drag of passing-flow and the entrainment of reversing-flow in the matrix fluid.The drag force caused by passing-flow in matrix fluid plays the dominant role in the moving of droplet,leading to the passing-over motion of the droplet during pairwise interaction.Conversely,the entrainment induced by reversing-flow in matrix fluid controls the droplets,resulting in the reversing motion of the droplets.Comparing the difference between the hydrodynamics of the single-phase droplet and double emulsion droplet during the binary droplet collision under shear flow,the mechanism of the enhancing and suppressing effect of an inner droplet on the deformation of the double emulsion droplet during the collision are revealed.Due to the unique structure of the double emulsion droplet,there are two kinds of effect mechanism of the inner droplet on the deformation of the outer droplet:(a)two 'throats' are formed between the inner and outer interfaces at the neck of the double emulsion droplets,where the low pressure decreases the pressure gradient of the outer interface at the neck,increasing the curvature radius at the interface along the half-breadth of the ellipse,which enhances the deformation of the double emulsion droplet;(b)high pressure region at the tip of the droplet moving to the half-breadth of the ellipse increases the pressure gradient of the outer interface at the neck,decreasing the curvature radius at the interface along the half-breadth of the ellipse,which suppresses the outer droplet deformation.Based on the combined experimental and numerical study,the effects of flow field configuration,fluid velocity,initial lateral separation and droplet size and structure on the hydrodynamic behaviors of droplets during the collision are investigated and the results are as follows:(1)With the increasing confinement,the reversing-flow region in matrix fluid turns to be expanded in the confined shear flow,leading to increasing role of reversing-flow on the motion of drops under pairwise interaction,which makes droplet motion under pairwise interaction transit from passing-over motion to reversing motion.Additionally,the increase in confinement accelerates the movement of the droplet which shortens the interactive time between two droplets,makes two droplets get closer at the lateral separation and enhances the droplet deformation during the collision.(2)With the increase of Ca,more volume of the droplet is exposed into the passing-over flow region due to the larger shear stress,enhancing the competitiveness of the drag force on the droplet,resulting in the transition from the reversing motion to passing-over motion.The increase in Ca accelerates the movement of the droplet which shortens the interactive time between two droplets and enlarges the deformation of the droplet due to the greater shear stress imposed on the droplet results in more stretched outer droplet during the collision.(3)As the initial lateral separation increases,the droplet is far from the zone of reversing-flow in matrix fluid and more exposed into the passing-flow region,resulting in the enlargement of the drag force imposed on the droplet by the passing-flow region,which leads to the transition from the reversing motion to passing-over motion.(4)With increasing the ratio of the inner droplet radius to the outer droplet radius k,the droplet deformation trends to be transited from overdamping to underdamping during the interaction process both in the passing-over motion and reversing motion.And the increase in k leads to smaller deformation of outer droplet while gives rise to larger deformation of inner droplet.In summary,the combined experimental,theoretical and numerical studies are undertaken to investigate the hydrodynamic characteristics of binary emulsion droplet collision in the shear flow.The droplet morphologies and underlying mechanisms of different motion behaviors during the collision is presented.The difference between the hydrodynamics of the single-phase droplet and double emulsion droplet during the binary droplet collision under shear flow are elucidated and the mechanism of the effect of an inner droplet on the deformation of the double emulsion droplet during the collision is revealed.The effects of flow field configuration,fluid velocity,initial lateral separation and droplet size and structure on the hydrodynamic behaviors of droplets during the collision are achieved.The above investigations,not only has significant academic meaning in hydrodynamic behaviors with multiple interfaces in the multiphase fluid area,but also provides an effective theoretical support for the design and optimization of droplet microfluidic technology.