电场与拉伸流场共同作用下双重乳液形变特性研究

本文采用基于Cahn-Hilliard方程的相场方法(Phase-field Method)捕捉液-液流体界面,并利用电流模型(Electric Currents)表征电场力作用,通过电场与多相流控制方程的双向耦合,建立了电场和拉伸流场共同作用下双重乳液形变动力学行为非稳态理论模型并开展数值模拟研究,揭示了流场和电场对双重乳液形变行为的调控机理。研究表明:双重乳液液滴处于单一拉伸流场中外液滴产生宽扁型形变,内液滴产生瘦长型形变;通过施加外加电场可以主动抑制双重乳液的形变,使液滴在拉伸流场中维持球形。     

双乳液液滴形变及破碎特性

本文基于VOF（Volume of Fluid）相界面追踪方法,建立了不可压缩W₁/O/W₂双乳液液滴动力学模型并进行了数值求解,模拟了双板平行剪切流条件下液滴在流场中的稳定变形与破碎过程。研究结果表明:液滴的稳定变形程度随着毛细数的增大而加剧,且双乳液内液滴的变形程度要明显小于液滴整体变形程度;液滴雷诺数为0.05时,存在一个0 57～0.58之间的临界毛细数,当液滴毛细数小于临界毛细数时,液滴只发生稳定变形,反之则发生破碎。     

流动聚焦微通道中双重乳液乳化行为研究

本文基于VOF(Volume of Fluid)相界面追踪方法,建立了流动聚焦微通道中双乳液液滴形成的模型,研究了互不相溶的三种流体在微流控系统中形成双重液滴的典型物理过程,给出了双重液滴形成的典型工况,分析了流动参数和工质物性对液滴形成过程的影响。结果表明:双重乳液液滴形成过程可分为滴式与喷式两种模式;外流体流量增大时液滴形成模式由滴式转变为喷式,所形成的双重液滴尺寸减小;中间流体流量变化对液滴形成模式、所形成的双重乳液内液滴尺寸影响不明显,外液滴随中间流体流量增大而增大。     

剪切流中液滴变形断裂机理的实验研究

本文描述了剪切流动中牛顿液滴在不相溶连续体相中的变形断裂的机理,采用逆向旋转透明Couette装置及微距摄相技术研究剪切流动中液滴的断裂.实验结果表明液滴的断裂取决于毛细数和粘度比,并通过三种机理得以实现:颈缩、端部夹断和毛细不稳定性.实验结果揭示了产生毛细断裂时的临界液线直径、无量纲毛细波长与剪切率和初始液滴尺寸关系.     

旋转环形液池内双组分溶液热毛细对流研究

通过毛细对流可视化实验系统和三维数值模拟研究了旋转环形液池内受Soret效应影响的双组分溶液热毛细对流,探究了泰勒数和液池深宽比对双组分溶液热毛细对流的稳定性及失稳后产生的振荡流型的影响。结果表明,旋转液池内受Soret效应影响的热毛细对流的基础流为逆时针流胞形成的轴对称稳态流动,液池旋转对基础流径向流动的影响较小,但会明显增大流体的相对周向流动。临界热毛细雷诺数随泰勒数的增大而增大,随深宽比的增大而减小。泰勒数和深宽比的变化会明显改变双组分溶液热毛细对流失稳后产生的振荡流型。随着泰勒数的增大,液池中会出现HTWs和内部波动共存的振荡流型,内部振荡由包含在基态流动中的旋转流胞产生。     

剪切流场中双重乳液稳态形变

建立了不可压缩双重乳液界面行为的理论模型,并可视化实验验证了所建立模型的正确性.基于所建立的理论模型,数值模拟研究了剪切流场中双重乳液的形变机理,探讨了内外液滴半径比及各相工质黏度对其形变特性的影响规律.研究结果表明:在剪切流场中,双重乳液的稳态形变程度随着毛细数的增大而加剧,且内液滴抗形变能力比外液滴更强;随着内外液滴半径比的增大,双重乳液内外液滴间相互作用增强,导致内液滴形变程度增大,同时内液滴抗形变效应逐渐凸显,造成外液滴形变程度减小;双重乳液自身黏性是液滴形变的一种阻力,随着内、外液滴黏度的升高,双重乳液整体形变程度均减小,并且乳液外液滴相黏度变化对双重乳液稳态形变程度的影响更为明显.     

荷电液滴捕集细颗粒物的数值模拟研究

对固定荷电液滴吸附细颗粒物进行了数值模拟计算。采用微漏电模型计算电场分布和作用在液滴上的电场力。利用VOF模型追踪液滴自由界面,考虑自由界面流体体积分数过渡引起的物性参数变化。通过求解牛顿方程计算固体细颗粒物的运动轨迹。计算模拟了细颗粒物在库仑力、电场力、流体黏阻以及重力的作用下脱离基底被荷电液滴捕集的全过程,分析了不同电场强度、颗粒粒径以及液滴粒径等因素对吸附效率的影响。计算结果与实验结果吻合较好。     

三相流体的轴对称格子Boltzmann模型及其在Rayleigh-Plateau不稳定性的应用

基于多组分相场理论提出了一类模拟三相流体流动的轴对称格子Boltzmann模型.该模型利用两个粒子分布函数来捕捉三种不同流体之间的相界面,另一个粒子分布函数来求解流体动力学方程以获得流场信息.为了刻画坐标变换引起的轴对称效应,巧妙地设计了演化方程中平衡态分布函数和外力项分布函数,从理论上保证本文模型可以正确恢复三相流体系统的宏观控制方程,并且轴对称效应产生的源项中不包含任何复杂的梯度项,从而比现有的轴对称格子Boltzmann模型更加简单高效.首先通过模拟一系列轴对称多相流的基准算例,包括静态的双液滴、液体透镜的扩展和二元流体Rayleigh-Plateau不稳定性,来验证本文模型的有效性与正确性.接下来,利用该模型研究了三相流体的Rayleigh-Plateau不稳定性的增长过程,定量分析了波数和液柱半径比对复合液体线程破裂过程中界面动力学行为、界面破裂时间以及生成子液滴尺寸的影响.可以发现复合的液体线程在波数较大时破裂生成一个复合主液滴和卫星液滴,而在波数较小时可以生成更多数量的卫星液滴,这导致复合主液滴和卫星液滴的尺寸随着波数的增加呈现先增大而后减少的趋势.另外,我们发现内部流体...     

微圆管内流动凝结换热的实验研究

本文对水平微圆管内凝结两相流流型进行了可视化观察,对流动和换热特性进行了实验研究。实验中只观察到 三种流型:柱塞状流、环状流和毛细泡状流。柱塞状流在质量小流量时才出现,流量较大时,流型以环状流和毛细泡状流 为主。实验的Nu数在某一Re数下具有最大值,而流动凝结的压降随Re数的增大单调增大。     

平板上蒸发液滴内表面张力作用下流动的研究

一、前言 相界面的传热、传质过程往往伴随着流体内部的对流运动。由界面张力(表面张力)驱动的流体流动,对于热、质交换的强化起着重要作用。人们对于平板上水平液膜,流体介质中的液滴等现象进行了广泛的研究,揭示了许多表面张力驱动流动的性质,以及它与传热、传质的关系。对于水平平板上厚度小于1mm的薄液膜,液膜内的流动主要是由于温差引起的表面张力梯度触发的Bénard细胞流。实验研究发现:在水平平板上蒸发的液滴内也会出现Bénard细胞流。但是,液滴与液膜存在形状上的差异,故内部流动形式也将不同。若把小接触角的液滴看作是各处厚度不等的液膜,可以预见滴内的细胞     

Y型微通道内双重乳液流动破裂机理

基于体积分数法建立了Y型微通道中双重乳液流动非稳态理论模型,数值模拟研究了Y型微通道内双重乳液破裂情况,详细分析了双重乳液流经Y型微通道时的流场信息以及双重乳液形变参数演化特性,定量地给出了双重乳液流动破裂的驱动以及阻碍作用,揭示了双重乳液破裂流型的内在机理.研究结果表明:流经Y型微通道时,双重乳液受上游压力驱动产生形变,形变过程中乳液两端界面张力差阻碍双重乳液形变破裂,两者正相关;隧道的出现将减缓双重乳液外液滴颈部收缩速率以及沿流向拉伸的速率,并减缓了内液滴沿流向拉伸的速率,其对于内液滴颈部收缩速率影响不大;隧道破裂和不破裂工况临界线可以采用幂律关系式l~*=βCa~b进行预测,隧道破裂和阻塞破裂工况临界线可以采用线性关系l~*=α描述;与单乳液运动相图相比,双重乳液运动相图各工况的分界线关系式系数α和β均相应增大.     

Field-induced formation and manipulation of a microscale liquid ball on an outside wall of the pulled pipette

We report a phenomenological observation of electric-field-induced formation and manipulation of liquid ball on an outer wall of the pulled pipette by using the quartz tuning fork-based atomic force microscope (QTF-AFM). The dye molecule solution with excitation wavelength of 488 nm and detection efficiency of 95% is used to investigate the movement characteristics of liquid droplets when the electric field is applied. The ejected liquid solution forms a microscale liquid droplet at the apex of the pipette by the application of electric field, containing dye molecules, which climbs up along the negatively charged outer surface of the pipette due to the electro-osmosis effect. With positive or negative bias voltages, we manipulate a liquid ball to slide upward or downward, respectively. This field-induced transport of a liquid droplet may be useful to nano-biotechnology or droplet-based microfluidic technology, for example, noncontact delivery and manipulation of liquid solution in the form of separated droplets.     

Ultrasonic surface acoustic wave-assisted separation of microscale droplets with varying acoustic impedance

In droplet-based microfluidic platforms, precise separation of microscale droplets of different chemical composition is increasingly necessary for high-throughput combinatorial chemistry in drug discovery and screening assays. A variety of droplet sorting methods have been proposed, in which droplets of the same kind are translocated. However, there has been relatively less effort in developing techniques to separate the uniform-sized droplets of different chemical composition. Most of the previous droplet sorting or separation techniques either rely on the droplet size for the separation marker or adopt on-demand application of a force field for the droplet sorting or separation. The existing droplet microfluidic separation techniques based on the in-droplet chemical composition are still in infancy because of the technical difficulties. In this study, we propose an acoustofluidic method to simultaneously separate microscale droplets of the same volume and dissimilar acoustic impedance using ultrasonic surface acoustic wave (SAW)-induced acoustic radiation force (ARF). For extensive investigation on the SAW-induced ARF acting on both cylindrical and spherical droplets, we first performed a set of the droplet sorting experiments under varying conditions of acoustic impedance of the dispersed phase fluid, droplet velocity, and wave amplitude. Moreover, for elucidation of the underlying physics, a new dimensionless number AR_D was introduced, which was defined as the ratio of the ARF to the drag force acting on the droplets. The experimental results were comparatively analyzed by using a ray acoustics approach and found to be in good agreement with the theoretical estimation. Based on the findings, we successfully demonstrated the simultaneous separation of uniform-sized droplets of the different acoustic impedance under continuous application of the acoustic field in a label-free and detection-free manner. Insomuch as on-chip, precise separation of multiple kinds of droplets is critical in many droplet microfluidic applications, the proposed acoustofluidic approach will provide new prospects for microscale droplet separation.     

Forming concentric double-emulsion droplets using electric fields

Double-emulsion droplets may be assembled into highly concentric shells using a uniform AC electric field to induce dipole/dipole interactions. The resulting force centers the inner droplet with respect to the outer shell if the outer droplet has a higher dielectric constant than the ambient, suspending liquid. The dielectric constant of the inner droplet does not influence this condition. Applying an electric field >10⁴ V_rms/m achieves centering of approximately 3–6 mm diameter droplets suspended in ～10 centipoise liquids within ～60 s. If the outer shell is electrically conductive, the effect depends strongly on frequency. In the case of the monomer-containing liquids requisite to forming foam shells for laser target fabrication, the electrical field frequency must be ～10 MHz or higher. Because of very stringent requirements imposed on the concentricity and sphericity of laser targets, electric field induced droplet distortion must be minimized. Consequently, the liquid constituents must be matched in density to ～0.1%.     

Dynamics of a dielectric droplet suspended in a magnetic fluid in electric and magnetic fields

The behavior of a microdrop of dielectric liquid suspended in a magnetic fluid and exposed to the action of electric and magnetic fields is studied experimentally. With increasing electric field, the deformation of droplets into oblate ellipsoid, toroid and curved toroid was observed. At the further increase in the electric field, the bursting of droplets was also revealed. The electrorotation of deformed droplets was observed and investigated. The influence of an additional magnetic field on the droplet dynamics was studied. The main features of the droplet dynamics were interpreted and theoretically examined.     

Hydrodynamic binary coalescence of droplets under air flow in a hydrophobic microchannel

Based on the volume of fluid(VOF) method, we conduct a numerical simulation to study the hydrodynamic binary coalescence of droplets under air flow in a hydrophobic rectangular microchannel. Two distinct regimes, coalescence followed by sliding motion and that followed by detaching motion, are identified and discussed. Additionally, the detailed hydrodynamic information behind the binary coalescence is provided, based on which a dynamic mechanical analysis is conducted to reveal the hydrodynamic mechanisms underlying these two regimes. The simulation results indicate that the sliding motion of droplets is driven by the drag force and restrained by the adhesion force induced by the interfacial tension along the main flow direction. The detachment(i.e., upward motion) of the droplet is driven by the lift force associated with an aerodynamic lifting pressure difference imposed on the coalescent droplet, and also restrained by the adhesion force perpendicular to the main flow direction. Especially, the lift force is mainly induced by an aerodynamic lifting pressure difference imposed on the coalescent droplet. Two typical regimes can be quantitatively recognized by a regime diagram depending on Re and We. The higher Re and We respectively lead to relatively larger lift forces and smaller adhesion forces acting on the droplet, both of which are helpful to detachment of the coalesced droplet.     

Surface-tension-confined droplet microfluidics

This article is a concise overview about the developing microfluidic systems named surface-tension-confined droplet microfluidics(STORMs). Different from traditional complexed droplet microfluidics which generated and confined the droplets by three-dimensional(3D) poly(dimethylsiloxane)-based microchannels, STORM systems provide twodimensional(2D) platforms for control of droplets. STORM devices utilize surface energy, with methods such as surface chemical modification and mechanical processing, to control the movement of fluid droplets. Various STORM devices have been readily prepared, with distinct advantages over conventional droplet microfluidics, which generated and confined the droplets by 3D poly(dimethylsiloxane)-based microchannels, such as significant reduction of energy consumption necessary for device operation, facile or even direct introduction of droplets onto patterned surface without external driving force such as a micropump, thus increased frequency or efficiency of droplets generation of specific STORM device, among others. Thus, STORM devices can be excellent alternatives for majority areas in droplet microfluidics and irreplaceable choices in certain fields by contrast. In this review, fabrication methods or strategies, manipulation methods or mechanisms,and main applications of STORM devices are introduced.     

Dripping to jetting transitions in coflowing liquid streams

A liquid forced through an orifice into an immiscible fluid ultimately breaks into drops due to surface tension. Drop formation can occur right at the orifice in a dripping process. Alternatively, the inner fluid can form a jet, which breaks into drops further downstream. The transition from dripping to jetting is not understood for coflowing fluid streams, unlike the case of drop formation in air. We show that in a coflowing stream this transition can be characterized by a state diagram that depends on the capillary number of the outer fluid and the Weber number of the inner fluid.     

Modeling simple-jet mode electrohydrodynamic-atomization droplets' trajectories and spray pattern for a single nozzle system

Electrohydrodynamic atomization (EHDA), or simply Electrospraying is the process of influencing the breakup of a liquid into droplets by using a strong electric field. There can be different modes of Electrospraying depending, basically, on the created electric field strength and the liquid flowrate, for a specified liquid. Among these modes, the so-called cone-jet mode is the most explored one. This is due to its ability to produce highly charged monodisperse droplets in the nano- to micro-meter size range. Another mode of interest, which can also produce monodisperse droplets is the simple-jet mode. This mode is less explored when compared to the former. Within the papers that were explored by the authors, Agostinho et al. (2012) were the first authors to carefully investigate and characterize this mode. In their work, the authors reported about the influence of the electric field and the liquid flowrate on the droplets' size and spray dispersion. They also pointed out that the charge on these droplets can be expressed as a certain percentage of their Rayleigh limit.So far, there has been no model proposed to describe the droplets' trajectories in the simple-jet mode. This paper describes the design and the implementation of a physical model for determining the droplet trajectories in this mode. The model is done, specifically, for a single nozzle/ring-up configuration. It is a two-dimensional model, which solves the force balance equation for each droplet breaking up from the jet. It takes into consideration; the initial droplet velocity, the force of gravity, the electric field force, the inter-droplet coulombic force and the drag force. The droplets' deformation and reorientation were hypothesized, from observations, to play a major role in initiating the droplets' dispersion. They were simulated by implementing periodic displacements on the droplets' center of charge from its center of mass. The calculated droplets' trajectories' envelope angle was fitted to the experimental envelope angle by adjusting the droplet charge around the values that were reported by Agostinho et al. (2012). The model was validated by comparing the shapes of the theoretical and experimental sprays.The model offers new possibilities of modeling the droplets' trajectories in complex geometries, and of introducing additional forces to manipulate their trajectories in the simple-jet mode.     

T型微通道中液滴半阻塞不对称分裂行为研究

液滴不对称分裂是获得不同尺寸微液滴的优选方法,研究液滴不对称分裂行为对于生物医学、能源化工及食品工程等领域具有重要意义.本文研制T型微通道芯片并设计搭建T型微通道液滴半阻塞不对称分裂行为可视化实验平台,研究流量调控对微液滴分裂比的影响规律,并建立理论模型对分裂比进行预测,得到以下结论:液滴不对称挤压分裂过程分为挤压前期、挤压后期和快速夹断阶段,在挤压前期,液滴颈部宽度随时间呈线性变化,在挤压后期,颈部宽度随时间呈指数关系,而在快速夹断阶段,液滴颈部向心收缩的界面附加压力占主导,液滴颈部宽度剧烈收缩,呈断崖式减小;调控分支通道流量可对液滴不对称分裂比进行调控,且调控作用受毛细数影响较大;基于液液流动压降模型的液滴分裂比预测模型能够有效预测液滴分裂比.