The microdroplet test: experimental and finite element analysis of the dependance of failure mode on droplet shape

The microdroplet technique is usually designed as a fibre embedded in a drop of resin and subsequently pulled out while the drop is being supported by two knife edges, resulting in either debonding of the droplets from the fibres, or breakage of the fibres before debonding can occur. In this study, the microdroplet technique was performed using a platinum ring with a 40 μm hole instead of the usual two knife edges, giving an axisymmetric geometry, load and stress distribution. Glass/phenolic and glass/polyester composite systems were tested experimentally and subsequent finite element modelling studies were performed to assess the variation of droplet size, and contact angle between the droplet and fibre. It was found that contact angle is of major influence in the proposed failure model. This study characterizes the influence of the contact angle between the droplet and the fibre on the subsequent stress distribution in the microdroplet specimen.     

Directional Motion of Droplets in a Conical Tube or on a Conical Fibre

Manipulating the directional movement of liquid droplets is of significance for design and fabrication of some microfluidic devices, An energy-based method is adopted to analyse the directional movement of a droplet deposited in a conical tube or on a conical fibre. We perform an experiment to investigate the directional motion of a droplet in an open conical tube. Our theoretical analysis and experimental observations both demonstrate that surface tension can drive the droplet to move in the conical tube. The critical condition of the liquid moving in the conical tube is presented. We also analyse a droplet on a conical hydrophilic fibre, which can move from the thinner to the thicker end.     

Weibull analysis of microbond shear strength at sisal fibre–polyester resin interfaces

An analysis has been made of the tensile strength of sisal fibres and the interfacial adhesion between fibres and polyester resin droplets. Density and microscopy methods were used to determine the cross-sectional area of the sisal fibres. The average tensile strength of treated sisal fibres decreased by a modest amount following treatment with 0.06 M NaOH. However, this treatment resulted in a substantial increase in the interfacial shear strength at the sisal fibre to polyester resin interface. Weibull analysis has been used successfully to analyse variability in tensile strengths and interfacial shear strength using probability of failure plots. Scanning electron microscopy has revealed the shape of resin droplets on the surface of treated and untreated sisal fibres and contact angles are much lower for droplets on treated fibres. Damage to the surface of fibres has been examined following shear testing. Weibull analysis is an effective tool for characterising highly variable fibre properties and evaluating the level of adhesion between polymer resin and the fibre surface.     

Microdroplet targeting induced by substrate curvature

Fundamental understanding of the wettability of curved substrates is crucial for the applications of microdroplets in colloidal science, microfluidics, and heat exchanger technologies. Here we report via lattice Boltzmann simulations and energetic analysis that microdroplets show an ability of transporting selectively to appropriate substrates solely according to substrate shape(curvature), which is called the substrate-curvature-dependent droplet targeting because of its similarity to protein targeting by which proteins are transported to the appropriate destinations in the cell. Two dynamic pathways of droplet targeting are identified: one is the Ostwald ripening-like liquid transport between separated droplets via evaporating droplets on more curved convex(or less curved concave) surfaces and growing droplets on less curved convex(or more curved concave) surfaces, and the other is the directional motion of a droplet through contacting simultaneously substrates of different curvatures. Then we demonstrate analytically that droplet targeting is a thermodynamically driven process. The driving force for directional motion of droplets is the surface-curvature-induced modulation of the work of adhesion, while the Ostwald ripening-like transport is ascribed to the substrate-curvature-induced change of droplet curvature radius. Our findings of droplet targeting are potentially useful for a tremendous range of applications, such as microfluidics, thermal control, and microfabrication.     

Droplet traffic at a simple junction at low capillary numbers

We report that, when a train of confined droplets flowing through a channel reaches a junction, the droplets either are alternately distributed between the different outlets or all collect into the shortest one. We argue that this behavior is due to the hydrodynamic feedback of droplets in the different outlets on the selection process occurring at the junction. A "mean field" model, yielding semiquantitative results, offers a first guide to predict droplet traffic in branched networks.     

微尺度下锥形微操作探针表面液滴形成的研究（英）

基于液滴的转移方法可实现微操作任务中微对象的拾取,锥形操作探针则常作为一种毛细力微操作执行工具。主要研究在空气冷凝模式下锥形探针端面的液滴形成。建立了微液滴形成的数学模型,主要包括初始液滴的形成、液滴的合并和液滴的移动,研究了影响操作液滴的关键参数,分析表明:过冷度决定最小液滴半径。对单液滴的生长机制进行理论分析,并通过数值求解的方法模拟了锥形操作探针端面的液滴形成。搭建实验测试平台,实验研究了微尺度下锥形微操作探针端面的液滴形成。实验结果表明:在空气冷凝模式下,操作探针端面能够形成微液滴。经过初始液滴的形成,液滴的合并和移动等过程最终可形成稳定的微液滴,且不同锥顶角下液滴的形成呈现多样化。     

双液滴撞击平面液膜的流动与传热特性

采用耦合的水平集-体积分数法(CLSVOF)对双液滴连续撞击恒定壁温壁面上的热液膜的流动和换热特性进行了数值模拟及分析,得到了双液滴撞击热液膜后形态演变的过程.分析了液滴垂直间距、撞击速度、液膜厚度以及液滴直径对双液滴撞击液膜后的流动与传热特性的影响,结果显示,壁面平均热流密度随液滴撞击速度的增大而增大,液滴垂直间距、液膜厚度和液滴直径对平均热流密度的影响较小,但会对热流密度在撞击区域和交界区的分布产生重要影响.     

Dynamical model for chemically driven running droplets

We propose coupled evolution equations for the thickness of a liquid film and the density of an adsorbate layer on a partially wetting solid substrate. Therein, running droplets are studied assuming a chemical reaction underneath the droplets that induces a wettability gradient on the substrate and provides the driving force for droplet motion. Two different regimes for moving droplets--reaction-limited and saturated regime--are described. They correspond to increasing and decreasing velocities with increasing reaction rates and droplet sizes, respectively. The existence of the two regimes offers a natural explanation of prior experimental observations.     

Determination of temperature and concentration of a vapor–gas mixture in a wake of water droplets moving through combustion products

Characteristic temperatures and concentrations of a vapor–gas mixture in a wake of water droplets moving through combustion products (initial temperature 1170 K) were determined using the Ansys Fluent mathematical modeling package. We investigated two variants of motion: motion of two droplets (with sizes from 1 mm to 3 mm), consecutive and parallel, and motion of five staggered droplets. The influence of the relative position of droplets and also of distances between them (varied from 0.01 mm to 5 mm) on temperatures and concentrations of water vapor was established. The distances determine the relation between the evaporation areas and the total volume occupied by a droplet aggregate in the gas medium. The results of modeling for conditions that take into account vaporization on the droplet surface at average constant values of evaporation rate and also with consideration of the change in the latter, depending on the droplet temperature field, are compared. We determined conditions under which the modeling results are comparable for the assumption of a constant vaporization rate and with regard to the dependence of the latter on temperature. The earlier hypothesis on formation of a buffer vapor layer (“thermal protection”) around a droplet, which decreases the thermal flow from the external gas medium, was validated.     

微管道内两相流数值算法及在电浸润液滴控制中的应用

叙述了一种模拟电介质电润湿(electrowetting on dielectric，EWOD)下的微液滴的运动的数值方法. 采用二阶投影法求解N-S方程和level set 函数，并利用零level set函数俘获液滴运动界面，在液体与固体接触的边界上，通过引入动态接触角表征电介质表面润湿性随电势的改变. 数值计算基于MAC网格，模拟了2维微管道内与固体壁面接触的变润湿性的两种液体的分界面形状、平板上的微液滴在不同电势作用下处于不同湿润性的形态，以及微管道内改变接触角液滴的运动变形过程等算例. 关键词： 电浸润 接触角 level set函数 投影法     

Dynamics of a bouncing droplet onto a vertically vibrated interface

Low viscosity (<100 cSt) silicon oil droplets are placed on a high viscosity (1000 cSt) oil bath that vibrates vertically. The viscosity difference ensures that the droplet is more deformed than the bath interface. Droplets bounce periodically on the bath when the acceleration of its sinusoidal motion is larger than a threshold value. The threshold is minimum for a particular frequency of excitation: droplet and bath motions are in resonance. The bouncing droplet has been modeled by considering the deformation of the droplet and the lubrication force exerted by the air layer between the droplet and the bath. Threshold values are predicted and found to be in good agreement with our measurements.     

Droplet on a fibre: Surface tension and geometry

The formation of the microdroplets on fibres for the MDT test offers an opportunity to carry out surface tension measurements based on the angle of contact between the resin and the fibre. Liquid microdroplets of epoxy were deposited on glass fibres, and their diameter, length and cone angle were measured. The equilibrium shape of the droplet can be derived from the principle of conservation of work and energy. If the surface tension of the polymer droplet and the specific surface energy of the polymer coated fibre are similar, then the interfacial tension is given simply by: _γint = (1-m) _γ , where m is the gradient of a linear plot of functions of the measured parameters and _γ is the liquid surface tension. The measurement of contact angles on a droplet/fibre system have been reported previously; however, our method proposes a different geometry and consequently a simpler solution to the problem, with an additional advantage of verification on the droplet equilibrium and accuracy of measurements.     

溶液液滴在热等离子体射流中的运动和蒸发

为考察溶液注入热等离子体喷涂过程中喷雾参数对涂层质量的影响,本文建立了溶液液滴在热等离子体中运动蒸发的数学模型。模拟了液滴在不同参数下的运动和蒸发的过程,考虑了液滴、热等离子气流及液滴表面气体混合物随温度及组分的物性变化以及斯蒂芬流的影响,得到液滴的运动轨迹,蒸发速率以及半径和表面温度的变化。结果表明在一定范围内增大液...     

矩形通道内横流喷雾掺混流场的实验研究

应用PIV系统对矩形通道内横流-喷雾掺混过程中液滴沿横流方向的流场进行了实验测量.分别获得了单喷嘴和双喷嘴下横流掺混中的流场结构,以及液滴的水平速度分布.横流作用下,液滴的最大水平速度为横流速度的两倍左右;反旋涡对降低了液滴的水平速度,提高了液滴在横流中的停留时间.双喷嘴条件下,横流截面上的液滴密集区域呈狭长的三角形,...     

A study of the optics and motion of oil droplets

Recent work has shown how study of the motion of oil films on vertical surfaces under gravity can be used in viscosity measurements. Over most of such a film, the profile is determined almost entirely by the viscosity but at both upper and lower edges the effect of surface tension is appreciable. This paper presents a study of droplets under conditions in which the surface tension effect is large. There is then an interaction between the droplet shape and the forces acting on it, resulting in some interesting optical properties which are illustrated in the paper.     

Laser-induced fluorescence imaging of acetone inside evaporating and burning fuel droplets

Laser-induced fluorescence was used to visualize acetone fields inside individual droplets of pure acetone as well as droplets composed of methanol or 1-propanol initially mixed with acetone. Droplets were supported on a horizontal wire and two vaporization conditions were investigated: (1) slow evaporation in room air and (2) droplet combustion, which leads to substantially faster droplet surface regression rates. Acetone was preferentially gasified, causing its concentration in droplets to drop in time with resultant decreases in acetone fluorescence intensities. Slowly vaporizing droplets did not exhibit large spatial variations of fluorescence within droplets, indicating that these droplets were relatively well mixed. Ignition of droplets led to significant variations in fluorescence intensities within droplets, indicating that these droplets were not well mixed. Ignited droplets composed of mixtures of 1-propanol and acetone showed large time-varying changes in shapes for higher acetone concentrations, suggesting that bubble formation was occurring in these droplets.     

横向静磁场对电磁悬浮液滴稳定性的影响

在利用电磁悬浮技术实现液滴悬浮的过程中,液滴内部往往存在剧烈对流、外部伴随快速旋转和质心的水平位移等不稳定因素;因此,实现液滴的稳定悬浮是完善电磁悬浮技术的关键.本文采用实验观测的方法,通过U形静磁场组件对液滴所在空间施加横向静磁场,利用高速相机记录了不同磁场强度下纯铜熔融液滴的振荡变形过程;分析了横向静磁场对悬浮铜液滴振荡频率、振幅以及旋转的影响.实验发现:对于熔融前的固态铜颗粒,若静磁场强度超过0.3 T,铜颗粒几乎以静止状态悬浮.熔融后,当施加0.15 T的静磁场,与未加静磁场时相比,液滴俯视图轮廓线拟合出的椭圆分别与x轴和y轴的交点坐标之差R-、椭圆面积A和椭圆长轴长度Dmax的振幅分别减小了25%,76%和60%;随着磁场强度的继续增加,振幅和频率继续减小,但在静磁场强度为0.3 T时,相比静磁场强度为0.2 T,频率增加了1 Hz.横向静磁场还抑制了悬浮铜液滴的旋转,当磁场强度增加到0.53 T时,悬浮液滴只在10?的角度范围内摆动.这些结果表明,施加横向静磁场能够有效提高悬浮液滴的稳定性.     

Self-propelled running droplets on solid substrates driven by chemical reactions

We study chemically driven running droplets on a partially wetting solid substrate by means of coupled evolution equations for the thickness profile of the droplets and the density profile of an adsorbate layer. Two models are introduced corresponding to two qualitatively different types of experiments described in the literature. In both cases an adsorption or desorption reaction underneath the droplets induces a wettability gradient on the substrate and provides the driving force for droplet motion. The difference lies in the behavior of the substrate behind the droplet. In case I the substrate is irreversibly changed whereas in case II it recovers allowing for a periodic droplet movement (as long as the overall system stays far away from equilibrium). Both models allow for a non-saturated and a saturated regime of droplet movement depending on the ratio of the viscous and reactive time scales. In contrast to model I, model II allows for sitting drops at high reaction rate and zero diffusion along the substrate. The transition from running to sitting drops in model II occurs via a super- or subcritical drift-pitchfork bifurcation and may be strongly hysteretic implying a coexistence region of running and sitting drops.     

贴壁风对脱硫反应器内液滴粘壁的影响

反应器内液滴粘壁对半干法烟气脱硫效率和运行稳定性具有重要影响.本文采用数值方法研究了不同贴壁风风速和风口高度对反应器内液滴粘壁现象的影响规律.结果表明,贴壁风对减轻液滴粘壁具有一定作用.贴壁风风速越高,液滴贴壁数量越少.贴壁风衰减较快,其风口高度存在最佳值,贴壁风风口与液滴喷射口的高度距离是按照液滴直线运动粘壁时液滴高度行程的1.2倍.     

Persistent droplet motion in liquid-liquid dewetting

When a nonvolatile liquid film dewets from a partly compatible liquid substrate, the advancing dewetting front leaves behind droplets formed through a Rayleigh instability mechanism at its rim. We have found that these droplets continue to move in the direction of the dewetting front for extended periods (of order one day) with an initial droplet velocity varying linearly with the droplet size, and a displacement varying logarithmically with time. We attribute this persistent motion to a transient surface tension gradient on the substrate liquid surface trailing the dewetting front.