Deformation and motion by gravity and magnetic field of a droplet of water-based magnetic fluid on a hydrophobic surface

Motion and deformation of a water-based magnetic fluid on a hydrophobic surface were investigated under gravity and a magnetic field. Surface energy and the resultant contact angle of the magnetic fluid depend on the surfactant concentration. The fluid viscosity is governed mainly by magnetite concentration. The front edge of the droplet moved under a weak external field. The rear edge required a higher external field for movement. The forces of gravity and the magnetic field for moving of the front edge are almost equal. However, those of the rear edge are different. The motion of magnetic fluids by an external field depends on concentrations of surfactants and magnetic particles, the external field, and experimental assembly.     

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.     

液滴撞击加热壁面传热实验研究

本文采用高速摄像仪对水滴和乙醇液滴撞击加热壁面后的蒸发过程进行了实验观测, 分析了液滴撞击加热壁面后的蒸发特性参数. 实验中, 两种液体初始温度均为20 ℃, 不锈钢壁面初始温度范围为68-126℃. 水滴初始直径为2.07 mm, 撞击壁面时Weber 数为2-44; 乙醇液滴初始直径为1.64 mm, Weber数为3-88. 结果表明, 液滴受到重力、表面张力及流动性的影响, 在蒸发过程的大部分时间内, 水滴高度持续降低而接触直径几乎不变; 蒸发后期, 液滴发生回缩, 水滴的接触直径、高度和接触角出现振荡现象. 乙醇液滴的接触角随时间的增加呈现先减小随后保持不变的趋势, 而接触直径和高度则持续减小, 直到液滴完全蒸发. 液滴蒸发总时长与液体物性和壁面温度有关, 随壁面温度的升高而减小, 与液滴撞击壁面时的Weber 数无关. 同时, 随着壁面温度的升高, 液滴显热部分占总换热量的比重增大, 显热部分能量不可忽略, 本文实验条件下得到水滴的平均热流密度为0.014-0.110 W·mm^-2.     

The shrinking instability of toroidal liquid droplets in the Stokes flow regime

We analyze the stability and dynamics of toroidal liquid droplets. In addition to the Rayleigh instabilities akin to those of a cylindrical droplet there is a shrinking instability that is unique to the topology of the torus and dominates in the limit that the aspect ratio is near one (fat tori). We first find an analytic expression for the pressure distribution inside the droplet. We then determine the velocity field in the bulk fluid, in the Stokes flow regime, by solving the biharmonic equation for the stream function. The flow pattern in the external fluid is analyzed qualitatively by exploiting symmetries. This elucidates the detailed nature of the shrinking mode and the swelling of the cross-section following from incompressibility. Finally the shrinking rate of fat toroidal droplets is derived by energy conservation.     

Mach–Zehnder interferometer by utilizing phase modulation of transmitted light through magnetic fluid films possessing tunable refractive index

Due to its diverse applications in photonics, bio-sensors, mechanics, etc., Mach–Zehnder interferometer becomes one of important devices. Hence, lots of efforts have been paid to develop advanced Mach–Zehnder interferometers. In this work, we explore new-model Mach–Zehnder interferometer, in which one of arm is consisted of magnetic fluid films. By utilizing the tunable refractive index of magnetic fluid films under external magnetic fields, the traveling phase of a propagating light through the magnetic fluid film is changed. This could lead to a variation in the interfered intensity of the Mach–Zehnder interferometer when an external magnetic field is applied. The modulation in the interfered intensity by the external magnetic field is demonstrated experimentally, and the relevant physical origin is also discussed.     

A simple method to estimate the magnetic moment of magnetic micro-particles

Micro-particles in suspension in a fluid are an example of a very low Reynolds number problem. In this case, no inertial effects are observed. Magnetic micro-particles with magnetic moment m, suspended in a fluid orient to applied external magnetic fields B due to the interaction between the field and the magnetic moment. In this work, we present a simple method to estimate the total magnetic moment of magnetic micro-organisms. The method is based on the application of an external oscillating magnetic field in the sites where the micro-organisms are. In this case, it is possible to obtain theoretically the solution of the equation of motion (rotation of the organism and its trajectory). The solution is a transcendental equation relating the orientation angle and m and can be solved by numerical methods. Changing the frequency and/or the field intensity, it is possible to obtain a situation in which the crystal rotates uninterruptedly (a resonance regime). This condition is related to the applied field intensity, to the frequency, to the medium viscosity, to the crystal dimension, and to the micro-crystal magnetic moment m. The method can be used to estimate the total cellular magnetic moment of magnetic micro-particles.     

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.     

Wetting of liquid droplets on two parallel filaments

Droplet wetting on two parallel filaments may assume a barrel-shaped morphology or a liquid bridge depending upon the filament diameter and spacing, droplet volume, and contact angle. This paper is aimed to examine the dependency of droplet wetting length upon the above parameters. In the process, morphology of either a barrel-shaped droplet or a liquid bridge sitting on two parallel filaments is determined numerically by using surface finite element method (SFEM). Variation of wetting length with contact angle is examined at varying droplet volume, filament spacing, and droplet morphology. It is found that the droplet wetting length increases with decreasing filament spacing ratio as well as contact angle while it also increases with the growth of droplet volume. The dependency of wetting length upon contact angle behaves sensitive to filament spacing in the case of stable liquid bridges, while it exhibits nearly constant sensitivity to the contact angle in the case of barrel-shaped droplets. The quantitative relations yielded in this study can be considered as characteristic curves applicable for a variety of droplet-on-filament systems, particularly useful to wetting property characterization of filaments, micro liquid delivery, biological cell manipulation, etc.     

Numerical simulation of two droplets impacting upon a dynamic liquid film

The impact of droplets on the liquid film is widely involved in industrial and agricultural fields. In recent years, plenty of works are limited to dry walls or stationary liquid films, and the research of multi-droplet impact dynamic films is not sufficient. Based on this, this paper employs a coupled level set and volume of fluid (CLSVOF) method to numerically simulate two-droplet impingement on a dynamic liquid film. In our work, the dynamic film thickness, horizontal central distance between the droplets, droplets' initial impact speed, and simultaneously the flow velocity of the moving film are analyzed. The evolution phenomenon and mechanism caused by the collision are analyzed in detail. We find that within a certain period of time, the droplet spacing does not affect the peripheral crown height; when the droplet spacing decreases or the initial impact velocity increases, the height of the peripheral crown increases at the beginning, and then, because the crown splashed under Rayleigh-Plateau instability, this results in the reduction of the crown height. At the same time, it is found that when the initial impact velocity increases, the angle between the upstream peripheral jet and the dynamic film becomes larger. The more obvious the horizontal movement characteristics, the more restrained the crown height; the spread length increases with the increase of the dynamic film speed, droplet spacing and the initial impact velocity. When the liquid film is thicker, more fluid enters the crown, due to the crown being unstable, the surface tension is not enough to overcome the weight of the rim at the end of the crown, resulting in droplets falling off.     

Magneto-optic fiber Sagnac modulator based on magnetic fluids

A magneto-optic modulator with a magnetic fluid film inserted into an optical fiber Sagnac interferometer is proposed. The magnetic fluid exhibits variable birefringence and Faraday effect under external magnetic field that will lead to a phase difference and polarization state rotation in the Sagnac interferometer. As a result, the intensity of the output light is modulated under the external magnetic field. Moreover, the modulator has a high extinction ratio and can easily be integrated in a single-mode fiber system. The performance of the modulator is not affected by ambient temperature variation from room temperature to 40 °C.     

Sonoprocessing of wetting of SiC by liquid Al: A thermodynamic and kinetic study

The sonoprocessing of droplet spreading during the wetting process of molten aluminum droplets on SiC ceramic substrates at 700 °C is investigated in this paper. When wetting is assisted by a 20 kHz frequency ultrasonic field, the wettability of liquid metal gets enhanced, which has been determined by the variations in thermodynamic energy and wetting kinetics. Wetting kinetic characteristics are divided into two stages according to pinning and depinning states of substrate/droplet contact lines. The droplet is static when the contact line is pinning, while it is forced to move when the contact line is depinning. When analyzing the pinning stage, high-speed photography reveals the evidence of oxide films being rapidly crushed outside the aluminum droplet. In this work, atomic models of spherical Al core being wrapped by alumina shell are tentatively built, whose dioxide microstructures are being transformed from face-centered cubic into liquid at the atomic scale. At the same time, the wetting experiment reveals that the oxide films show changes in the period of sonoprocessing from 3rd to 5th second.During the ultrasonic spreading behavior in the late stage, there is a trend of evident expansion of the base contact area. The entire ultrasonic process lasts for no longer than 10 s. With the aid of ultrasonic sinusoidal waves, the wettability of metal Al gets a rapid improvement. Both molecular dynamic (MD) investigations and the experiments results reveal that the precursor film phenomenon is never found unless wetting is assisted by ultrasonic treatments. However, the precursor film appears near the triple line after using ultrasonics in the droplet wetting process, whose formation is driven by ultrasonic oscillations. Due to the precursor film, the ultrasonic wetting contact angle is lower than the non-ultrasonic contact angle. In addition, the time-variant effective ultrasonic energy has been quantitatively evaluated. The numerical expressions of thermodynamic variables are well verified by former ultrasonic spreading test results, which altogether provide an intrinsic explanation of the fast-decreasing contact angle of Al/SiC.     

Confined ferrofluid droplet in crossed magnetic fields

When a ferrofluid drop is trapped in a horizontal Hele-Shaw cell and subjected to a vertical magnetic field, a fingering instability results in the droplet evolving into a complex branched structure. This fingering instability depends on the magnetic field ramp rate but also depends critically on the initial state of the droplet. Small perturbations in the initial droplet can have a large influence on the resulting final pattern. By simultaneously applying a stabilizing (horizontal) azimuthal magnetic field, we gain more control over the mode selection mechanism. We perform a linear stability analysis that shows that any single mode can be selected by appropriately adjusting the strengths of the applied fields. This offers a unique and accurate mode selection mechanism for this confined magnetic fluid system. We present the results of numerical simulations that demonstrate that this mode selection mechanism is quite robust and “overpowers” any initial perturbations on the droplet. This provides a predictable way to obtain patterns with any desired number of fingers.     

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.     

单液滴撞击倾斜液膜飞溅过程的耦合Level Set-VOF模拟

采用耦合水平集--体积分数法(CLSVOF)对液滴撞击倾斜表面液膜后液膜的形态演化及飞溅过程进行数值模拟, 并对液滴撞击液膜过程中形成的空气卷吸现象进行研究并探讨了撞击角对此的影响, 分析了液滴撞击后液体内部的压力和速度分布, 对液滴撞击倾斜表面液膜的飞溅过程进行讨论, 并与实验结果进行了对比, 验证了CLSVOF方法研究液滴撞击倾斜液膜的可行性. 结果表明, 液滴撞击倾斜液膜时前后两部分飞溅现象产生的机理不同, 前半部分飞溅是由于压差引起的颈部射流, 而后半部分则是由液膜径向流动产生的飞溅现象. 随着撞击角的增大, 空气卷吸气泡数量减少.     

In-plane magnetic anisotropy and coercive field dependence upon thickness of CoFeB

The structural and magnetic properties of as-grown 5–50 nm thin ion-beam sputter deposited transition metal–metalloid Co₂₀Fe₆₀B₂₀ (CFB) films are reported in this communication. A broad peak observed at 2θ∼45° in the glancing angle X-ray diffraction pattern revealed the formation of very fine nano-sized grains embedded in majority amorphous CFB matrix. Although no magnetic field is applied during deposition, the longitudinal magneto-optic Kerr effect measurements performed at 300 K in these as-grown films clearly established the presence of in-plane uniaxial magnetic anisotropy (K_u). It is argued that this observed anisotropy is strain-induced. This is supported by the observed dependence of direction of K_u on the angle between applied magnetic field and crystallographic orientation of the underlying Si(100) substrate, and increase in the coercivity with the increase of the film thickness.     

The dynamic behavior of magnetic fluid adsorbed to small permanent magnet in alternating magnetic field

The dynamic behavior of a magnetic fluid adsorbed to a small NdFeB permanent magnet subjected to an alternating magnetic field was studied with a high speed video camera system. The directions of alternating magnetic field are parallel and opposite to that of the permanent magnet. It was found that the surface of magnetic fluid responds to the external alternating magnetic field in elongation and contraction with a lot of spikes. Generation of a capillary magnetic fluid jet was observed in the neighbourhood of a specific frequency of alternating field. The effect of gravitational force on surface phenomena of magnetic fluid adsorbed to the permanent magnet was revealed.     

Formation and mobility of droplets on composite layered substrates

A mesoscale fluid film placed on a solid support may break up and form droplets. In addition, droplets may exhibit spontaneous translation by modifying the wetting properties of the substrate, resulting in asymmetry in the contact angles. We examine mechanisms for droplet formation and motion on uniform and terraced landscapes, i.e., composite substrates. The fluid film stability, droplet formation and velocity are studied theoretically in the isothermal case using a lubrication approach in one spatial dimension. The droplet properties are found to involve contributions from both the terraced layer thickness and molecular interactions via the disjoining potential.     

Dynamic magnetic susceptibility of a two-layer film in a strong magnetic field

A theoretical study is reported of the effect of interlayer exchange coupling on the resonance properties of a two-layer magnetic film with “easy-axis” and “easy-plane” anisotropic layers in a strong tilted magnetic field. The dependence of the resonance fields on the tilting angle of the external magnetic field to the film has been obtained, the tensor of integrated high-frequency film susceptibility has been found, and its dependence on the strength and orientation of the external field, as well as on layer thickness, has been analyzed. The results obtained agree with the available experimental data.     

受限胶体液滴蒸发过程中胶体颗粒沉积过程观察

亲水玻璃基片在掩模板的保护下, 通过喷涂超疏水层, 得到了被疏水层包围的圆形亲水区域. 胶体液滴在这一区域被很好地限制, 并且液滴体积可以在较大范围内变化, 体积的变化可以改变液滴与基片的表观接触角. 通过显微观察手段原位观察了表观接触角为疏水的受限胶体液滴蒸发过程中粒子沉积行为. 在整个蒸发过程中, 受限液滴边界被钉扎在亲疏水交界处. 粒子沉积过程中, 驱动粒子的液滴内部流动会发生变化. 粒子沉积图案形成过程由三种流体行为控制, 最初, Marangoni效应占主导作用, 驱动粒子在液滴表面聚集, 随之沉积到液滴边缘; 随着蒸发进行, 当接触角变小(<60°)时, 由于边界蒸发速度更快导致的毛细补偿流使得粒子直接向边界沉积. 在干燥的最后阶段, 亲水区域内的液层变得很薄, 只有一单层粒子存在于这一薄液层中, 蒸发继续进行时, 薄液层发生失稳使得粒子迅速聚集而形成网络化图案, 由于粒子间距变小, 球间的液桥毛细力也会参与到这一聚集过程中.     

基于MRT-LB伪势模型的液滴撞击液膜数值研究

运用改进的格子玻尔兹曼（LB）伪势多松弛多相模型，研究单/双液滴撞击液膜的流动特性.考察单液滴在不同气液相密度比时撞击液膜的发展.随着密度比的减小，冠状水花顶端开始向内弯曲，且底部半径显著减小.在大密度比情况下研究双液滴撞击液膜.结果表明：双液滴撞击液膜有中心射流的形成；液滴水平间距的增大，延缓中心射流的出现，并降低初始中心射流的高度；随Re数的增加，中心射流的高度明显增大.