倾斜均质表面上等径水滴聚合过程的可视化实验

对倾斜均匀表面上等直径水滴的聚合过程及特性进行了可视化实验研究,获得了水滴直径和表面倾角等参数对液滴聚合过程中液滴液桥半径、接触角和接触线变化特性的影响,分析了水滴聚合对其运动的影响.实验结果表明:表面倾角越大,下滑的临界半径越小;液滴的直径越大,液滴聚合后越容易下滑;液滴聚合可以加快液滴的运动,使下滑临界半径减小.     

水平均质表面上液滴聚合过程的可视化实验研究

对水平均匀表面上液滴的聚合过程及特性进行了可视化实验研究,获得了液滴半径和液滴物性等参数对液滴聚合过程中液滴液桥半径和接触角变化特性的影响规律。实验结果表明:液滴聚合中液桥半径和接触角都呈衰减振荡变化; 聚合前液滴半径越小,液桥半径振荡频率越大,振幅越小,振荡时间越短;液滴的粘度越大,液桥半径的振荡频率越小, 振幅越小,振荡时间越短;液滴聚合前的接触角明显大于聚合液滴静止后的接触角,其差值与固体界面状况和气、固、液物性相关。     

倾斜均质表面上非等径液滴的聚合特性

对倾斜均匀表面上非等径液滴的聚合过程及特性进行了可视化实验研究,获得了液滴半径和表面倾角等参数对液滴聚合过程中液滴液桥半径、接触角和接触线变化特性的影响规律,进一步说明了倾斜表面上液滴聚合可以加快液滴的运动.     

Some effects of the electrostatic interaction of water drops in the atmosphere

The electric field at the surface of two conducting spherical charged particles and their interaction force are calculated. It is shown that as particles carrying like charge approach each other, the force changes sign and becomes attractive. The case where the charge on each particle varies as the square of its radius is an exception (repulsion at any distance between the particles). Self-similar asymptotic solutions for the interaction force and energy are found for particles of identical size. For a pair of charged water drops falling simultaneously in the atmosphere, a numerical simulation shows that a drop formed by coalescence of the pair may be subject to the Rayleigh instability. Zh. Tekh. Fiz. 69, 12–17 (December 1999)     

On the coalescence of sessile drops with miscible liquids

Sessile drops sitting on highly wettable solid substrates fuse in qualitatively different ways after contact, depending on the surface tension gradients between the mixing droplets. In early time evolution the drop coalescence can be fast or delayed (intermittent). In long time evolution a secondary drop formation can occur. We study numerically droplet dynamics during coalescence in two and three spatial dimensions, within a phase field approach. We discuss criteria to distinguish different coalescence regimes. A comparison with recent experiments will be done.     

Ultrafast x-ray phase-contrast imaging of the initial coalescence phase of two water droplets

We report an ultrafast x-ray phase-contrast imaging study of the early merging dynamics of two water drops in air. Owing to the edge-enhancement capability, the high penetrability, and the unprecedented temporal and spatial resolutions offered by this new x-ray technique, the coalescence singularity of two water drops was revisited. A finite initial contact radius was identified and the evolvement of the trapped toroidal air bubble was studied for the first time. Despite the existence of this finite initial contact radius, the subsequent meniscus radius followed power laws which agree with theoretical predictions for the inviscid regime.     

Deformation and breakup of aqueous drops in viscous oil under a uniform AC electric field

Electric coalescence in alternating current (AC) electric fields is an important electrical dehydration technology. The deformation and breakup of water drops are crucial to the application of this process. In this study, these procedures were examined experimentally in an AC electric field using a high-speed camera. The deformation and breakup of drops depend on the intensity and frequency of this field. Deformation is aggravated by the increase in frequency under a constant electric field strength. Furthermore, the electric strength of breakup weakens as the frequency increases. Thus, understanding the deformation process can help advance electrocoalescencer design.     

Effects of high-frequency and high-voltage pulsed electric field parameters on water chain formation

Separation methods utilizing high-frequency and high-voltage pulsed DC electric fields have been used extensively in the oil and petroleum industries, where the occurrence of water-in-oil dispersions is highly unwelcome because of physical constraints and the high maintenance costs required to treat these dispersions. This paper reports the results of studies of the effects of applied electric field parameters, including electric field strength, frequency, and duty ratio, on water chain formation in water-in-oil emulsions. The investigations were performed in a rectangular Perspex^® cell. The results of the studies show that dipole–dipole forces dominate the process of water chain formation. At low electric field strength, frequency, or duty ratio, dipole–dipole forces are negligible; therefore, the process of water chain formation and aqueous drop coalescence are inconspicuous. However, at high electric field strength, frequency, or duty ratio, significant dipole–dipole forces give rise to water chain formation and aqueous drop coalescence. At extremely high electric field strength, frequency, or duty ratio, aqueous drops are excessively polarized and disintegrate, inhibiting the processes of water chain formation and aqueous drop coalescence. The optimum electric field parameters for separation of water-in-oil dispersions are as follows: electric field strength, 3.80 kV cm⁻¹; frequency, 4.0 kHz; and duty ratio, 0.65.     

Electrical properties of water drops inside the dropwise cluster

The Letter shows that inside a dropwise cluster formed over the heated water surface, water drops are electrically charged. The charge of a separate drop reaches 10³ units of an elementary charge. The drops are positioned from each other at the distance of double Debye radius length. It is fixed up that drops levitate over the water surface in consequence of the Stokes force acting from the side of gas-vapor flow rising from water surface. The Stokes force thousand times exceeds the Coulomb drop repulsion force from the water surface.     

A simplified model of electrocoalescence of two close water droplets in oil

The conditions of electrocoalescence of close droplets of conducting liquid suspended in insulating oil under the action of an electric field are investigated. Previous results are first recalled and discussed. A physical picture allows justifying the asymptotic laws obtained for coalescence conditions of very close drops under a given potential difference. An extrapolation to the coalescence conditions of close droplets subjected to a uniform electric field is proposed and shortly discussed.     

Role of dimensionality and axisymmetry in fluid pinch-off and coalescence

We present data on the pinch-off and coalescence of thin liquid alkane lenses floating on water. Pinch-off in quasi-2D lenses is distinctly different from pinch-off in axisymmetric 3D drops and involves a cascade of satellite droplets which extends to micron length scales. In contrast, coalescence of lenses is qualitatively similar to coalescence of 3D drops. Coalescence is predicted to involve entrainment of the exterior fluid as the droplets merge. This reentrant folding is obscured in 3D droplets but is clearly visible in coalescence of thin lenses.     

Lateral vibration of a water drop and its motion on a vibrating surface

The resonant modes of sessile water drops on a hydrophobic substrate subjected to a small-amplitude lateral vibration are investigated using computational fluid dynamic (CFD) modeling. As the substrate is vibrated laterally, its momentum diffuses within the Stokes layer of the drop. Above the Stokes layer, the competition between the inertial and Laplace forces causes the formation of capillary waves on the surface of the drop. In the first part of this paper, the resonant states of water drops are illustrated by investigating the velocity profile and the hydrostatic force using a 3d simulation of the Navier-Stokes equation. The simulation also allows an estimation of the contact angle variation on both sides of the drop. In the second part of the paper, we investigate the effect of vibration on a water drop in contact with a vertical plate. Here, as the plate vibrates parallel to gravity, the contact line oscillates. Each oscillation is, however, rectified by hysteresis, thus inducing a ratcheting motion to the water droplet vertically downward. Maximum rectification occurs at the resonant states of the drop. A comparison between the frequency-dependent motion of these drops and the variation of contact angles on their both sides is made. The paper ends with a discussion on the movements of the drops on a horizontal hydrophobic surface subjected to an asymmetric vibration.     

碰撞聚合引起液滴数目变化的生灭过程模型

液滴碰撞聚合是气体/液滴两相流动中的重要现象.为研究碰撞聚合引起的液滴数目变化,考虑液相仅包含大小两类尺寸液滴的情形,计算不同尺寸的两个液滴间的碰撞概率,对固定观察区域内液滴的碰撞聚合、迁入及迁出因素作随机分析,建立描述液滴数目变化的生灭过程模型.由无条件概率模型方程,得出模型方程的相应平稳分布.以雨滴下落过程的质量增长为例,说明所得分布在气体液滴两相流动中的应用.     

Spreading of gallium on the surface of a contact layer of variable composition from the Pb–Bi system

An approach for obtaining substrates of variable composition of metal systems is proposed. The spreading of drops of gallium of uniform size on the surfaces of areas of variable composition, obtained via contact melting of the Bi-Pb system is studied. A change in the diameter of a spot is observed, depending on the position of the drop on the surface of the substrate. A change in the shape of the spot is observed, depending on the composition along the interface region. The need to use force to separate the crystallized drops from the surface of the substrate, depending on the positions of the gallium drops (i.e., the substrate’s composition) confirms the wettability upon spreading.     

Droplet transport and coalescence kinetics in emulsions subjected to acoustic fields

A method to aid the separation of the oil phase from aqueous emulsions using a low-intensity, resonant ultrasonic field has recently been developed. The density and compressibility difference between the dispersed and continuous phases within the emulsion results in a net force on the oil drops that pushes them toward the pressure antinodes of the standing-wave field, where coalescence subsequently occurs. A trajectory model is developed to predict the relative motion of drops subjected to the acoustic field. Such trajectories are sensitive to the physical properties and relative size of interacting drops, the initial configuration of the drops, and acoustic field parameters. Model predictions are validated by comparing experimentally observed trajectories with those predicted by the model. The modeling approach is then extended to determine the temporal evolution of the size of the region surrounding a target drop cleared by coalescence as a function of physical and acoustic field parameters. These results form the basis of a population balance model that attempts to track the size-evolution of a drop population coalescing under the influence of an acoustic field.     

Viscous to inertial crossover in liquid drop coalescence

Using an electrical method and high-speed imaging, we probe drop coalescence down to 10?ns after the drops touch. By varying the liquid viscosity over two decades, we conclude that, at a sufficiently low approach velocity where deformation is not present, the drops coalesce with an unexpectedly late crossover time between a regime dominated by viscous and one dominated by inertial effects. We argue that the late crossover, not accounted for in the theory, can be explained by an appropriate choice of length scales present in the flow geometry.     

Coalescence of spreading droplets on a wettable substrate

We investigate experimentally and theoretically the coalescence dynamics of two spreading droplets on a highly wettable substrate. Upon contact, surface tension drives a rapid motion perpendicular to the line of centers that joins the drops and lowers the total surface area. We find that the width of the growing meniscus bridge between the two droplets exhibits power-law behavior, growing at early times as t1/2. Moreover, the growth rate is highly sensitive to both the radii and heights of the droplets at contact, scaling as ho3/2/Ro. This size dependence differs significantly from the behavior of freely suspended droplets, in which the coalescence growth rate depends only weakly on the droplet size. We demonstrate that the scaling behavior is consistent with a model in which the growth of the meniscus bridge is governed by the viscously hindered flux from the droplets.     

微通道内不混溶气液两相二氧化碳界面动力学行为的格子Boltzmann研究

将高精度的二氧化碳状态方程与气液两相流格子Boltzmann方法中的伪势模型耦合,研究微通道内二氧化碳气液两相流动的界面动力学行为,包括二氧化碳气泡和液滴的分裂、合并、变形,以及气液两相二氧化碳在演化过程中的质量交换.研究发现：当分裂和合并行为达到平衡,并且两相之间不发生质量交换时流动达到稳态.稳态时的流型主要依赖于表面张力,惯性力,管道的润湿性,以及初始体积分数.当表面张力较大时,微通道内形成的二氧化碳气泡或液滴会收缩成圆形,此时二氧化碳气泡或液滴会堵塞微通道,形成段塞流;随着表面张力的减小,形成的气泡或液滴不容易收缩,在微通道内更容易发生变形,出现泡状流或环状流.当壁面润湿性为强疏水性时,二氧化碳在微通道中的流动为环状流,其它润湿性下,流型为段塞流.体积分数较小时,二氧化碳两相流动的流型为段塞流,体积分数较大时,流型为环状流.     

Superhydrophobic wind turbine blade surfaces obtained by a simple deposition of silica nanoparticles embedded in epoxy

Samples of wind turbine blade surface have been covered with a superhydrophobic coating made of silica nanoparticles embedded in commercial epoxy paint. The superhydrophobic surfaces have a water contact angle around 152°, a hysteresis less than 2° and a water drop sliding angle around 0.5°. These surfaces are water repellent so that water drops cannot remain motionless on the surface. Examination of coated and uncoated surfaces with scanning electron microscopy and atomic force microscopy, together with measurements of water contact angles, indicates that the air trapped in the cavity enhances the water repellency similarly to the lotus leaf effect. Moreover, this new coating is stable under UVC irradiation and water pouring. The production of this nanoscale coating film being simple and low cost, it can be considered as a suitable candidate for water protection of different outdoor structures.     

Nucleation and growth on a superhydrophobic grooved surface

The growth dynamics of water drops condensed on a superhydrophobic geometrically patterned surface were studied. Drop size evolution at early and intermediate times is self-similar. Drop growth laws do not differ for a flat surface because of a reduction of both drop and substrate dimensionality. A striking observation is the instantaneous drying of the top surface of grooves at a point in time due to coalescence of the drops with a completely filled channel. At late times, only a few large drops grow connected to the channels, in a mixed Wenzel-penetration regime.