Inkjet droplet deposition dynamics into square microcavities for OLEDs manufacturing

The dynamics of inkjet deposition in square microcavities are investigated utilizing a three-dimensional multi-relaxation-time pseudopotential lattice Boltzmann (LB) model with large density ratios. A geometric scheme is considered within the pseudopotential LBM framework to obtain the desired contact angles. The effects of wettability, density ratios, droplet viscosity and impact velocity are explored to reveal the droplet–microcavity interactions. With the contact angles of microcavity increasing, the physical outcomes including the crown-like shape with a small round dot, circular hollow core, uniform film and convex film are identified and analyzed. At a lower density ratio ρ_r?=?11.6, the surrounding denser gas resists the droplet recoiling flow resulting in an increasing hollow core. The appropriate higher droplet viscosity and decreasing impact velocity are preferred which could eliminate the hollow core in the recoiling phase and accelerate the inkjet deposition process straightforward. The revelation of droplet-microcavity dynamics is beneficial for optimizing inkjet deposition process and fabricating uniform OLEDs panels.     

Droplets coalescence and mixing with identical and distinct surface tension on a wettability gradient surface

The influence of identical and distinct surface tensions on the coalescence and mixing of droplets after a direct collision on a wettability gradient surface (made from a self-assembled monolayer, SAM technique) was investigated. The results indicate that their mixing is driven sequentially by interior convection and diffusion; the convection endures less than 100 ms but dominates more than 60 % of the mixing. If the stationary droplet has a large surface tension (73.28 mN × m^?1), whether the moving droplet has a large surface tension (73.28 mN × m^?1) or a small surface tension (38.63 mN × m^?1), the mushroom-shaped mixing pattern is generated within the coalesced droplet that enhances the convective mixing and also significantly enlarges the interface for mass diffusion. The mixing index of these two cases was greater than 0.8 at 120 s after the collision. For the cases in which the stationary droplet with a small surface tension collided by the moving droplet with a large surface tension, a mixing pattern with a round-head shape developed, which was insufficient to benefit the mixing. When the stationary and moving droplets both had small surface tension, the moving droplet was unable to merge with stationary droplet and had poor mixing quality due to the small surface Gibbs energy of both stationary and moving droplets. For the collision of droplets of identical surface tension, the surface tension affects the coalescence behavior; for the collision of droplets with distinct surface tension, the coalescence behavior and mixing quality depend on the colliding arrangement of stationary and moving droplets.     

Chemical reaction and mixing inside a coalesced droplet after a head-on collision

We investigated the phenomena of a chemical reaction inside a coalesced droplet after a direct (head-on) collision. A droplet containing an alkaline solution collided with a droplet containing a pH indicator on a surface with a wettability gradient. We used a high-speed camera to observe the color-changing reaction inside the coalesced droplet. Compared with a traditional dye-mixing test, the chemical reaction inside the coalesced droplet facilitated the mixing of two counter-reactive fluids and was more than 100 times as efficient as for unreactive fluids mixing inside the coalesced droplet. Instead of mere mixing, a chemical reaction inside a coalesced droplet is valuable for applications in a digital microfluidic open system. In droplet coalescence, the characteristics of the fluids and the ratio of volumes of two droplets caused a varied profile of the droplet coalescence, especially the neck curvature that affects the shape of the material interface between the two droplets at an initial phase. We observed the evolution of the chemical reaction with a varying radius of neck curvature inside the coalesced droplet. For the case of a small radius of neck curvature, the small interfacial area between two reactive fluids accumulated an intense heat of reaction and induced a rapid growth of the fingers. For the case of a large radius of neck curvature, the growth of fingers was slight and the interface was uniform across the large interfacial area. Our work illustrates a correlation between the rate of chemical reaction and the profile of a coalesced droplet, which is a significant reference in droplet-based microfluidic systems for biochemical applications.     

Analysis of single droplet dynamics on striped surface domains using a lattice Boltzmann method

In this paper, the behavior of a micron-scale fluid droplet on a heterogeneous surface is investigated using a two-phase lattice Boltzmann method (LBM). The two-phase LBM permits the simulation of the time dependent three-dimensional motion of a liquid droplet on solid surface patterned with hydrophobic and hydrophilic strips. A nearest-neighbor molecular interaction force is used to model the adhesive forces between the fluid and solid walls. The solid heterogeneous wall is a uniform hydrophilic substrate painted with hydrophobic strips. The model is validated by demonstrating the consistency of the simulation results with an exact solution for capillary rise and through qualitative comparison of computed dynamic contact line behavior with experimentally measured surface properties and observed surface shapes of a droplet on a heterogeneous surface. The dependence of spreading behavior on wettability, the width of hydrophobic strip, initial location of the droplet relative to the strips, and gravity is investigated. A decrease in contact angle of the liquid on a hydrophilic surface may lead to breakup of the droplet for certain substrate patterns. The simulations suggest that the present lattice Boltzmann (LB) model can be used as a reliable way to study fluidic control on heterogeneous surfaces and other wetting related subjects.     

Lattice Boltzmann simulations of the motion induced by variable surface tension

Methods for implementing variable surface tension in the multiphase Lattice Boltzmann model with the color model and Shan-Chen scheme are tested by analyzing the models’ abilities to reproduce a theoretical result by Levich and Kuznetzov. If the surface tension around a droplet is asymmetrical, the droplet moves towards the side where the surface tension is lower. The droplet’s velocity is proportional to the surface tension gradient, the droplet’s radius, and the inverse of the viscosity. The model is tested to determine whether the simulated droplets move in the manner predicted by theory. Although the discreteness of the underlying lattice causes a spurious oscillation to the velocity, the numerical results concerning the average velocity show a good correspondence between theory and the model in regards to the surface tension gradient and droplet size. The color model also produces good simulations in the scenarios with different viscosities, while the diffusive properties and unknown relationships between the parameters and surface tension in the Shan-Chen model make the numerical results of that model more dubious, even though several of the results are qualitatively in agreement.     

Water droplet properties on periodically structured superhydrophobic surfaces: a lattice Boltzmann approach to multiphase flows with high water/air density ratio

Surface roughness affects the contact angle (CA) due to the increased area of solid–liquid interface and due to the effect of sharp edges of rough surfaces. Roughness may also lead to another non-wetting regime, by forming a composite solid–liquid–air interface between the water and the textured surface; this composite interface exhibits strong water repellency due to the various pockets of air entrapped between the surface textures. The contact between water and a hydrophobic textured surface leads to one of these two regimes depending on the thermodynamics stability of the regimes. In this study, the projection method of lattice Boltzmann method is used to analyze the large density difference at the air and water interface. The method is applied to simulate two-phase flows with the density ratio of up to 1,000. A numerical model is presented to provide a relationship between roughness and CA, which is used to develop optimized texture topography and create a biomimetic superhydrophobic surface. The numerical models encompass the effects of contact area, solid–liquid–gas composite interface and shape edges. The models are reused to analyze different possible roughness distributions and to calculate the effect of the cross-sectional area of pillars, including rectangular, triangular, cross, and pyramidal pillars. The energy barrier is investigated to predict the position of the transition between the Cassie and Wenzel regime observed for each roughness parameter as well as a theoretical free surface energy model.     

Numerical prediction of three-dimensional time-dependent viscoelastic extrudate swell using differential and algebraic models

The dynamic behavior of a droplet on a solid surface is simulated by the lattice Boltzmann method (LBM) for two-phase fluids with large density differences; the wetting boundary condition on solid walls is incorporated in this simulation. By using the method, the dynamic behavior of a droplet impinging on a horizontal wall is investigated in terms of various Weber numbers. The dynamic contact angle, the contact line velocity, and the wet length are calculated, and found to be in good agreement with available experimental data. In addition, the method is applied to simulations of the collision of a falling droplet with a stationary droplet on a solid surface. The behavior of the droplets and the mixing process during their collision are simulated in terms of various impact velocities and several static contact angles on the solid surface. It is seen that mixing occurs around the rim of the coalescent droplet due to the circular flows. Also, the relationship between the mixing rate of the primary coalescent droplet and Weber number is investigated.     

Isotropic color gradient for simulating very high-density ratios with a two-phase flow lattice Boltzmann model

This study presents the integration of isotropic color gradient discretization into a lattice Boltzmann Rothman–Keller (RK) model designed for two-phase flow simulation. The proposed model removes one limitation of the RK model, which concerns the handling of O(1000) large density ratios between the fluids for a wide range of parameters. Taylor’s series expansions are used to characterize the difference between an isotropic gradient discretization and the commonly used anisotropic gradient. The proposed color gradient discretization can reduce, by one order of magnitude, the spurious current problem that affects the interface between the phases. A set of numerical tests is conducted to show that a rotationally invariant discretization enables widening of the parameter range for the surface tension. Surface tensions from O(10⁻²) to O(10⁻⁸), depending on the density ratio, are accurately simulated. An extreme density ratio of O(10,000) is successfully tested for a steady bubble with an error of 0.5% for Laplace’s law across a sharp interface, with a thickness of about 5–6 lattice units.     

Simulation of incompressible two-phase flows with large density differences employing lattice Boltzmann and level set methods

A hybrid lattice Boltzmann and level set method (LBLSM) for two-phase immiscible fluids with large density differences is proposed. The lattice Boltzmann method is used for calculating the velocities, the interface is captured by the level set function and the surface tension force is replaced by an equivalent force field. The method can be applied to simulate two-phase fluid flows with the density ratio up to 1000. In case of zero or known pressure gradient the method is completely explicit. In order to validate the method, several examples are solved and the results are in agreement with analytical or experimental results.     

A cache‐efficient implementation of the lattice Boltzmann method for the two‐dimensional diffusion equation

The lattice Boltzmann method is an important technique for the numerical solution of partial differential equations because it has nearly ideal scalability on parallel computers for many applications. However, to achieve the scalability and speed potential of the lattice Boltzmann technique, the issues of data reusability in cache‐based computer architectures must be addressed. Utilizing the two‐dimensional diffusion equation, , this paper examines cache optimization for the lattice Boltzmann method in both serial and parallel implementations. In this study, speedups due to cache optimization were found to be 1.9–2.5 for the serial implementation and 3.6–3.8 for the parallel case in which the domain decomposition was optimized for stride‐one access. In the parallel non‐cached implementation, the method of domain decomposition (horizontal or vertical) used for parallelization did not significantly affect the compute time. In contrast, the cache‐based implementation of the lattice Boltzmann method was significantly faster when the domain decomposition was optimized for stride‐one access. Additionally, the cache‐optimized lattice Boltzmann method in which the domain decomposition was optimized for stride‐one access displayed superlinear scalability on all problem sizes as the number of processors was increased. Copyright © 2004 John Wiley & Sons, Ltd.     

Three-dimensional lattice Boltzmann simulations of high density ratio two-phase flows in porous media

A three-dimensional multiphase lattice Boltzmann model is implemented to study the spontaneous phase transport in complex porous media. The model is validated against the analytical solution of Young’s and Laplace’s laws. Afterward, three-dimensional porous layers are randomly generated to investigate droplet penetration into a substrate, liquid transport in a porous channel as well as extraction of a droplet from a porous medium. Effects of several geometrical and flow parameters such as porosity, density ratio, Reynolds number, Weber number, Froude number and contact angle are considered. A parametric study of the influence of main non-dimensional parameters upon the impact of liquid drops on permeable surface is performed. Results show that while increasing Froude number causes spreading of the droplet on the surface, increasing Reynolds number, Weber number, porosity and liquid-air density ratio will enhance the penetration rate into the surface. Furthermore, increasing the contact angle decreases both the spreading and the penetration rate at the same time. In the same way, for the liquid transport in a porous channel, it is found that increasing the porosity and Reynolds number will result in increasing penetration rate in the channel. For the extraction of a droplet from a porous medium, it is shown that by increasing the gravitational force and/or porosity the droplet extracts faster from the substrate.     

Measurement of vorticity in the surface layer using an acoustic echo sounder array

The earth's atmospheric surface layer is usually defined as that region of the lower atmosphere (generally below about 10 m above the earth's surface) where surface friction causes vertical fluxes of heat, moisture and momentum to be constant with height. Within the surface layer, either in response to surface friction or to the atmosphere above, horizontal circular eddies often develop. These circular motions may provide the source of rotation for dust devils so often seen on hot and dry days particularly in desert regions. Also, at larger scales ($\text{～ 1 km in diameter}$) regions of warm and therefore buoyant upward moving air, called thermal plumes, may acquire rotation. These plumes may extend from the earth's surface to more than a kilometer in height on a warm afternoon. Fluid dynamicists quantify this horizontal rotation with a parameter known as the vertical component of vorticity. Vorticity is very difficult to measure in the earth's atmosphere at scales of close to a kilometer because the calculation involves wind-speed differences over horizontal distances of about 500 m. The winds must be measured quite accurately because the differences can be quite small and, therefore, the errors in these measurements are often quite large. This work describes a method of measuring vertical vorticity at scales down to 500 m using an array of three acoustic sounders about 2 m above the earth's surface which overcomes some of the accuracy problems mentioned above. We relate these vorticity measurements to other atmospheric parameters and compute temporal spectra of these quantities to help explain the relationship between vorticity, thermal plume activity, and the smaller-scale dust devils.     

Lattice Boltzmann simulation of viscous fingering phenomenon of immiscible fluids displacement in a channel

In this paper, the viscous fingering phenomenon of two immiscible fluids in a channel is studied by applying the lattice Boltzmann method (LBM). The fundamental physical mechanisms of a finger formation or the interface evolution between immiscible fluids are described in terms of the relative importance of viscous forces, surface tension, and gravity, which are quantifiable via the dimensionless quantities, namely, capillary number, Bond number and viscosity ratio between displaced fluid and displacing fluid. In addition, the effect of wettability on flow behaviour of fluids is investigated for the cases with and without consideration of gravity, respectively. The numerical results provide a good understanding of the mechanisms of viscous fingering phenomenon from a mesoscopic point of view and confirm that the LBM can be viewed as a promising tool for investigating fluid behaviour and other immiscible displacement problems.     

一种基于多信息综合的人脸跟踪算法

随着智能监视系统的发展需求,人脸跟踪作为智能监视系统中的一类特殊对象的跟踪方法,在跟踪时对环境的适应性要求很高。文章提出了一种基于多信息综合特征的,在变动场景下,对人脸进行跟踪的算法。为了使人脸跟踪在环境发生变化的情况下,可以进行有效跟踪,算法中用人脸区域内部的颜色信息、亮度梯度信息等获得综合的判别特征。在跟踪时,对于人脸的大小、朝向的变化以及光线变化,背景物等的干扰均有较强的鲁棒性。实验结果表明,算法在摄像机转动、仰俯、光线变化、人头晃动以及背景颜色分布与人脸近似等情况下均可完成有效跟踪。     

基于梯度与颜色信息融合的水文资料图像分割

针对纸质水文资料数字化应用,对相机拍摄的水文资料图像进行分割,提出基于梯度和颜色信息融合的水文图像分割方法。首先利用图像在CIELab空间上的颜色分量特征分割出曲线,然后进行分块处理,利用梯度算子在水平和垂直方向分别判别属于网格线上的目标像素点,统计这些像素点的颜色信息,利用颜色分量关系对网格线进行初步提取,之后加入水平和垂直方向的腐蚀,合并两方向的结果得到最终的网格二值化图像,最终由曲线图像和网格图像合并后得到水文图像的分割结果。对多幅水文图像进行分割的实验结果表明,本文方法能自适应地完成对多幅图像有效的分割,并且能够减少相机拍摄光照不均的影响,有较好的鲁棒性和较低的计算复杂度。     

Effect and experiment of curvature radius of 3‐D printed conformal load‐bearing antenna array on EM performance

Prototypes of a special conformal load‐bearing antenna array (CLAA) which has nondevelopable surface, are designed, fabricated, and tested, and the effect of the substrate curvature radius on its EM performance is also researched in this work. A novel three‐dimensional (3‐D) printing technology and fabrication equipment based on micro‐droplet spraying and metal laser sintering are proposed to create patch array and divider network on a non‐developable curved rigid substrate. In order to compare with conventional technology (such as chemical etching), a planar CLAA prototype with two patches, operating frequency at 5GHz, is designed and fabricated by two different technologies, the surface roughness, fabrication tolerance, and EM performance are tested and compared. Finally, a spherical CLAA prototype with eight patches, operating frequency at 13GHz, is designed and fabricated by the novel 3D printing, measured EM performance demonstrate the applicability of additive manufacturing for this special CLAA.     

Two dimensional coupled oscillators array with rhombus structure and its application in active antenna array

Beam scanning and forming can be achieved by coupled oscillators array without phase shifter. Active antenna array based on coupled oscillators array has the virtue of low cost, high integration, and high efficiency. Traditional two dimensional coupled oscillators array has been arranged on rectangular lattices, and phase difference of adjacent elements is limited to [-90°, 90°]. Therefore, the beam scanning range is limited to [-30°, 30°] from normal for half wavelength element spacing. A new two dimensional coupled oscillators array with rhombus structure is presented. Phase control method and phase error of the array are also provided. Stability of the array is analyzed, and stable condition is given. When this coupled oscillators array with rhombus structure is used in active antenna array, theoretical results show that phase difference of adjacent elements reach the limit of [-180°, 180°] along the horizontal and vertical directions. Therefore, it has wider beam scanning range than that of a rectangular lattice structure.     

一种改进的活动轮廓图像分割技术

图像分割是由图像处理到图像分析的关键步骤，也是一种基本的计算机视觉技术。针对传统的活动轮廓外力模型均存在一些难以克服的缺点，提出了一种改进的活动轮廓图像分割技术，并首先介绍了用活动轮廓进行目标分割的基本原理，即一条曲线在其内部能量和外部能量的共同作用下，可以移动到所期望的位置，并且当曲线到达目标位置的时候，活动曲线所具有的能量达到最小。在传统的活动轮廓中，外部能量通常由目标点的梯度势能场给出，但是由于梯度势能场存在着一些难以克服的缺点，即不能够很好地指导曲线的移动，为此，对其进行了改进，即采用一种梯度向量流场作为外部能量场的方法，从而有效地克服了传统梯度势能场捕捉范围小以及难以处理凹平面的缺点，并通过实验证明了该方法的有效性。     

Roughness optimization for biomimetic superhydrophobic surfaces

For non-wetting liquids the contact angle with a rough surface is greater than with a flat surface and may approach 180°, as reported for leaves of water-repellent plants, such as lotus. Roughness affects the contact angle due to the increased area of solid–liquid interface and due to the effect of sharp edges of rough surfaces. High roughness may lead to composite solid–liquid–air interface, which may be either stable or unstable. A comprehensive analytical model is proposed to provide a relationship between local roughness and contact angle, which is used to develop roughness distribution and to create biomimetic superhydrophobic surfaces. Various roughness distributions are considered, including periodic and surfaces with rectangular, hemispherically topped cylindrical, conical and pyramidal asperities and the random Gaussian height distribution. Verification of the model is conducted using experimental data for the contact angle of water droplet on a lotus leaf surface. For two solid bodies in contact, for wetting liquids, wetting leads to the meniscus force, which affects friction. Dependence of the meniscus force on roughness, previously ignored, is considered in the paper and it is found that with increasing roughness meniscus force can grow due to scale effect.

基于组合型表面模型的运动目标跟踪

运动目标跟踪是计算机视觉领域研究的难点课题,提出了一种基于组合型表面模型的视频运动目标跟踪算法。研究了目标的颜色特征空间和梯度特征空间,通过梯度特征和颜色特征的组合,以直方图的形式来建立目标表面模型,之后使用CamShift算法来完成一帧一帧的跟踪。实验结果表明,在图像背景复杂且目标出现遮挡的情况下,该方法仍能有效的跟踪目标。