An improved method of sessile drop for determining the surface tension of liquids

An improved procedure is developed for the processing of images of meridional section of a liquid drop, obtained as a result of realization of the sessile drop method for determining the surface tension of liquid. The procedure provides for the scanning of digital image of droplet, for numerical solution of the Young-Laplace equation, and for the calculation of surface tension, wetting angle, and volume of the drop.     

EHD喷印技术相关参数数值分析

目的对基于电-液耦合动力学原理(简称EHD)喷印技术的相关参数,进行锥射流和滴落模式下的数值分析研究。方法在锥射流模式中,对毛细凝结加热液体喷射的物理模型进行调整,以适用于EHD喷印;对滴落模式的液滴沉积过程,引入表面张力概念进行数值分析。运用数值法得到了锥射流的轮廓图,集中讨论了流量和净高度对射流直径的影响;重点研究了液滴直径与电压频率和液体表面张力之间的关系,并将理论结果与实验和经验数据进行了比较分析。结果锥射流模式在一定条件下,射流直径随着流量和净高度的增大而增加;在滴落模式中,得到的液滴直径随电压频率和流体表面张力的增大而减小,且数值分析得到的结果与实验结果相一致。结论对相关参数进行数值分析是对实验研究的补充,为EHD喷印技术研究提供了理论参考。     

Microstructure Evolution of Cu-Pb Monotectic Alloys Processed in Drop Tube

Rapid solidification of Cu-Pb monotectic alloys has been accomplished during free fall in a 3m drop tube.Both macrosegregated and uniformly dispersed structures are observed in Cu-40wt pct Pb alloy droplets,whereas droplets of composition Cu-64wt pct Pb exhibit only macrosegregation morphologies.The microstructures are strongly dependent on droplet size.The higher undercooling tends to facilitate liquid phase separation and results in more extensive macrosegregation in smaller droplets.There exists a pronounced tendency for the Pb-rich liquid to occupy the surface of the droplets of both compositions,resulting from the quite lower surface tension of the Pb-rich phase and cauing a Pb-rich layer at the surface of the solidified droplet.The nucleation of monotectic cells in the Cu-40 wt pct Pb droplets with dispersed structures preferentially occurs at the droplet surface.A single nucleation event takes place more frequently as droplet size is reduced.     

Parameters controlling solidification of molten wax droplets falling on a solid surface

An experimental study was done to identify parameters that determine the shape of splats formed by droplets of paraffin wax impacting and freezing on a polished aluminum surface. Impact velocity was varied from 0.5 to 2.7 m/s and surface temperature from 23 to 73 °C. Droplet impact was photographed, and the splat diameter and liquid-solid contact angle measured from photographs. A simple energy conservation model was used to predict the maximum extent of droplet spread and the rate of droplet solidification. The extent of droplet solidification was found to be too small to affect droplet impact dynamics. Photographs showed liquid recoiling in the droplet center following impact on a cold surface (23 °C); the height of recoil diminished if either substrate temperature or impact velocity was increased. Droplet recoil was attributed to surface tension pulling back the periphery of the splat. Reducing the surface temperature increased surface tension, promoting recoil. At sufficiently large impact velocities droplets fragmented. A model based on the Rayleigh-Taylor instability was used to predict the number of satellite droplets that broke loose after impact.     

Spreading Characteristics of liquids

We encounter the spreading phenomenon in the fusing of powder images in Electrophotographic Imaging.

This paper presents a model for droplet spread on a substrate based on Frenkel’s theory. The model yields a set of master curves for spreading characteristics of the droplet. The curves describe the contact angle θ between the boundaries of droplet and the substrate as a function of the normalized time. This normalized time is a function of surface tension and viscosity of the liquid and the size of the droplet.

The other variable is the equilibrium contact angle between the liquid and substrate.

Experiments were carried out to check the validity of this model for a spreading droplet. Both experimental observations and the model verify the functional relationship, θ = f (normalized time, equilibrium contact angle).     

Droplet Image Feedback Control System in Evaporation Experiment

In order to realize the steady-state droplet evaporation, image feedback control system is designed based on DSP. The system has three main functions: to capture and store droplet images during the experiment; to calculate droplet geometrical and physical parameters such as volume, surface area, surface tension and evaporation velocity at a high-precision level; to keep the droplet volume constant. The DSP can drive an injection controller with the PID control to inject liquid so as to keep the droplet volume constant. The evaporation velocity of droplet can be calculated by measuring the injected volume during the evaporation. The structure of hardware and software of the control system, key processing methods such as contour fitting and experimental results are described.     

Surface tension and density data for Fe–Cr–Mo,Fe–Cr–Ni,and Fe–Cr–Mn–Ni steels

The temperature dependence of surface tension and density for Fe–Cr–Mo (AISI 4142), Fe–Cr–Ni (AISI 304), and Fe–Cr–Mn–Ni TRIP/TWIP high-manganese (16 wt% Cr, 7 wt% Mn, and 3–9 wt% Ni) liquid alloys are investigated using the conventional maximum bubble pressure (MBP) and sessile drop (SD) methods. In addition, the surface tension of liquid steel is measured using the oscillating droplet method on electromagnetically levitated (EML) liquid droplets at the German Aerospace Centre (DLR, Cologne). The data of thermophysical properties for Fe–Cr–Mn–Ni is of major importance for modeling of infiltration and gas atomization processes in the prototyping of a “TRIP-Matrix-Composite.” The surface tension of TRIP/TWIP steel increased with an increase in temperature in MBP as well as in SD measurement. The manganese evaporation with the conventional measurement methods is not significantly high within the experiments (?_Mn < 0.5 %). The temperature coefficient of surface tension (dσ/dT) is positive for liquid steel samples, which can be explained by the concentration of surface active elements. A slight influence of nickel on the surface tension of Fe–Cr–Mn–Ni steel was experimentally observed where σ is decreased with increasing nickel content. EML measurement of high-manganese steel, however, is limited to the undercooling state of the liquid steel. The manganese evaporation strongly increased in excess of the liquidus temperature in levitation measurements and a mass loss of droplet of 5 % was observed.     

A model of a liquid droplet impinging on a high-temperature solid surfaces

The paper deals with a model of a liquid droplet vertically impinging on a heated solid surface. The model uses the following assumptions. The value of the wall temperature is taken to be such that the droplet-wall interaction would proceed via gas-vapor interlayer (T 400 °C). The droplet liquid is incompressible and nonviscous. The droplet surface is taken to be free, with its deformation caused by the effect of external pressure distributed over the droplet surface. The pressure is made up by two components, of which the first one is the surface tension pressure due to the curvature of the droplet surface; the second component is the pressure of vapor between the droplet and wall, which is determined by analyzing the process of vapor escape from the vapor interlayer. The motion of liquid within the droplet is taken to be potential and axisymmetric. The equations of droplet motion are solved relative to the potential of the vector field of velocity. The suggested model is used to perform numerical calculations of the droplet collision process, and the obtained results are compared with the data of other authors.Translated from Teplofizika Vysokikh Temperatur, Vol. 42, No. 6, 2004, pp. 921–927.Original Russian Text Copyright © 2004 by A. V. Gulikov, I. I. Berlin, and A. V. Karpyshev.     

Role of Surface Energy in the Vapor–Liquid–Solid Growth of Silicon

The conditions of vapor-phase Si whisker growth are examined, and the role of the surface Gibbs energy in the vapor–liquid–solid process is evaluated. The mechanism responsible for the catalytic activity of the liquid phase on the tip of Si whiskers is elucidated. Experimental surface tension data are used to estimate the driving force acting on the three-phase line of contact upon a displacement of the liquid droplet in the course of whisker growth.     

Surface tension and viscosity from damped free oscillations of viscous droplets

Damped oscillations of a viscous droplet in vacuum or in an inert gas of negligible density are considered. The dependence of the complex decay factor on the properties of the liquid is investigated for the first time, and numerical results are compared with earlier studies for special cases. A new method is developed to determine both surface tension and viscosity from a single experiment in which the damping rate and frequency of oscillations are measured. The procedure to determine surface tension and viscosity from oscillating levitated liquids is outlined, and results are presented for various modes of shape oscillations.     

基于SPH方法的低韦伯数下三维液滴碰撞聚合与反弹数值模拟研究

光滑粒子流体动力学方法（SPH方法）作为纯拉格朗日粒子方法,可以有效避免网格法在模拟大变形过程中带来的网格扭曲等缺陷,适合模拟含大变形的液滴碰撞聚合与反弹过程。该文基于Ott和Schnetter提出的修正SPH方法,利用有限差分与SPH一阶导数相结合的方法处理粘性项中的二阶导数问题,进行Couette流算例验证,数值解...     

Capillary absorption of metal nanodroplets by single-wall carbon nanotubes

We present a simple model that demonstrates the possibility of capillary absorption of nonwetting liquid nanoparticles by carbon nanotubes (CNTs) assisted by the action of the Laplace pressure due to the droplet surface tension. We test this model with molecular dynamics simulation and find excellent agreement with the theory, which shows that for a given nanotube radius there is a critical size below which a metal droplet will be absorbed. The model also explains recent observations of capillary absorption of nonwetting Cu nanodroplets by carbon nanotubes. This finding has implications for our understanding of the growth of CNTs from metal catalyst particles and suggests new methods for fabricating composite metal-CNT materials.     

Nano-web formation by the electrospinning at various electric fields

Electrospinning is a process in which an electrified liquid jet is ejected by the interaction between the surface tension and the exerted electric force on the droplet surface. It is important to understand the effects of an electric field on the path of the ejected jet from the droplet to the opposite electrode in the electrospinning process. The effects of electric fields on the formation of nano-webs are presented in this paper. As the design of the electrodes varies, the ejected jets were deposit on the screen, exhibiting different or chareateristics. The design of the electric field is a significant parameter in the attempt to control nano-web formation.     

Surface Oscillations of an Electromagnetically Levitated Droplet

The natural oscillation frequency of freely suspended liquid droplets can be related to the surface tension of the material, and the decay of oscillations to the liquid viscosity. However, the fluid flow inside the droplet must be laminar to measure viscosity with existing correlations; otherwise the damping of the oscillations is dominated by turbulent dissipation. Because no experimental method has yet been developed to visualize flow in electromagnetically levitated oscillating metal droplets, mathematical modeling can assist in predicting whether or not turbulence occurs, and under what processing conditions. In this paper, three mathematical models of the flow: (1) assuming laminar conditions, (2) using the k−ɛ turbulence model, and (3) using the RNG turbulence model, respectively, are compared and contrasted to determine the physical characteristics of the flow. It is concluded that the RNG model is the best suited for describing this problem when the interior flow is turbulent. The goal of the presented work was to characterize internal flow in an oscillating droplet of liquid metal, and to verify the accuracy of the characterization by comparing calculated surface tension and viscosity values to available experimental results.     

Profile determination of a drop of liquid under surface tension,gravity and centrifugal forces

The steady-state shape of a drop of liquid under the action of surface tension, gravity and centrifugal forces is determined in this paper. The governing equations are derived from a variational principle on the total energy of the droplet. The steady-state shape is assumed to be axially symmetric, which allows describing it by means of its generator curve. This is approximated by a cubic parametric spline with suitable end conditions, and unknown supporting points. These are determined via the nonlinear least-square approximation of the arising overdetermined nonlinear algebraic system of equations. The procedure is illustrated with an example from the glass industry.     

Surface tension of γ-TiAl-based alloys

Within the Integrated Project IMPRESS, funded by the EU, a concerted action was taken to determine the thermophysical properties of a γ-TiAl-based alloys, suitable for casting of large turbine blades for aero-engines and stationary gas turbines. The challenge was to develop a castable alloy, free of grain refiners and susceptible to heat treatment. Owing to the high reactivity of this class of alloys, many difficulties were encountered to process the liquid phase in a crucible. This prevented also the measurements of specific heat, viscosity and electrical conductivity in the liquid phase. However, surface tension and density could be measured using container-less techniques. For the surface tension determination, both the oscillating droplet method by the electromagnetic levitation as well as a combined method using two methodologies in one test (i.e. the pendant drop and sessile drop) by an advanced experimental complex that has been designed for investigations of high temperature capillarity phenomena were applied. All the quantities have been obtained as a function of temperature, in some cases also in the undercooled liquid. In this article, we report a comparative discussion on the results obtained for the surface tension of Ti–Al–Nb and Ti–Al–Ta alloys, together with the corresponding theoretical values calculated by thermodynamic models.     

Development and characterization of an ionization technique for analysis of biological macromolecules: liquid matrix-assisted laser desorption electrospray ionization

We have developed an atmospheric pressure ionization technique called liquid matrix-assisted laser desorption electrospray ionization (liq-MALDESI) for the generation of multiply charged ions by laser desorption from liquid samples deposited onto a stainless steel sample target biased at a high potential. This variant of our previously reported MALDESI source does not utilize an ESI emitter to postionize neutrals. Conversely, we report desorption and ionization from a macroscopic charged droplet. We demonstrate high mass resolving power single-acquisition FT-ICR-MS analysis of peptides and proteins ranging from 1 to 8.6 kDa at atmospheric pressure. The liquid sample acts as a macroscopic charged droplet similar to those generated by electrospray ionization, whereby laser irradiation desorbs analyte from organic matrix containing charged droplets generating multiply charged ions. We have observed a singly charged radical cation of an electrochemically active species indicating oxidation occurs for analytes and therefore water; the latter would play a key role in the mechanism of ionization. Moreover, we demonstrate an increase in ion abundance and a concurrent decrease in surface tension with an increase in the applied potential.     

DOD 式压电喷墨打印系统液滴形成过程的数值模拟

目的研究DOD式压电喷墨打印系统液滴形成过程。方法简化压电式喷墨打印头的几何模型,建立基于流体体积法的流体仿真模拟系统,模拟给定压力条件下液滴生成的全过程,并以此进行数值实验来评估油墨的表面张力和黏度对喷墨中液滴形成形态的影响。结果给出不同瞬间液滴的形态,得出适合于给定条件的最优参数组合。引入流体力学中的无量纲常数Z,评价表面张力和油墨黏度在耦合作用下液滴的成形状态。确定表面张力约为40 mN/m、黏度约为10 Pa·s、无量纲常数Z为4.56的油墨能够得到理想的单液滴喷射形态。结论将仿真的结果应用到实验中,对比分析可知吻合较好,证明了数值模拟的可靠性。     

A new method for simultaneous measurement of surface tension and viscosity

A new method for the simultaneous mesurement of the surface tension and viscosity of a liquid was developed by combining the principle of the oscillating drop method with a microgravity environment. This new method can be used in an ordinary laboratory. A droplet falls for 1.5 m in approximately 0.55 s. During this short period, the surface oscillation of the droplet is recorded by two high speed line sensors equipped with a laser backlight and cylindrical lenses. The recording speed and resolution of the line sensors are 84000 line/s and 2048 pixels, respectively. The laser backlight forms a shadow of the droplet, and each of the cylindrical lenses makes the shadow into be a line, allowing the maximum diameter to be precisely measured by a line sensor. Before focusing the laser column to a line, it was split into two columns and each of them is forcused into a different line in order to determine the changes in the diameters in two right-angled directions. The measured oscillations show only a single peak for the n = 2 mode in the Fourier spectrum. This fact guarantees that the surface oscillation is almost ideal, and the simple equations for a spherical droplet can be used without any corrections.