Experimental study on fluid selection for a stable Taylor cone formation via micro-PIV measurement

In this study, the visualization of the flow inside a Taylor cone formed during an electrohydrodynamic (EHD) spraying is conducted to analyze its stability among five liquid candidates. A micro-PIV with a micro-nozzle is used for the visualization, and the physical properties as well as measured values are utilized in the analysis. First, in forming the Taylor cone, the electrohydrodynamic force is required to be sufficiently large in order to overcome the surface tension of the liquid. Thus, among the five liquids tested here, three, in this case IPA, EtOH, and MeOH, form a Taylor cone due to the relatively low surface tension levels as compared to the others. Once electrohydrodynamic jetting occurs, the average and maximum velocities become monotonically proportional to the average current. As the velocities are the smallest in using IPA, the circulation flow becomes superior to the extrusive flow, which yields the stable formation of a Taylor cone. Also, low fluctuation of the instantaneous currents supports the stable formation of IPA. Consequently, IPA shows the most stable formation of the Taylor cone in our condition due to the lowest average current and low-level surface tension. Eventually, micro-PIV would be a good tool in choosing an optimal fluid for stable EHD spraying.     

Selective spreading and jetting of electrically driven dielectric films

We study the electrically driven spreading of dielectric liquid films in wedge-shaped gaps across which a potential difference is applied. Our experiments are in a little-studied regime where, throughout the dynamics, the electrical relaxation time is long compared to the time for charge to be convected by the fluid motion. We observe that at a critical normal electric field the hump-shaped leading edge undergoes an instability in the form of a single Taylor cone and periodic jetting ensues, after which traveling waves occur along the trailing thin film. We propose a convection-dominated mechanism for charge transport to describe the observed dynamics and rationalize the viscosity dependence of the self-excited dynamics.     

Volume-Charged Cones on a Liquid Interface in an Electric Field

In this study, we explore a novel type of slender conical liquid meniscus arisen in high electric field, which carries surface charge and net bulk charge of opposite sign. Stability of such dissipative structure is ensured by the balance between capillary and electrostatic forces and competition between the surface and bulk electric currents. The bulk charge is governed by the applied voltage being generated by the electric field of the cone due to dissociation/associations reactions at its apex. The effect of the physical parameters of the liquid on the microcone structure is elucidated. It is shown that the cone angle cannot exceed a critical value, which is a function of dielectric permittivity of the liquid. The electric current through the cone is found to be proportional to the square of the applied voltage. The obtained results can be applied for analysis of atomization processes of various liquids.     

New formula for calculating coulomb energies in the liquid-drop model

A new transformation of double volume integrals into double surface integrals is presented. A simple regular method for deriving integrands in a surface integral is proposed. This method is used to calculate the Coulomb energy of a nucleus within the model of a liquid drop with a sharp boundary. Numerical results obtained on the basis of the new formula are compared with those calculated by one of the formulas employed previously.     

冷凝器壁面滴状冷凝的热力学机理及最佳接触角

应用自由能判据证明了冷凝器壁面液滴的力学平衡条件与接触角有关,进而证明了当壁面液滴与壁面的接触大于90°时,冷凝器壁面液滴的化学势将小于同曲率的球形液滴的化学势;另外,在相同饱和比下,壁面液滴的临界半径也小于球形液滴的临界半径.而在接触角小于90°时,情况则恰好相反.正是由于这双重原因,使得滴状冷凝的传热性能大大优于膜状冷凝的传热性能,并由此求出了冷凝器壁面液滴的最佳接触角. 关键词：     

Sb induced (7×7) to (1×1) surface phase transformation of the Si(111) surface

Adsorption of Sb at a very low flux rate results in an epitaxial layer-by-layer growth on Si(111) surface held at room-temperature. Band-bending is not observed for submonolayer Sb coverages while sharp changes in the photoemission features are observed for 1.0 monolayer (ML) Sb adsorption. Changes in the core level binding energy and width in X-ray photoelectron spectroscopy, surface related feature in Electron energy loss spectroscopy and spot intensity ratios in Low energy electron diffraction studies suggest a surface phase transition upon adsorption of 1.0 monolayer of Sb. A plausible model is proposed to explain the abrupt metal-semiconductor transformation at this critical coverage of 1.0 ML.     

强激光与双锥靶作用加速准单能质子束

提出了一种新型的双锥靶结构用于准单能质子束加速。利用二维PIC粒子模拟程序研究了强激光与双锥靶作用加速产生质子束的物理过程以及质子束品质。双锥靶产生的质子束在峰值能量和发散角度等方面都明显优于相同激光条件下单锥靶和平面靶的结果。尤其与平面靶相比,双锥靶质子束的峰值能量提高了5倍以上,而且很好地保持准单能性。一方面双锥靶的内锥部分是临界密度材料,提高了激光的吸收效率;另一方面双锥靶内形成了更强的准静态磁场,可以约束引导更多的超热电子传输过锥尖,进而增强加速质子束的鞘层电场。     

On the Global Existence and Stability of a Multi-Dimensional Supersonic Conic Shock Wave

We establish the global existence and stability of a three-dimensional supersonic conic shock wave for a compactly perturbed steady supersonic flow past an infinitely long circular cone with a sharp angle. The flow is described by a 3-D steady potential equation, which is multi-dimensional, quasilinear, and hyperbolic with respect to the supersonic direction. Making use of the geometric properties of the pointed shock surface together with the Rankine–Hugoniot conditions on the conic shock surface and the boundary condition on the surface of the cone, we obtain a global uniform weighted energy estimate for the nonlinear problem by finding an appropriate multiplier and establishing a new Hardy-type inequality on the shock surface. Based on this, we prove that a multi-dimensional conic shock attached to the vertex of the cone exists globally when the Mach number of the incoming supersonic flow is sufficiently large. Moreover, the asymptotic behavior of the 3-D supersonic conic shock solution, which is shown to approach the corresponding background shock solution in the downstream domain for the uniform supersonic constant flow past the sharp cone, is also explicitly given.     

Stress waves propagation in solids under high-speed liquid impact

The high-speed liquid/solid impact phenomenon is often seen in many technicalfields such as, water jet cutting technology[1], rain erosion of aviation vehicles[2] and ro-tor blades erosion in large steam turbines[3]. Recently, China Aerodynamics ResearchCenter reported experimental results of rain erosion of hypervelocity projectile[4]; Xi’anJiaotong University reported numerical simulation of high-speed liquid drop impact on asolid surface[5]. In analyzing the mechanism of material’s dama…     

金属熔体中气泡形核的理论分析

引入界面接触角，考虑表面张力对气泡形貌的影响，以熔体中均质形核、夹杂物的平表面上异质形核和圆锥形凹坑内异质形核三种典型模型对气泡形核机理进行理论研究.研究发现，三种形核模型下具有相等的微米量级的气泡临界形核半径，并随气压的增大而减小.结果表明，用以制备藕状规则多孔金属的Gasar工艺中能够形成的最小气孔的直径为微米量级(0.1—1.0 MPa气压).在圆锥形凹坑内异质形核时存在最佳圆锥顶角(对应最小气泡体积)，其值与气压无关，只随接触角的增大而增大.在接触角处于90°—180°范围内，最佳圆锥顶角下圆锥形 关键词： 气泡 形核 多孔金属 Gasar     

Electrohydrodynamics of a Cone–Jet Flow at a High Relative Permittivity

We have proposed a new solution of the electrohydrodynamic equations describing a novel cone–jet flow structure formed at a conductive liquid meniscus in an electric field. Focusing on the liquids characterized by a high relative permittivity and using the slender body approximation, the cone–jet transition profiles and their characteristic radii are predicted in relation to the material parameters. The stable value of the cone angle is obtained using the Onsager’s principle of maximum entropy production. Three different regimes of the cone–jet flow behavior are identified depending on the relative importance of capillary, viscous and inertial stress contributions. The presented complete analytical solutions for the cone–jet transition zone and the far jet region yield several different laws of algebraic decrease for the radius, surface charge, and electric field of the jet.     

Phase field theory of heterogeneous crystal nucleation

The phase field approach is used to model heterogeneous crystal nucleation in an undercooled pure liquid in contact with a foreign wall. We discuss various choices for the boundary condition at the wall and determine the properties of critical nuclei, including their free energy of formation and the contact angle as a function of undercooling. For particular choices of boundary conditions, we may realize either an analog of the classical spherical cap model or decidedly nonclassical behavior, where the contact angle decreases from its value taken at the melting point towards complete wetting at a critical undercooling, an analogue of the surface spinodal of liquid-wall interfaces.     

Correlations to predict droplet size in ultrasonic atomisation.

In conventional two fluid nozzles, the high velocity air imparts its energy to the liquid and disrupts the liquid sheet into droplets. If the energy for liquid sheet fragmentation can be supplied by the use of ultrasonic energy, finer droplets with high sphericity and uniform size distribution can be achieved. The other advantage of ultrasound induced atomisation process is the lower momentum associated with ejected droplets compared to the momentum carried by the droplets formed using conventional nozzles. This has advantage in coating and granulation processes. An ultrasonic probe sonicator was designed with a facility for liquid feed arrangement and was used to atomise the liquid into droplets. An ingenious method of droplet measurement was attempted by capturing the droplets on a filter paper (size variation with regard to wicking was uniform in all cases) and these are subjected to image analysis to obtain the droplet sizes. This procedure was evaluated by high-speed photography of droplets ejected at one particular experimental condition and these were image analysed. The correlations proposed in the literature to predict droplet sizes using ultrasound do not take into account all the relevant parameters. In this work, a truly universal correlation is proposed which accounts for the effects of physico-chemical properties of the liquid (flow rate, viscosity, density and surface tension), and ultrasonic properties like amplitude, frequency and the area of vibrating surface. The significant contribution of this work is to define dimensionless numbers incorporating ultrasonic parameters, taking cue from the conventional numbers that define the significance of different forces involved in droplet formation. The universal correlations proposed are robust and can be used for designing ultrasonic atomisers for different applications. Among the correlations proposed here, those ones that are based on the dimensionless numbers and Davies approach predict droplet sizes within acceptable limits of deviation. Also, an empirical correlation from experimental data has been proposed in this work.     

Variational problems on the contact of heterogeneous phases

The variational formulation of general problems of capillary surface equilibrium (problems on a drop on a solid surface, drop on the surface of a drop of another liquid, partly faceted capillary surface) is considered. For the problem of a partly faceted capillary surface, the Legendre transformation between the equilibrium form and surface energy density is used.     

Mechanism and characteristics of steam laser patterning

Steam laser patterning of thin films and/or solid surfaces has been studied by jetting a beam of steam, such as water vapor, onto a sample surface to form a thin liquid film on it and patterning the sample by laser etching along predetermined path. In steam laser patterning, bubbles are formed in a thin liquid film on a sample surface irradiated by a pulsed laser. When the collapsed shock wave generated at the moment of bubble collapse and the high-speed liquid jet formed during bubble collapse are strong enough, cavitation erosion of the sample surface takes place. Compared to dry laser patterning, the etching rate can be greatly enhanced and no shoulder-like structure is formed at the rim of the laser-irradiated spot in steam laser patterning due to this cavitation erosion effect. PACS 81.65.cf; 52.38.Mf; 79.20.Ds; 42.62.-b; 62.50.+p     

Simulation on parameter optimization of gold nano-antenna attached to semiconductor ring resonator for heat-assisted magnetic recording device

Heat-assisted magnetic recording is a technology to improve recording density for hard disks. The authors’ group has proposed a device, in which a gold nano-antenna as a near-field transducer is attached to a semiconductor ring resonator as an integrated light source. Localized surface plasmon resonance at the tip of nano-antenna excites near-field light to form small recorded marks to increase recording density. In this study, to improve the device performance, the dependence of spot size and energy density of near-field light on tip curvature, length, and bottom diameter of nano-antenna was investigated through a numerical simulation. Cylinder type and cone type nano-antennas were considered. For both types, as the tip curvature of nano-antenna increased, the spot size decreased and the energy density increased. It was possible to reduce the spot size to 18 × 18 nm². For cylinder type, there was an optimal length of nano-antenna where the energy density became maximum, and the optimal length changed depending on the presence or absence of recording medium. This was because of the difference in plasmon resonance condition. Moreover, for cone type, there was an optimal bottom diameter of nano-antenna where the energy density became maximum, and the optimal bottom diameter changed depending on the length of nano-antenna.

     

Laser-induced jet formation and droplet ejection from thin metal films

An experimental study of femtosecond laser-induced jet formation and droplet ejection from thin metal films is presented. These processes are compared to liquid jet formation during laser-induced forward transfer of viscous liquids. As a result of this comparison, a mechanism explaining the main features of laser processing of thin metal films is proposed. According to this mechanism, laser-induced generation of a molten bump and its collapse are similar to the collapse of cavitation bubbles on a liquid?Cair interface. Material criteria required for realization of the jetting process are discussed and supported by experimental observations.     

Thickness of a boundary layer attributed to the wave motion on the charged free surface of a viscous liquid

It is shown that the analytical estimator for the boundary layer thickness that contains the wave frequency in the denominator and is proposed for approximate calculation of the wave motion on the free surface of a viscous liquid cannot be formally applied to the wave motion on the uniformly charged liquid surface. The fact is that, when the surface charge density attains a value critical in terms for the Tonks-Frenkel instability, the wave frequency tends to zero. From the analysis of liquid motions near the electric charge critical density, a technique is proposed for calculating the thickness of a boundary layer attributed to flows of various kinds. It is found that the thickness of the boundary layer due to aperiodic flows with amplitudes exponentially growing with time (such flows take place at the stage of instability against the surface charge) does not exceed a few tenths of the wavelength, whereas the thickness of the boundary layer due to exponentially decaying liquid flows is roughly equal to the wavelength.     

毛细作用的变分法理论

以圆柱形毛细管内壁、毛细管内的液体和气体为研究对象.设毛细管液体的表面为旋转面,写出了系统的自由能(表面能及重力势能)的泛函.使自由能泛函取极值的欧拉方程就是关于附加压强的拉普拉斯公式.泛函取极值时在可动边界(毛细管内壁)上的边界条件就可导出关于接触角的杨氏方程.此二方程是自由能取极值的密不可分的两个必要条件.     

Building designed granular towers one drop at a time

A dense granular suspension dripping on an imbibing surface is observed to give rise to slender mechanically stable structures that we call granular towers. Successive drops of grain-liquid mixtures are shown to solidify rapidly upon contact with a liquid absorbing substrate. A balance of excess liquid flux and drainage rate is found to capture the typical growth and height of the towers. The tower width is captured by the Weber number, which gives the relative importance of inertia and capillary forces. Various symmetric, smooth, corrugated, zigzag, and chiral structures are observed by varying the impact velocity and the flux rate from droplet to jetting regime.