Dynamic structure formation at the fronts of volatile liquid drops

We report on instabilities during the spreading of volatile liquids, with emphasis on the novel instability observed when isopropyl alcohol is deposited on a monocrystalline Si wafer. This instability is characterized by emission of drops ahead of the expanding front, with each drop followed by smaller, satellite droplets, forming the structures which we nickname "octopi" due to their appearance. A less volatile liquid, or a substrate of larger heat conductivity, suppresses this instability. We formulate a theoretical model that reproduces the main features of the experiment.     

Steric interactions in cellular structures formed in a water/oil/surfactant/cosurfactant mixture

Dynamics of dense microemulsion droplets forming ordered cellular phases as temperature increases are investigated by neutron spin echo spectroscopy (NSE). The shape fluctuations of droplets are shown using a specific contrast. Their relaxation time tau(f) is obtained by analysis of the NSE curves, which also reveals the short-range Brownian motion (diffusion constant D0) of the "caged" droplets. The thermal dependence of D0 and tau(f) supports the notion of entropically driven interdroplet steric repulsion stabilizing the cellular phases.     

组分混合磷脂球胶束化动力学

两亲性磷脂分子能够形成各种不同形态的胶束,其结构形成不仅依赖于磷脂分子结构和组成,还依赖于两亲性分子的自组装路径. 本工作采用粗粒化分子动力学方法模拟研究了二棕榈酰磷脂酰胆碱(DPPC)与十六烷基磷酸胆碱(HPC)混合磷脂球胶束化行为. 通过调节DPPC/HPC的组分比例和磷脂球尺寸,观察到多种不同胶束结构形成,例如：球形和非球形(扁平或长椭球)囊泡、盘形胶束、单环或双环胶束和蠕虫状胶束. 研究发现,由于原位胶束化作用,采用磷脂球作为初始态有利于形成囊泡和环形拓扑结构胶束. 模拟结果表明,结合初始态结构设定同时调节磷脂分子组成是一种有效调控磷脂胶束结构的方法.     

Formation of ordered micro-porous membranes

Regular micro-porous polymeric membranes have recently been discovered by rapidly evaporating a solution of CS₂ containing poly(p-phenylene)-block-polystyrene [#!ref1!#]. 1,2-dichloroethane (a chlorated solvent in which polystyrene gel phase has never been observed) is also found to produce ordered structures, which definitively excludes eventual effect of the gelation process during the membrane formation. The observation of the solution surface during the solvent evaporation reveals the growing of micron-sized water droplets trapped at the surface and forming compact aggregates. The study of the solution/water interface shows that the water droplets profile is in agreement with the pore shape observed in the membranes. Moreover, the copolymer was found to precipitate at the interface, forming a layer encapsulating the droplets and preventing their coalescence. In that way, the final structure results from the droplets stacking under the action of large surface currents. Finally, we argue that the decisive element in the formation of ordered structures is the ability of the polymer to precipitate at the solution/water interface, which seems to be related the star-polymer microstructure. Received: 18 August 1998     

Spray Forming: Controlling the Atomization Result with Regard to Particle Properties

Spray forming is a new production process for manufacturing semi‐finished metal products. The procedure combines the liquid metal atomization with the compaction event of the generated metal droplets on a substrate. During spray forming, the heat emission of the molten metal particles is one of the most important transfer operations defining the thermal conditions in the spray and deposit. Definite thermal conditions throughout the manufacturing process lead to fine equi‐axed grain structures and therewith to characteristic and desired material properties. Thus, in particular, the disintegration of the molten metal occurring during spray forming is an essential step.     

Planar Droplet Sizing for the Characterization of Droplet Clusters in an Industrial Gun‐Type Burner

An important problem in spray combustion deals with the existence of dense regions of droplets, called clusters. To understand their formation mechanism, the droplet dynamics and fuel concentration profile are investigated by means of planar laser techniques in an industrial gun‐type burner. The simultaneous measurement of elastic Mie scattering and Laser Induced Fluorescence (LIF) allows the instantaneous measurement of the Sauter Mean Diameter (SMD), after proper calibration. Using two different CCDs to get the two signals requires a detailed calibration of the CCD response before getting absolute diameters. Pixels are binned 6 by 6 to obtain the final SMD map, this is a compromise between spatial accuracy and noise. Velocity field is measured on both sets of images using standard Particle Image Velocimetry (PIV) algorithms. The comparison of cross‐correlation technique with PDA results shows that the velocity measured on the LIF images are close to the velocity based on D₃₀, whereas the Mie scattering results are similar to D₂₀. On Mie scattering images, regions of high interfacial area forming clusters can be detected. A special tracking scheme is used to characterize their dynamics in terms of velocity and diameters by ensuring that the same volume of fluid is tracked. It is shown that the clusters have a velocity similar to the velocity of droplets with the same diameter as the mean SMD of the cluster. It is also shown that an increase of pressure tends to trigger the appearance of such a group of droplets, due to a smaller diameter of the droplets caused by the increase of pressure discharge. Uncertainties for the different techniques used are discussed.     

超声作用下圆板表面液滴铺展雾化特性

对超声激励下圆板表面液滴铺展及雾化行为进行了可视化观测并基于ANSYS Workbench对超声激励下平板表面等效应力分布进行数值模拟,结合等效应力分布特性分析了不同位置液滴雾化行为差异并归纳总结了液滴铺展雾化的三种典型行为,研究结果表明:超声作用可使圆板表面液滴瞬间雾化且表面会形成与应力分布相一致的间隔交替的"雾化环...     

Influence of the initial parameters of spray water on its motion through a counter flow of high-temperature gases

The motion of spray water through a counter flow of high-temperature gases is experimentally studied on a macroscopic level using optical techniques for diagnostics of two-phase liquid-gas and vapor-liquid flows. It is found that the initial temperature, concentration of typical impurities, and dispersity of water influence the component composition of the forming gas-vapor-droplet mixture. The integral characteristics of evaporation of solitary droplets with initial sizes (conditional characteristic radii) of 3–5 mm and a spray water flow with droplets less than 0.5 mm across through a high-temperature gaseous medium are compared.     

液滴碰撞Janus颗粒球表面的行为特征

为研究液滴碰撞Janus颗粒(双亲性)球表面的独特行为特征,以粒径为5.0 mm铜球为材料制备了Janus颗粒,用直径为2.0 mm的液滴,在韦伯数(We)为2.7,10,20,30的测试情况下对Janus颗粒球表面进行了碰撞实验.结果表明:液滴碰撞Janus颗粒球表面后的运动可分为铺展、回缩、振荡和回弹4个过程.在不...     

Direct numerical simulation of diluted combustion by evaporating droplets

Diluted combustion has been studied using DNS in a three-dimensional temporally developing reacting shear-layer with the oxidizer stream laden with evaporating droplets. The gaseous phase is described in the Eulerian frame while the discrete droplet phase is treated in the Lagrangian frame, with strong two-way coupling between the two phases through mass, momentum and energy exchange. Grid-resolution-independent results have been obtained in cases without and with droplets. A comprehensive parametric study has been conducted by varying the initial Stokes number (St₀) and mass loading ratio (MLR₀). Detailed field analysis has been conducted to examine the complex nonlinear interactions among droplet dynamics, evaporation, turbulence and combustion, and so on. Effects of evaporating droplets on averaged flow and combustion quantities have also been presented. In particular, the conditional scalar dissipation rate is found to be enhanced by evaporating droplets, which suggests that they can promote micromixing and combustion under certain conditions, in addition to their roles in combustion suppression. The transport equation for the mixture fraction variance has been analyzed, with a focus on the vaporization-related source terms. Such source terms exhibit more complex local variations in the present shear-flow non-premixed flame configuration, compared with the case in the homogeneous decaying turbulence configuration of Réveillon and Vervisch (2000).     

Pattern formation in binary fluids confined between rough, chemically heterogeneous surfaces

Using a mesoscale model for hydrodynamics, we simulate driven flow of AB binary fluids past surfaces that contain well-defined roughness or asperities. The geometry and wetting properties of the asperities are found to have a dramatic effect on the flow patterns. We isolate conditions where the A fluid forms vertical bands that bridge the asperities and an imposed shear (or pressure gradient) drives the system to form monodisperse droplets of A within the B fluid. The size of the droplets can be tailored by varying the morphology of the asperities. The surfaces needed to create this rich dynamical behavior are used as the stamps in microcontact printing; thus, the parameter space can readily be accessed experimentally, and the predictions suggest an efficient method for forming emulsions with well-controlled morphologies.     

Oscillatory stability of quantum droplets in PT-symmetric optical lattice

We study stability and collisions of quantum droplets(QDs) forming in a binary bosonic condensate trapped in parity-time (PT)-symmetric optical lattices. It is found that the stability of QDs in the PT-symmetric system depends strongly on the values of the imaginary part W_0 of the PT-symmetric optical lattices, self-repulsion strength g, and the condensate norm N. As expected,the PT-symmetric QDs are entirely unstable in the broken PT-symmetric phase. However, the PT-symmetric QDs exhibit oscillatory stability with the increase of N and g in the unbroken PT-symmetric phase. Finally, collisions between PT-symmetric QDs are considered. The collisions of droplets with unequal norms are completely different from that in free space. Besides, a stable PT-symmetric QDs collides with an unstable ones tend to merge into breathers after the collision.     

Droplet traffic at a simple junction at low capillary numbers

We report that, when a train of confined droplets flowing through a channel reaches a junction, the droplets either are alternately distributed between the different outlets or all collect into the shortest one. We argue that this behavior is due to the hydrodynamic feedback of droplets in the different outlets on the selection process occurring at the junction. A "mean field" model, yielding semiquantitative results, offers a first guide to predict droplet traffic in branched networks.     

Method for producing nanomaterials in the plasma of a low-pressure pulsed arc discharge

A new method for the production of nanomaterials in the plasma of a low-pressure arc discharge is developed and experimentally studied. This method can be used to synthesize nanoparticles 5–10 nm in size with a narrow size distribution. In this method, a low-pressure arc discharge is used to melt a material, to disperse the molten material, to deliver liquid material droplets to the plasma, to cool the liquid nanoparticles forming in the plasma up to their solidification, and to deposit the solidified nanoparticles onto a substrate.     

Self-propelled droplets

Self-propelled droplets are a special kind of self-propelled matter that are easily fabricated by standard microfluidic tools and locomote for a certain time without external sources of energy. The typical driving mechanism is a Marangoni flow due to gradients in the interfacial energy on the droplet interface. In this article we review the hydrodynamic prerequisites for self-sustained locomotion and present two examples to realize those conditions for emulsion droplets, i.e. droplets stabilized by a surfactant layer in a surrounding immiscible liquid. One possibility to achieve self-propelled motion relies on chemical reactions affecting the surface active properties of the surfactant molecules. The other relies on micellar solubilization of the droplet phase into the surrounding liquid phase. Remarkable cruising ranges can be achieved in both cases and the relative insensitivity to their own ‘exhausts’ allows to additionally study collective phenomena.     

The ranges of the aerodynamic drag coefficient of water droplets moving through typical gas media

The integral characteristics of the deformation processes of liquid (water) droplets moving through a gas medium (air at a temperature of about 300 K, kerosene combustion products with a temperature of about 1100 K) were experimentally investigated. The initial sizes (characteristic radii) of the droplets varied from 50 μm to 2.5 mm, and the initial velocities, from 0.5 m/s to 5 m/s. Velocities of the gas counter (relative to the direction of droplets displacement) flow weremaintained about 1.5 m/s by means of a special-purpose pressure system. Characteristic “deformation cycles” of droplets, their number, durations, and lengths, and also maximal amplitudes of the deformation process were identified. The ranges of numerical values of the aerodynamic drag coefficients c _d for the characteristic deformation cycles were determined. The influence of droplets velocities and sizes, and also of the gas medium temperature on these parameters was established. Characteristic times of preserving the corresponding droplet forms and c _d values within the range of the most typical deformation cycles were found.     

Droplet traffic in microfluidic networks: a simple model for understanding and designing

We propose a simple model to analyze the traffic of droplets in microfluidic "dual networks." Such functional networks which consist of two types of channels, namely, those accessible or forbidden to droplets, often display a complex behavior characteristic of dynamical systems. By focusing on three recently proposed configurations, we offer an explanation for their remarkable behavior. Additionally, the model allows us to predict the behavior in different parameter regimes. A verification will clarify fundamental issues, such as the network symmetry, the role of the driving conditions, and of the occurrence of reversible behavior. The model lends itself to a fast numerical implementation, thus can help designing devices, identifying parameter windows where the behavior is sufficiently robust for a device to be practically useful, and exploring new functionalities.     

液态三元Fe-Sn-Si/Ge偏晶合金相分离过程的实验和模拟研究

本文采用自由落体实验技术和格子玻尔兹曼计算方法研究了低重力条件下液态Fe-Sn-Si/Ge合金的相分离过程. 实验发现, 二种合金液滴在自由下落过程中均发生显著的液相分离, 形成了壳核和弥散组织. 当Fe-Sn-Si合金中的Si元素被等量的Ge元素替换后, 壳核组织中富Fe区和富Sn区的分布次序会发生反转. 计算表明, 在液相分离过程中冷却速率、Marangoni对流和表面偏析对壳核构型的选择和弥散组织的形成起决定性作用.     

Laser direct writing of biomolecule microarrays

Protein-based biosensors are highly efficient tools for protein detection and identification. The production of these devices requires the manipulation of tiny amounts of protein solutions in conditions preserving their biological properties. In this work, laser induced forward transfer (LIFT) was used for spotting an array of a purified bacterial antigen in order to check the viability of this technique for the production of protein microarrays. A pulsed Nd:YAG laser beam (355 nm wavelength, 10 ns pulse duration) was used to transfer droplets of a solution containing the Treponema pallidum 17 kDa protein antigen on a glass slide. Optical microscopy showed that a regular array of micrometric droplets could be precisely and uniformly spotted onto a solid substrate. Subsequently, it was proved that LIFT deposition of a T. pallidum 17 kDa antigen onto nylon-coated glass slides preserves its antigenic reactivity and diagnostic properties. These results support that LIFT is suitable for the production of protein microarrays and pave the way for future diagnostics applications. PACS 87.14.Ee; 81.15.Fg; 07.07.Df