液体火箭有机凝胶喷雾液滴蒸发模型及仿真研究

凝胶推进剂虽然兼具有液体推进剂流量可控和固体推进剂长期可储存等优点, 但凝胶喷雾液滴蒸发燃烧问题却一直困扰着凝胶推进剂研制及燃烧室设计工作, 阻碍了凝胶推进剂实际工程应用.设计实现了凝胶单液滴蒸发燃烧实验系统, 通过某型有机凝胶偏二甲肼(UDMH)单液滴在四氧化二氮蒸气中的蒸发燃烧实验现象, 进一步深入分析了凝胶液滴蒸发燃烧机理.根据实验中凝胶单液滴在不同阶段的蒸发特性, 建立了有机凝胶喷雾液滴在胶凝剂膜形成、膨胀、破裂三个不同蒸发阶段的多组分蒸发模型, 采用初步选定的模型参数及物性参数对凝胶单液滴在高温气体环境中的蒸发全过程进行了仿真计算, 并与常规液体液滴的仿真结果进行了对比分析.结果表明,凝胶喷雾液滴表面胶凝剂含量在蒸发初期增加比较缓慢, 但在某临界时刻后的极短时间内迅速升高至形成胶凝剂膜的质量分数95%, 导致表面质量流率迅速下降至0,表面温度则快速上升至UDMH推进剂沸点.胶凝剂膜形成后, 液滴半径及表面UDMH蒸气质量分数出现了实验现象中凝胶液滴反复膨胀-破裂的震荡现象, 液滴表面温度维持在略高于沸点的某温度范围内,凝胶液滴内部的沸腾蒸发明显强于液体液滴表面稳态蒸发流率, 使得凝胶喷雾液滴生存时间小于常规液体液滴.     

Persistent droplet motion in liquid-liquid dewetting

When a nonvolatile liquid film dewets from a partly compatible liquid substrate, the advancing dewetting front leaves behind droplets formed through a Rayleigh instability mechanism at its rim. We have found that these droplets continue to move in the direction of the dewetting front for extended periods (of order one day) with an initial droplet velocity varying linearly with the droplet size, and a displacement varying logarithmically with time. We attribute this persistent motion to a transient surface tension gradient on the substrate liquid surface trailing the dewetting front.     

Capillarity driven instability of a soft solid

We report the observation of a Plateau instability in a thin filament of solid gel with a very small elastic modulus. A longitudinal undulation of the surface of the cylinder reduces its area thereby triggering capillary instability, but is counterbalanced by elastic forces following the deformation. This competition leads to a nontrivial instability threshold for a solid cylinder. The ratio of surface tension to elastic modulus defines a characteristic length scale. The onset of linear instability is when the radius of the cylinder is one-sixth of this length scale, in agreement with theory presented here.     

Absorption of solar radiation by charged water droplets

We consider the absorption of solar radiation by charge accumulated on the surface of an otherwise neutral water droplet. We estimate the order of magnitude of the additional contribution from the surface charges to the photoabsorption cross-section. The approximate location and strength of major spectral lines is indicated. This spectral information requires determination of the quantum mechanical energy levels and wave functions of the absorbing system. An approximate calculation is carried out using a simple model for electrons on the surface of the droplet. The implications for atmospheric applications are considered.     

Modelling drying pathways of an evaporating soft matter droplet

Micro-droplets of soft matter solutions have different morphologies upon drying, and can become wrinkled, buckled or cavitated particles. We investigate the morphology evolution of a drying soft matter droplet in this work: at the early stage of drying, wrinkling or cavitation instability can occur in the droplet, depending on the comparison between the critical wrinkling and cavitation pressure; at a later stage of drying, no wrinkles will appear if cavitation happens first, while cavitation can still occur if wrinkling happens first. A three-dimensional phase diagram in the space of elastic length, gel layer thickness and weight loss is provided to illustrate the drying pathways of a soft matter droplet. This diagram can help guide future fabrications of micro-particles with desired morphologies.     

Rosensweig instability of ferrogel thin films or membranes

We derive the dispersion relation of surface waves for magnetic gel membranes or thin films at the interface between two fluids in the presence of an external magnetic field normal to the free surface. Above a critical field strength surface waves become linearly unstable with respect to a stationary pattern of surface protuberances. This linear stability criterion generalizes that of the Rosensweig instability for ferrofluid and ferrogel free surfaces to take into account bending elasticity and intrinsic elastic and magnetic surface properties of the film or membrane, additionally. The latter is of interest for uniaxial ferrogel film or membranes, which show a locked-in permanent magnetization.     

An accurate adaptive solver for surface-tension-driven interfacial flows

A method combining an adaptive quad/octree spatial discretisation, geometrical Volume-Of-Fluid interface representation, balanced-force continuum-surface-force surface-tension formulation and height-function curvature estimation is presented. The extension of these methods to the quad/octree discretisation allows adaptive variable resolution along the interface and is described in detail. The method is shown to recover exact equilibrium (to machine accuracy) between surface-tension and pressure gradient in the case of a stationary droplet, irrespective of viscosity and spatial resolution. Accurate solutions are obtained for the classical test case of capillary wave oscillations. An application to the capillary breakup of a jet of water in air further illustrates the accuracy and efficiency of the method. The source code of the implementation is freely available as part of the Gerris flow solver.     

A new surface-tension formulation for multi-phase SPH using a reproducing divergence approximation

In this paper, we propose a new surface-tension formulation for multi-phase smoothed particle hydrodynamics (SPH). To obtain a stable and accurate scheme for surface curvature, a new reproducing divergence approximation without the need for a matrix inversion is derived. Furthermore, we introduce a density-weighted color-gradient formulation to reflect the reality of an asymmetrically distributed surface-tension force. We validate our method with analytic solutions and demonstrate convergence for different cases. Furthermore, we show that our formulation can handle phase interfaces with density and viscosity ratios of up to 1000 and 100, respectively. Finally, complex three-dimensional simulations including breakup of an interface demonstrate the capabilities of our method.     

硫酸镁微液滴水和重水交换动力学的微区拉曼研究

本文利用微区拉曼技术,研究硫酸镁液滴水和重水交换的动力学.在低湿度时,由接触离子对连接形成的链状结构使硫酸镁液滴表面形成胶态结构,阻碍其与环境之间的水交换,造成表面和内部的结构差异.拉曼光谱的高空间分辨能力为观测这一特殊的表面结构提供了便利.沉积在聚四氟乙烯疏水基底上的硫酸镁重水液滴呈球形,可以实现对液滴表面和中心的两...     

表面张力对固壁旁空泡运动特性影响的理论和实验研究

表面张力是影响空泡脉动及空蚀的一个重要因素.对五种不同表面张力液体中空泡脉动（膨胀和收缩）过程进行了研究,并将实验结果与基于空泡生长和溃灭理论的计算结果进行了对比.实验中,用激光作为测试光源,采用光偏转测试系统研究了不同表面张力液体中空泡泡壁运动规律及泡壁速度的变化.结果表明：表面张力对空泡膨胀过程起抑制作用,故液体表面张力愈大,空泡能达到的最大直径越小;表面张力对空泡的收缩过程则起加速作用,液体表面张力愈大,收缩越迅速,空泡泡壁运动速度越大,其所产生的瞬时溃灭压强越大,空化效果越好. 关键词： 表面张力 空泡 光偏转     

The shrinking instability of toroidal liquid droplets in the Stokes flow regime

We analyze the stability and dynamics of toroidal liquid droplets. In addition to the Rayleigh instabilities akin to those of a cylindrical droplet there is a shrinking instability that is unique to the topology of the torus and dominates in the limit that the aspect ratio is near one (fat tori). We first find an analytic expression for the pressure distribution inside the droplet. We then determine the velocity field in the bulk fluid, in the Stokes flow regime, by solving the biharmonic equation for the stream function. The flow pattern in the external fluid is analyzed qualitatively by exploiting symmetries. This elucidates the detailed nature of the shrinking mode and the swelling of the cross-section following from incompressibility. Finally the shrinking rate of fat toroidal droplets is derived by energy conservation.     

Amoeba-like motion of an oil droplet

In this paper, we demonstrate our recent attempt to construct a chemical model system of amoeboid motion. The system is intended to mimic biological motility based on the generation and collapse of an elastic aggregate; it is composed of oil, water, and surfactants. In this chemical system, the oil–water interface shows extension and retreat of spherical extrusions accompanied by the generation of aggregate on the interface. This instability of the oil–water interface can cause autonomous splitting and motion of a floating oil droplet. The current mathematical model based on the generation of a passive elastic gel is explained, as well as the discrepancy between the model and the experiments. We further describe recently observed microscopic characteristics of the aggregate formation process that might cause the interfacial instability. Finally, we discuss the disadvantage of a chemical model system compared with active colloid and in vitro biological systems, and also mention its potential advantages.     

Rayleigh instability of charged droplets in the presence of counterions

The classical instability of a charged spherical droplet is reconsidered in the presence of counterions. An ensemble of such droplets is studied within a simplified cell model. Screening of the electric field by the counterions is found to increase the equilibrium droplet size. Furthermore, if the ions can enter the droplet, a first-order phase transition occurs upon increasing Bjerrum length, surface tension or droplet density, leading to a phase separation. Simple scaling properties of the free energy give the shape of the phase boundary and show the system to be scale-invariant there. Pearl-necklace structures of hydrophobic polyelectrolytes are discussed as an application. Received 30 August 2001     

Mechanisms of melt droplets and solid-particle ejection from a target surface by pulsed laser action

We suggest an explanation of the effect of melt droplets and solid particle ejection from a target surface under the impact of laser radiation with intensity 10⁸–10¹⁰ W/cm². We consider the capillary wave instabilities on the evaporating surface of melt, which lead to growth of large-scale surface structures and ejection of macroparticles. The instability increments and characteristic droplet sizes are determined. Conditions are found for droplet-free evaporation in terms of the dynamic pressure of evaporated matter.     

液滴在非互溶大黏度比液体表面的铺展特性

利用基于润滑理论的液滴在非互溶液体表面铺展的物理模型,探讨大黏度比情形下黏度比对液滴演化过程和平衡形状的影响,分析液滴厚度、铺展半径等特征参数的变化。结果表明：接触线附近液—液界面的变形受黏度比和表面张力比的影响;提高黏度比将导致铺展速率降低、时间常数减小,进而延长演化历程,但不影响液滴最终稳定形状;铺展半径与时间的关系满足x_max= 1 - 0.2 exp(- βt);大黏度比液滴铺展后期并未出现小黏度比时的惯性振荡现象。     

浸润接触线的摩擦性质与固体表面张力的Wenzel行为

依据Adam和Jessop关于固-液-气三相接触线具有静摩擦性的观点，能从浸润接触角数据推算 出固体的表面张力系数，但结果显然会与摩擦条件的具体形式有关.以报道的不锈钢和聚丙 烯实验数据为例，通过对比Mises和Amonton两种摩擦定律给出的表面张力系数，后者给出的 张力才具有总是随表面粗糙性的增大而增加的Wenzel效应，以及当固-液，固-气界面的张力 系数之差等于液-气界面的张力系数时，退后角将变为零的全浸润条件.这似乎表明用Amonto n定律描写接触线的静摩擦要更为合理. 关键词： 浸润角 静摩擦 固体表面张力     

波纹基底上含不溶性活性剂液滴的铺展稳定性

针对波纹基底上含不溶性活性剂液滴的铺展历程, 采用润滑理论建立了液滴铺展数理模型, 推导出基态和扰动态下液膜厚度和活性剂浓度的演化方程组, 基于非模态稳定性理论分析了液滴铺展的稳定性及参数的影响规律. 研究表明: 扰动量在液滴中心及铺展前沿处很小, 在液膜最薄处达到最大值且活性剂浓度的负扰动现象比较明显; 扰动波数可增强液滴铺展稳定性, 但随扰动波数增加, 该稳定性逐渐下降甚至转变为不稳定. 增加 Marangoni数将导致液滴铺展不稳定性加剧; 增大基底高度具有增强液滴铺展稳定的作用, Peclet数和基底波数取适中值时有利于液滴铺展的稳定性. 关键词： 活性剂液滴 非平整基底 铺展 非模态稳定性     

共焦拉曼光谱对疏水基底上Mg(CH3COO)2液滴与CO2非均相反应动力学过程的观测

本文利用共焦拉曼光谱在球形液滴的表面和中心可聚焦两次的特性, 观测了不同RH(relative humidity, 相对湿度)下疏水基底上Mg(CH3COO)2液滴与CO2在液滴表面和中心的反应情况。结果表明, 反应生成的产物为碱式碳酸镁。当RH约73%时, 液滴表面和中心生成的产物的量差别不大; 当RH约65%时, 表面和中心的差别较RH约73%时明显; 而当RH降至约58%时, 表面和中心的差别则变得非常明显。由于Mg(CH3COO)2液滴在较低相对湿度下会形成胶态结构, 引起传质受阻, 从而使得CO2由液滴表面扩散到内部变得越来越困难, 最终导致液滴表面生成的产物比中心多。     

The optimal faucet

The production of small fluid droplets relies on an instability of solutions to the Young-Laplace equation. We ask whether smaller droplets can be produced by changing the shape of the nozzle. At a given critical pressure, the circular nozzle actually produces the largest droplet. The droplet volume can be decreased by up to 18% using a triangular nozzle with stretched corners.     

The influence of temperature gradients on the kinetic boundary layer problem for a condensing droplet

We extend an earlier method for solving kinetic boundary layer problems to the case of particles moving in aspatially inhomogeneous background. The method is developed for a gas mixture containing a supersaturated vapor and a light carrier gas from which a small droplet condenses. The release of heat of condensation causes a temperature difference between droplet and gas in the quasistationary state; the kinetic equation describing the vapor is the stationary Klein-Kramers equation for Brownian particles diffusing in a temperature gradient. By means of an expansion in Burnett functions, this equation is transformed into a set of coupled algebrodifferential equations. By numerical integration we construct fundamental solutions of this equation that are subsequently combined linearly to fulfill appropriate mesoscopic boundary conditions for particles leaving the droplet surface. In view of the intrinsic numerical instability of the system of equations, a novel procedure is developed to remove the admixture of fast growing solutions to the solutions of interest. The procedure is tested for a few model problems and then applied to a slightly simplified condensation problem with parameters corresponding to the condensation of mercury in a background of neon. The effects of thermal gradients and thermodiffusion on the growth rate of the droplet are small (of the order of 1%), but well outside of the margin of error of the method.