High-speed roll coating with complex rheology fluids

The flow of high solids content suspensions and coating colors through the deformable gap of counter-rotating rolls at high speed was investigated. Measurements of nip pressure profiles in a laboratory film coater were conducted and flow visualization experiments downstream the nip were carried out. A high-speed video system allowed showing the formation, elongation and break up of filaments that yield the misting droplets. The role of rheology on misting was also investigated using shear and extensional data. Results show that misting, mainly generated by film splitting, is reduced when the extensional viscosity of the color is larger.     

Filament-breaking length—a measure of elasticity in extension

A technique has been developed to assess elasticity in extension of fluids which are weakly elastic. The technique is based on stretching a fluid sample held between two small closely spaced co-axial disks until the fluid bridge or filament breaks. The distance between the disks on breakup, ‘the breaking length’, is measured. When the fluid is elastic, this length is greater than that of an equivalent inelastic fluid. An inelastic baseline was established by measuring the breaking lengths of a wide variety of Newtonian fluids. Measurements with weakly elastic fluids reveal that the extra breaking length increases with polymer type and concentration and with the rate of stretching, the expected behavior for elastic liquids. The breaking lengths of model paper coatings were measured and found to correlate with the degree of misting in a roll coating machine, indicating that droplet formation is related to extensional elasticity.     

Rheological behavior of compatibilized and non-compatibilized PA6/EPM blends

The rheological properties of PA-6/EPM polymer blends, non-compatibilized and compatibilized with grafted ethylene propylene rubber (EPM-g-MA), have been investigated. Linear and non-linear (relaxation both in shear and extension) experiments were realized. Stress relaxation experiments coupled with scanning electron microscopy (SEM) analysis showed the existence of one relaxation time and non-deformed droplets for the immiscible blend, and two relaxation times and deformed droplets for the compatibilized ones, the second relaxation being more pronounced for higher compatibilizer contents. These results clearly indicate that, despite the high viscosity and elasticity ratios, if high amounts of compatibilizer are added to the blend, interfacial slip is suppressed and a high-enough adhesion between the phases is achieved for the high-viscosity dispersed phase to be deformed. Paper presented at the 3rd Annual European Rheology Conference, April 27–29, 2006, Crete, Greece     

流场中聚合物共混体系液滴形变的理论模型

讨论了两相聚合物共混体系中，悬浮于另一种牛顿（或粘弹）液体中的牛顿（或粘弹）液滴的形变理论模型．影响液滴形变的主要因素有两相的组成、粘度比和弹性比、动态界面张力、临界界面张力系数，外流场形式及其强度．对于两相均为牛顿流体的体系，理论预测能够与实验相符；对于两相（或其中一相）为粘弹流体的体系，由于弹性的影响而使液滴形变的研究变得复杂，理论模型尚需完善．建立完整的液滴形变理论模型还需深入研究界面层、微观分子形变、液滴之间及液滴和连续相介质之间的相互作用对液滴形变的影响     

Flow of elasto-viscoplastic liquids through a planar expansion–contraction

The steady flow of incompressible elasto-viscoplastic liquids through a planar expansion–contraction is investigated. A novel constitutive model is employed to describe the mechanical behavior of the flowing liquids. Numerical solutions of the constitutive and conservation equations were obtained via a finite element method to investigate the role of elasticity, yield stress, and inertia. The fields of velocity, stress, elastic strain, and rate of strain were obtained for different combinations of the governing parameters. It was observed that these fields, as well as the shape and position of the yield surface, are all strong functions of elasticity, yield stress, and inertia. The trends observed agree well with previous numerical and visualization results available in the literature. The present work offers a detailed study on the effects of elasticity, presenting, in particular, the fields of elastic strain.     

Hydrodynamics of a drying multicomponent liquid droplet

Using microscopy methods on light and dark fields, the flow patterns developing in drying droplets of pure transparent liquids, solutions, and suspensions of micro- and nanoparticles are investigated. The flow patterns inside drying droplets of real, colloid, and mixed solutions containing nanoand microparticles-markers are studied by means of video and photo registration of microscopic images. The analysis of particle displacements indicates the existence of a global convective flow which forms a toroidal circulation with an ascending jet at the droplet center. The typical types of the structures depending on the droplet composition are distinguished. It is shown that the intensity of the flow inside the droplet affects the surface convection. The effect of the hydrodynamic flow on the transport of a substance, forming the dry-deposit texture, is studied.     

Two-dimensional study of drop deformation under simple shear for Oldroyd-B liquids

The effect of viscoelasticity on the deformation of a circular drop suspended in a second liquid in shear is investigated with direct numerical simulations. A numerical algorithm based on the volume-of-fluid method for interface tracking is implemented in two dimensions with the Oldroyd-B constitutive model for viscoelastic liquids. The code is verified against a normal mode analysis for the stability of two-layer flow in a channel; theoretical growth rates are reproduced for the interface height, velocity and stress components. Drop simulations are performed for drop and matrix liquids of different viscosities and elasticities. A new feature is found for the case of equal viscosity, when the matrix liquid is highly elastic and surface tension is low; hook-like structures form at the drop tips. This is due to the growth of first normal stress differences that occur slightly above the front tip and below the back tip as the matrix elasticity increases above a threshold value.     

Thermocapillary migration of liquid droplets in a temperature gradient in a density matched system

An experimental investigation of thermocapillary flow in droplets of a vegetable oil (partially hydrogenated soybean oil) immersed in silicone oil was conducted in a test cell with a heated top wall and a cooled bottom wall. The liquids are nearly immiscible and have equal densities at a temperature below the room temperature, thus providing a simulation of low-gravity conditions by reducing the buoyancy forces. The interfacial tension between the two oils was measured in the temperature range 20–50°C using a capillary tube and d/dT was determined to be negative. Droplets ranging in sizes from 3 mm to 1 cm diameter were injected into the silicone oil. The vertical temperature profile in the bulk liquid (silicone oil) produces temperature variations along the interface which induce variations in the interfacial tension. The flow inside the droplet driven by the resulting interfacial shear stresses was observed using a laser light-sheet flow visualization technique. The flow direction is consistent with the sign of d/dT. The observed maximum surface velocities are compared to the theoretical predictions of Young et al. (1959).For short times after injection, the droplets were driven by this flow towards the hot wall above the matched-density temperature until the droplets reached a point where the forces due to the flow and buoyancy were equal. After longer times, the droplets moved to the cold side due to suspected density changes caused by mass transfer from the droplets to the silicone oil. This was confirmed by tests under isothermal conditions, where it was observed that droplets of all sizes fell to the cold bottom eventually.Thus, even though the thermocapillary flow inside the droplets persisted for long times in spite of the mass transfer, the migration of droplets towards the hot side was eventually affected by uncontrolled buoyancy forces resulting from density changes due to mass transfer. While additional liquids are being tried, it is suggested from the present experience that reduced gravity experiments will probably be necessary to provide unambiguous data for the migration of droplets.     

Motion of a slender body in quiescent polymer solutions

The motion of a slender body falling in quiescent polymer solutions is investigated experimentally. It represents the simplest model of motion of single fibers in the flow of fiber suspensions. The fall behavior in quiescent polymer solutions is compared with that in water. It is demonstrated that a slender body falling in Newtonian liquids rotates to adopt a horizontal orientation, whereas in non-Newtonian liquids it rotates towards a vertical orientation but for less concentrated solutions is not able to reach the vertical orientation and moves sideways with a constant orientation angle. The effects of shear thinning and elasticity on the motion of the body are discussed.     

Droplet relaxation in blends with one viscoelastic component: bulk and confined conditions

Using a counter rotating parallel plate shear flow cell, the shape relaxation of deformed droplets in a quiescent matrix is studied microscopically. Both the effects of geometrical confinement and component viscoelasticity are systematically explored at viscosity ratios of 0.45 and 1.5. The flow conditions are varied from a rather low to a nearly critical Ca number. Under all conditions investigated, viscoelasticity of the droplet phase has no influence on shape relaxation, whereas matrix viscoelasticity and geometrical confinement result in a slower droplet retraction. Up to high confinement ratios, the relaxation curves for ellipsoidal droplets can be superposed onto a master curve. Confined droplets with a sigmoidal shape relax in two stages: the first consists of a shape change to an ellipsoid with a limited amount of retraction, and the second is the retraction of this ellipsoid. The latter stage can be described by means of one single relaxation time that can be obtained from the relaxation of initially ellipsoidal droplets. The experimental results are compared to the predictions of a recently published phenomenological model for droplet dynamics in confined systems with viscoelastic components (Minale et al., Langmuir 26:126–132, 2010). However, whereas the model predicts additive effects of geometrical confinement and component viscoelasticity, the experimental data reveal more complex interactions.     

流动聚焦实验

介绍了一种制备微纳米量级颗粒的流动聚焦技术,它的最基本的特点是从毛细管流出的液体由高速运动气体驱动经小孔聚焦形成稳定的锥,锥顶端射出的微射流因不稳定性破碎成小颗粒.实验在自行设计的装置上完成.分析了流动聚焦技术中影响锥-射流以及颗粒形貌的因素,总结了过程中装置的结构参数以及气体压力降、液体流量和物性等参数对射流的直径的影响.结果显示该技术制备的颗粒单分散性好,直径达到微米和亚微米量级,在工业方面有重大应用前景.     

Stress relaxation behavior of PMMA/PS polymer blends

In this work, the stress relaxation behavior of PMMA/PS blends, with or without random copolymer addition, submitted to step shear strain experiments in the linear and nonlinear regime was studied. The effect of blend composition (ranging from 10 to 30 wt.% of dispersed phase), viscosity ratio (ranging from 0.1 to 7.5), and random copolymer addition (for concentrations up to 8 wt.% with respect to the dispersed phase) was evaluated and correlated to the evolution of the morphology of the blends. All blends presented three relaxation stages: a first fast relaxation which was attributed to the relaxation of the pure phases, a second one which was characterized by the presence of a plateau, and a third fast one. The relaxation was shown to be faster for less extended and smaller droplets and to be influenced by coalescence for blends with a dispersed phase concentration larger than 20 wt.%. The relaxation of the blend was strongly influenced by the matrix viscosity. The addition of random copolymer resulted in a slower relaxation of the droplets.     

全氟聚醚羧酸铵离子液体的润滑性能及润滑机理研究

设计制备了三种全氟聚醚羧酸铵离子液体,在微动振动摩擦磨损试验机上考察了其在不同温度下对钢/铜锡合金以及钢/钢摩擦副的润滑性能,并与全氟聚醚(PFPE)和全氟聚醚羧酸(PFPEC)进行了对比. 通过测量接触角表征了所制备离子液体对金属表面的润湿性,通过测试摩擦试验过程中接触电阻的变化分析了摩擦过程中摩擦膜的变化;采用扫描电镜和X射线光电子能谱仪分别对磨斑表面形貌和元素状态进行了表征. 结果表明:作为钢/铜锡合金摩擦副的润滑剂时,全氟聚醚羧酸铵离子液体在常温下的润滑性能与PFPE和PFPEC相差不大,但在高温条件下表现出更为优异的减摩抗磨性能;而作为钢/钢摩擦副的润滑剂,其在常温、高温条件下均表现出优于PFPE以及PFPEC的减摩抗磨性能. 多种物理化学表征研究表明该类离子液体优异的减摩抗磨性能归因于其在金属表面优异的吸附性能以及稳定摩擦化学反应膜的形成.      

Abnormal rotation of a deformed liquid crystal droplet immersed in an isotropic fluid after transient flow

The morphology evolution of liquid crystal droplets immersed in an isotropic fluid in flow field is found to be different from flexible polymer droplets. In this paper, we investigated the retraction of a liquid crystal droplet after transient flow. It is found that the liquid crystal droplet will rotate during the shape recovery, which has never been observed for an isotropic droplet. The factors that influence the rotational angle of a single liquid crystal droplet during retraction progress were studied, including the temperature, the dimension of the droplets, the time of shear flow, the shear rate, the flow type, and the properties of liquid crystal molecules. The rotation of liquid crystal droplet during shape recovery is ascribed to both the bulk elasticity of liquid crystal droplets and the anisotropic properties of the interface between liquid crystal and isotropic fluid.     

Simulation of collisions of two droplets containing two different liquids using incompressible smoothed particle hydrodynamics method

Theoretical and Computational Fluid Dynamics - The present study aims to simulate a collision of two droplets containing immiscible liquids by employing a three- dimensional incompressible smoothed...     

Temperature measurements on droplets in monodisperse stream using laser-induced fluorescence

This paper presents a novel technique based on laser-induced fluorescence in liquids, allowing the temperature of 200-μm diameter monodisperse droplets to be measured. The droplets are seeded with an organic dye (rhodamine B), and the temperature dependence of the fluorescence quantum yield is used to determine temperature. The use of LDA optics and a single argon laser source allows to obtain an additional simultaneous velocity measurement. The method appears particularly interesting for the validation of numerical models of evaporating and combusting droplets in the field of design of the combustion chambers of aeronautical and automotive engines, where fuel is injected in droplet form. The measurement technique and data processing are extensively described in the paper. The method is demonstrated on a heated monodisperse droplet stream: the temperature and velocity distribution along the jet were determined. Received: 28 May 1999/Accepted: 13 December 1999     

Flow seeding with an air nebulizer

 In the framework of studies on anemometric measurements or tomographic visualizations, the seeding of a reactive flow by a nebulizer has been analyzed. In order to determine the performances of this apparatus, different liquids have been tested to evaluate the influence of their physical properties on the droplets size distribution. Measurements have been made using a Phase Doppler Anemometer. It is shown that the mean D ₁₀ diameter lies between 2 and 3 μm, whatever the liquid may be, except pure water, and that, within a certain range, viscosity and surface tension have little influence on the size distribution. The ability of these liquids to seed reactive mixture is also discussed and especially their interaction in premixed flames. Received: 4 May 1998/Accepted: 15 February 1999     

From dynamic modulus via different relaxation spectra to relaxation and creep functions

The main goal of the paper is to compare predictive power of relaxation spectra found by different methods of calculations. The experimental data were obtained for a new family of propylene random copolymers with 1-pentene as a comonomer. The results of measurements include flow curves, viscoelastic properties, creep curves and rubbery elasticity of copolymer melts. Different relaxation spectra were calculated using independent methods based on different ideas. It lead to various distributions of relaxation times and their “weights”. However, all of them correctly describe the frequency dependencies of dynamic modulus. Besides, calculated spectra were used for finding integral characteristics of viscoelastic behaviour of a material (Newtonian viscosity, the normal stress coefficient, steady-state compliance). In this sense all approaches are equivalent, though it appears impossible to estimate instantaneous modulus. The most crucial arguments in estimating the results of different approaches is calculating the other viscoelastic function and predicting behaviour of a material in various deformation modes. It is the relaxation and creep functions. The results of relaxation curve calculations show that all methods used give rather similar results in the central part of the curves, but the relaxation curves begin to diverge when approaching the high-time (low-frequency) boundary of the relaxation curves. The distributions of retardation times calculated through different approaches also appear very different. Meanwhile, predictions of the creep curves based on these different retardation spectra are rather close to each other and coincide with the experimental points in the wide time range. Relatively slight divergences are observed close to the upper boundary of the experimental window. All these results support the conclusion about a rather free choice of the relaxation time spectrum in fitting experimental data and predicting viscoelastic behaviour of a material in different deformation modes. Received: 15 March 2000 Accepted: 18 September 2000     

Oldroyd-B黏弹性液滴碰撞过程的数值模拟

复杂的流变特性使凝胶推进剂的雾化过程存在一定困难,这制约了它的发展.聚合物胶凝剂的加入使凝胶推进剂具有黏弹性,从而在雾化时会产生黏弹性液滴,因此为了进一步认识凝胶推进剂的雾化机理、提高凝胶推进剂的雾化性能,对黏弹性液滴的碰撞行为进行数值模拟研究.针对凝胶推进剂雾化过程中出现的液滴撞击现象,考虑流体具有的黏弹性效应,采用...     

Novel shear modulus equations for concentrated emulsions of two immiscible elastic liquids with interfacial tension

Starting from the shear modulus equation for a dilute emulsion system of two immiscible liquids with interfacial tension, four new equations have been developed for the shear modulus of concentrated emulsions using a differential scheme. The continuous phase and the dispersed droplets are treated as elastic liquids in the derivation. Out of the four models developed in the paper, two models predict the relative shear modulus (ratio of emulsion modulus to continuous phase modulus) to be a function of three variables: elastocapillary number, modulus ratio (dispersed phase modulus to continuous phase modulus) and volume fraction of dispersed phase. The remaining two models include an additional parameter, i.e. the maximum packing volume fraction of droplets. The proposed models are evaluated using three sets of experimental data on high frequency shear modulus of concentrated polymer-thickened oil-in-water emulsions.