Reserve ability of parallel system with failed elements

The aim of the analysis is to define for a system with parallel elements conditions at which the failure of one or several elements does not cause immediate failure of the whole system. The critical number of failed elements of such a system is assessed based on the strength probability properties of its structural elements. The condition of the catastrophic failure of the whole system is defined by the shape and scale parameters of the strength probability function and the number of parallel elements. The proposed probabilistic model allows estimating the ability of the parallel systems to continue operation when some of its elements are broken.     

Viscoelastic properties of polymerlike reverse micelles

A dramatic increase in the viscosity of reverse micellar solutions of lecithin in a variety of organic solvents of up to a factor of 10⁶ upon the addition of a small amount of water can be observed. The formation of viscoelastic solutions can be explained by a water-induced aggregation of lecithin molecules into flexible cylindrical reverse micelles and the subsequent formation of a transient network of entangled micelles. The viscoelastic properties of these solutions are characterized as a function of water content and temperature for different organic solvents by means of dynamic shear viscosity measurements. The results are interpreted by making analogies to the behavior of semidilute polymer solutions and living polymers.Dedicated to Prof. Dr. J. Meissner on the occasion of his 60th birthday.     

Preface

This special issue of PARTICUOLOGY is devoted to the first UK-China Particle Technology Forum taking place in Leeds, UK, on 1-3 April 2007. The forum was initiated by a number of UK and Chinese leading academics and organised by the University of Leeds in collaboration with Chinese Society of Particuology, Particle Technology Subject Group （PTSG） of the Institution of Chemical Engineers （IChemE）, Particle Characterisation Interest Group （PCIG） of the Royal Society of Chemistry （RSC） and International Fine Particle Research Institute （IFPRI）. The forum was supported financially by the Engineering and Physics Sciences Research Council （EPSRC） of United Kingdom,     

Viscoplasticity of agglomerated suspensions

This work is a theoretical study on the effects of agglomeration on the fluidity and plasticity of a suspension of neutrally buoyant particles in a Newtonian fluid. The dynamics of a cluster of permanently attached spherical particles in a simple shear field is analyzed. The viscous and plastic components of the drag force acting on each of the agglomerated particles is then calculated and found to depend on the size of the individual particle unit, its location being relative to the center of the cluster and the material properties of the engulfing fluid. This information in conjunction with the knowledge of the interparticle cohesive forces is used to establish criteria for the agglomerate size reduction during dispersive mixing. From the kinematics of the cluster movement and the forces acting on its particulate components the rate of energy dissipation is calculated and utilized to evaluate the viscosity and yield stress of the suspension. These rheological parameters depend on the volume fraction and architecture of the agglomerate, the number of fused particles per cluster, and the viscosity of the suspending fluid. The analysis is also extended to include the case of polydispersity of agglomerate sizes.     

La notion de moyenne en turbulence. Quelques réflexions selon une perspective historique

The historical evolution of the averaging concept in turbulence is presented according to a two-fold analysis, taking into consideration the physical meaning and the mathematical formulation. After having placed in their historical context some symptomatic characteristics of turbulent flows, the question of their interpretation as a turbulence syndrome, leading to a unitary identification of the phenomenon, is discussed. We then deal with the emergence of the notion of mean as a tool of physical understanding of such a unitary approach, and its relation with the progression of the “experimental” evidence. We conclude by relating this notion to the theoretical aspects of a determinism on average of the turbulent regime, its finality and some attempts to develop a probabilistic theory of turbulence.     

Nonlinear study of the dynamic behavior of a string-beam coupled system under combined excitations

In this paper,the nonlinear dynamic behavior of a string-beam coupled system subjected to external,parametric and tuned excitations is presented.The governing equations of motion are obtained for the nonlinear transverse vibrations of the string-beam coupled system which are described by a set of ordinary differential equations with two degrees of freedom.The case of 1:1 internal resonance between the modes of the beam and string,and the primary and combined resonance for the beam is considered.The method of multiple scales is utilized to analyze the nonlinear responses of the string-beam coupled system and obtain approximate solutions up to and including the second-order approximations.All resonance cases are extracted and investigated.Stability of the system is studied using frequency response equations and the phase-plane method.Numerical solutions are carried out and the results are presented graphically and discussed.The effects of the different parameters on both response and stability of the system are investigated.The reported results are compared to the available published work.     

Evolution of viscoelastic properties of polyurethane in the course of curing

Simultaneous measurements of the components of dynamic modulus at the set of frequencies were made by Fourier-Transform Mechanical Spectroscopy (FTMS) to investigate the curing process of a segmented polyurethane.The whole process of curing is treated as consisting of three stages. In the first stage, the material can flow and this stage is complete at the gel-point. In the second stage, the curing proceeds in the network and ends with phase separation of aggregating segments (blocks) of polymeric chains. In the third stage, the curing continues in the two-phase system. The object of investigation was the transition from the second to the third stage. It was shown that the position of the maximum of loss tangent depends on frequency and does not coincide with different characteristic points marking the time dependencies of G and G. This is due to the formation of micro-particles of the newly formed phase. The FTMS method is used for separation of the modulus changes into parts related to formation of chemical and physical networks.Special means of treating calorimetric measurements identified a weak maximum heat output at the moment corresponding to the transition point.     

Anisotropic conduction of heat in a flowing polymeric material

In this paper a theory is presented which relates the thermal conductivity tensor of an amorphous polymeric material to the history of deformation of the material. The basis of the theory is formed by the network theory for polymeric materials. It will be shown that the results obtained here are in good agreement with experimental results on rubber. The effect of anisotropic heat conduction on the flow of a polymeric material will be demonstrated by the simple example of viscous heating in shear flow.Presented at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.     

Flow birefringence and rheological measurements on shear induced micellar structures

Aqueous solutions of cationic surfactant systems with strongly binding counterions show the striking phenomenon of shear induced phase transitions. At low shear rates or angular frequencies, the solutions exhibit Newtonian flow. At high rates of shear, however, the rheological properties change dramatically. Above a well defined threshold value of the velocity gradient, a supermolecular structure can be formed from micellar aggregates. This shear induced structure (SIS) behaves like a gel and exhibits strong flow birefringence. The formation of the shear induced structure is very complicated and depends on the specific conditions of the surfactant system. In this paper we discuss new results which have been obtained from rheological measurements and from flow birefringence data. We examine the stability of the shear induced state as a function of temperature, surfactant concentration and salt concentration and we analyse the effect of solubilisation of alcohols and hydrocarbons. The results are interpreted in terms of a kinetic model which accounts for the observed behavior.Dedicated to the 60. birthday of Prof. H. Harnisch, Hoechst AGPartly presented at the 2nd Conference of European Rheologists, Prague, June 17–20, 1986     

The effect of dissolved polymers on the rheological properties of coating colours

Coating colours used for the coating of paper and board consist mainly of a mineral pigment, which is very often clay, a synthetic binder such as a styrenebutadiene latex, dispersion agents and water retention aids. The latter are often water soluble polymers. These polymers have a very strong influence on the rheological properties of the coating colours, both on the strain rate dependence of the apparent viscosity and on the viscoelasticity. The effects of two different grades of carboxymethylcellulose (CMC) and one grade of hydroxyethylcellulose (HEC), on the rheological properties at room temperature of a clay-based coating colour at pH 8, have been investigated. It is concluded that the high values of the dynamic modulus of the colours are due to interactions between the cellulose derivatives and the solid particles, i.e. mainly the clay particles. For HEC this interaction is associated with adsorption of the polymeric molecules on the clay particles. In the case of CMC, the adsorption is strongly retarded by the presence of the dispersant (a polyacrylate salt). It is suggested that the marked elasticity of the CMC-containing colour in addition to a possible polymer adsorption may be due to charge interactions and/or depletion flocculation. The effect of CMC and HEC on the water-retention properties of the colour is also discussed.     

ARCJET ABLATION EXPERIMENT TO SIMULATE THE CHELYABINSK ASTEROID ENTRY 1)

烧蚀是小行星极高速进入地球大气层后最重要的现象之一,在很大程度上决定了小行星的质量/尺寸变化、飞行轨迹、甚至光辐射特性. 为观测小行星材料在超高速高温流场中的烧蚀现象,在电弧加热器上开展了模拟Chelyabinsk小行星事件典型弹道状态(速度约5.6 km/s,高度17 km,流星体直径1 m)的烧蚀实验. 试件为钝头外形,头部半径20 mm,半锥角18$^\circ$. 作为对比,试件分别采用玄武岩和碳钢材料. 成功记录了清晰的烧蚀动态过程,观察到两种材料试件表面的熔融损失流动、以及玄武岩试件的蒸发喷射和崩裂剥落等现象,全程测得烧蚀气体发射光谱、试件实时外形变化、表面热图变化等数据. 分析结果显示了两种材料明显不同的烧蚀现象和质量损失机制：碳钢在高温气流冲击作用下溅射成大量微小液滴,跟随气流高速流失;玄武岩质量损失以熔融物剪切流动为主,并伴随少量块状剥落及蒸发喷射. 烧蚀时间为4 s,玄武岩和碳钢的质量损失及驻点后退量分别为37.9 g,72.7 g以及7.3 mm,13.1 mm,估算玄武岩材料的有效烧蚀焓约为2.6 MJ/kg,两种材料的烧蚀光谱测量组分与电镜能谱扫描结果吻合.     

On the role of stress-induced migration on time-dependent terminal velocities of falling spheres

There is experimental evidence to suggest that even under steady-state conditions the velocity of solid spheres or bubbles moving through viscoelastic fluids can become time dependent. One of the possible explanations offered for interpreting this phenomenon has been the generation of a polymer depleted layer in the line of passage of the particles, which disappears due to the counterbalancing effect of molecular diffusion in the long range. We have done some careful experiments and measured these concentrations to show that no such depletion layers are formed. Alternative explanations of the phenomenon have been examined and the importance of the possible effects of microstructures generated through temporary associations has been emphasised.     

Embracing the Random in the Self-Organizing Psyche

The history of the idea of randomness in the Nineteenth Century is presented as a backdrop to Freud's understanding of randomness in the psyche. Next, the role of the random event in nonlinear dynamical systems theory is discussed. Then, a revision of the traditional psychoanalytic view of the synthetic function of the ego is proposed based on the process of self-organization in which random elements are utilized in the emergence of new systemic structures. A theory of dreaming is elaborated which transcends the traditional dichotomy of dreams as either random events or meaningful psychological contents. Finally, remarks about psychoanalytic treatment as a process of self-organizing incorporation of the random are offered.     

An overview of the formulation,existence and uniqueness issues for the initial value problem raised by the dynamics of discrete systems with unilateral contact and dry friction

In the end of the seventies, Schatzman and Moreau undertook to revisit the venerable dynamics of rigid bodies with contact and dry friction in the light of more recent mathematics. One claimed objective was to reach, for the first time, a mathematically consistent formulation of an initial value problem associated with the dynamics. The purpose of this article is to make a review of the today state-of-art concerning not only the formulation, but also the issues of existence and uniqueness of solution.     

Ultrasonic devulcanization of waste rubbers: Experimentation and modeling

Continuous ultrasonic devulcanization of ground tire rubber (GRT) and styrene-butadiene rubber (SBR) is considered. Experiments are performed under various processing conditions. Two recipes of SBR with different amounts of polysulfidic linkages are utilized. Gel fraction and crosslink density of devulcanized rubbers are measured and a unique relationship between them is established. Die characteristics with and without imposition of ultrasonic waves are determined. Devulcanized samples are revulcanized and mechanical properties are measured. In some cases, properties of revulcanized SBR samples exceeded those of virgin vulcanizates. This is explained based on the presence of a double network in the revulcanized rubber. A modification of acoustic cavitation and flow modeling of ultrasonic devulcanization of SBR and GRT is proposed using a concept of effective viscosity characterizing the flow of vulcanized particles before devulcanization combined with a shear rate, temperature and gel fraction-dependent viscosity of devulcanized rubber. Velocity, shear rate, pressure, and temperature field along with gel fraction, crosslink density and number of bonds broken are simulated. Predicted data on gel fraction, crosslink density, and pressure using the present modification of the model are found to be closer to experimental data then previously reported.Dedicated to the memory of Professor Tasos C. Papanastasiou     

Dynamics and Breakup of Viscoelastic Liquids (A Review)

The phenomena of hydrodynamic breakup of liquid jets, drops, films, bridges, and filaments are reviewed for liquids with viscoelastic properties. The reasons for breakup are capillary instabilities, collisions with rigid obstacles, and other forms of dynamic action. The relationship between the properties of the liquids and the features of the breakup process is discussed.     

Newtonian stratified flow through an abrupt expansion

The present paper is devoted to the numerical simulations of stratified generalized Newtonian flow. The main feature of the algorithm is to include the coordinates of the interface as primitive unknowns of the flow problem together with the nodal velocity components and pressures calculated on a deformable finite element mesh. Newton-Raphson's iterative method is used for solving the non-linear problem. Special attention is given to the location of the intersection between the interface and a solid boundary, when it is unknown at the outset. The method is applied to the creeping stratified flow of two Newtonian fluids through a one-to-four abrupt expansion; the results remain valid for the contraction problem. It is found that, under appropriate conditions, the small Newtonian corner vortex can increase by a large factor, both in size and intensity; the degrees of freedom of the simulation are the ratios of flow-rate and of viscosity of the fluids.     

Sequences of orbits and the boundaries of the basin of attraction for two double heteroclinic orbits

The boundaries of the basin of attraction are usually assumed to be rather elementary for Hamiltonian systems with autonomous perturbations. In the case of one saddle point, the sequences of orbits before capture are unique for each basin. However, we show that for two saddle points each with double heteroclinic orbits, there is an infinite number of different sequences of nearly homoclinic orbits before capture depending on the four heteroclinic parameters. The probabilities of capture are independent of the capture sequence.     

Physiology,mechanics, and rheology

Biological systems possess rather specialized mechanical properties, acquired as part and parcel of the evolutionary development of the system as a whole. Their optimization permits the system to function physiologically in the context of a biologically essential, but mechanically often widely varying environment with adequate efficiency. The system's environment is its source of food and shelter; it represents the space in which it forages or preys on other creatures and in which it has to defend itself against still others. Thus, the system has to develop an adequately pliant, rheologically matched, energy-use efficient, mechanical interface between it and its surroundings. This must be an interface that both effectively excludes, but also effectively admits, the external. Internally, as well, it has to adapt the mechanical properties of cell and connective tissue to physiological function and the efficient performance of useful work. This will be illustrated by way of examples. Blood rheology is briefly discussed and put into the context of clinical hemorheology and epithelial protection; and function, by way of a mucus coating or a mucociliary clearance system, is reviewed in some detail. The importance of all aspects of rheological matching is demonstrated.Delivered as a Keynote Lecture at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.