Linear viscoelastic properties of emulsions and suspensions with thermodynamic and hydrodynamic interactions

We present a new approach to describe the rheological properties of dispersions with non-hydrodynamic interactions (steric, electrostatic and Van der Waals interactions) in the linear viscoelastic domain. Our model is based on the calculation of additional stresses resulting from interaction potentials between spheres and Brownian motion. We start from the statistical mechanical approaches which have been developed by Batchelor and Green and later Lionberger and Russel, to model the viscoelastic properties of emulsions and suspensions. We have extended their calculations to the more general case of viscoelastic deformable inclusions in a viscoelastic matrix. Our contribution lies in the computation of the hydrodynamic functions involved in the term describing interaction stresses. This computation is based on Palierne's results on the deformation field around a viscoelastic inclusion embedded in a viscoelastic matrix. We have also rewritten the conservation equation in the case of interest, over the whole frequency domain. We finally express the complex shear modulus of the dispersion as the sum of two terms : Palierne's complex shear modulus gives the purely hydrodynamic contribution; the interaction contribution depends on both the hydrodynamic properties and the interaction potential.     

Evolution of the rheological properties during the reactive processing of thermoplastic/thermoset epoxy

The melt state reaction, or fusion process of bisphenol-A and the diglycidyl ether of bisphenol-A can produce both linear phenoxy backbone chains and crosslinked network structures. The linear chains can be thought of as thermoplastic polymer, while the crosslinked molecular matrix is a thermoset; therefore, this resin system can be termed a thermoplastic/thermoset epoxy.Rheological analysis has been performed on this system to determine the occurrence of the crossover of the storage and loss moduli during the reaction using three techniques: isothermal cure, interval frequency sweep, and interval stress relaxation.Determination of the activation energy of the reaction by consideration of the conventional gel-point determination prove unsatisfactory as compared to that which is determined by FTIR. An alternative technique is presented which yields a value in good agreement with that obtained by following epoxide consumption. The thermoplastic/thermoset nature of this material leads to the deviation from traditional thermoset cure behavior.     

Linear and non-linear viscoelastic properties of ethylene vinyl acetate/nano-crystalline cellulose composites

This paper reports on the melt rheological properties of ethylene vinyl acetate containing between 0 and 10 wt.% of nano-crystalline cellulose (NCC). A complete set of rheological tests including frequency sweeps, shear transients, and uniaxial elongations was performed. Frequency sweeps showed that at low frequencies, a pseudo solid-like behavior was obtained for NCC concentrations higher than 5%. This behavior was related to hydrogen bonding between NCC particles and the creation of particle networks as the result of particle–particle interactions. For transient shear tests, all compositions presented a stress overshoot at high shear rates before reaching a steady state. It was found that the amplitude of this overshoot depends on both NCC content and shear rate. On the other hand, the time to reach the maximum was found to be highly shear rate dependent but concentration dependence was rather weak. For uniaxial extensional flow, higher extensional viscosity was observed with increasing NCC content. On the other hand, strain hardening was found to decrease with increasing NCC content.     

Studying linear and nonlinear vibrations of fractional viscoelastic Timoshenko micro-/nano-beams using the strain gradient theory

Nonlinear Dynamics - In this paper, the vibrational behavior of micro- and nano-scale viscoelastic beams under different types of end conditions in the linear and nonlinear regimes is investigated...     

Influence of high pressure processing on the linear viscoelastic properties of egg yolk dispersions

The effect of high-pressure treatments on the gelation of egg yolk dispersions was studied by using differential scanning calorimetry (DSC) and small amplitude oscillatory shear (SAOS). The influence of pressure of processing and pH were analysed. The DSC results suggest a progressive decrease in thermal denaturation enthalpy as pressure level increases related to protein denaturation. SAOS was used to evaluate the effect of different pressure levels on the linear viscoelastic behaviour of egg yolk dispersions. An increase in the pressure level produces a dramatic change in the linear viscoelastic behaviour undergoing a sol–gel transition. High hydrostatic pressure (HHP) processing was also analysed as a function of pH and solids content. The results obtained confirm that the impact of high pressure on aggregation and network formation can be modulated by pH. HHP processing of egg yolk systems is highly affected by protein concentration particularly when it is compared to heat processing.     

Data processing methodology in the characterization of the mechanical properties of terrain

The methodology for processing load-sinkage and shear test data obtained using the bevameter is examined. A weigthed least squares technique is proposed for deriving terrain values from experimental data. To streamline data processing time, a portable automatic data processing unit for the bevameter has been developed. The basic features and functions of the unit are described. The automatic data processing unit, together with associated bevameter, has undergone extensive field trials. They have been employed to measure the properties of various types of natural terrain. The results indicate that this new system can provide a basis for the development of a reliable, convenient and standardized technique for identifying terrain properties in relation to off-road mobility.     

The influence of mechanical strain on the optical properties of spherical gold nanoparticles

We utilize classical Mie scattering theory to investigate the effects of tensile and compressive mechanical strain on both the far field (absorption, scattering and extinction efficiencies) and near field (surface enhanced Raman scattering) optical properties of spherical gold nanoparticles with diameters ranging from 10 to 100 nm. By accounting for the strain effects on both the ionic core (bound) and conduction (free) electrons through appropriate modifications of the bulk dielectric functions, we find that gold nanoparticles are relatively sensitive to the effects of mechanical strain due to the fact that the plasmon resonance wavelength for spherical gold particles, which occurs around , is nearly coincident with the interband transitions of the core electrons. Specifically, we find that tensile strain leads to significant enhancements ranging from 60% to 120% in the far field optical efficiencies, while compressive strain leads to similar decreases, and that the plasmon resonance wavelength can be red or blueshifted up to 100 nm due to the applied strain. Finally, we find that tensile strain also strongly enhances the local electric (E)-field at the surface of the nanoparticles, which is of considerable interest for surface-enhanced Raman scattering applications; 5% tensile strain is found to enhance the |E|⁴ intensity by 63%. The present results demonstrate the potential of mechanical strain, and specifically that of tensile mechanical strain in enhancing and tailoring the optical properties of gold nanoparticles.     

Linear and non-linear viscoelastic rheology of hybrid nanostructured materials from block copolymers with gold nanoparticles

A polystyrene-b-poly-4-vinypyridine (PS-b-P4VP) diblock copolymer is modified with a gold precursor to obtain an organic–inorganic (hybrid) block copolymer in bulk with gold nanoparticles selectively incorporated in the P4VP block. In the linear viscoelastic regime, temperature sweep tests over a series of these hybrid block copolymer systems revealed consistent shifts (ΔT) in the glass transition temperatures (both T _g\text-PS_{\rm g\text{-}PS} and T _g\text-P4VP_{\rm g\text{-}P4VP}) of the hybrid materials in comparison to the pristine polymers. Studying different volume fractions of the pyridine block, a level-off point was found for block copolymers with f _P4VP > 0.26, where the shifts in T _g\text-P4VP_{\rm g\text{-}P4VP} consistently increased up to ΔT = 25°C. By artificially increasing the volume fraction of the pyridine block, the nanoparticles reduce the transition regime determined in master curves. At higher volume fractions of the pyridine block, crossover frequencies were not detected after the entanglement regime, indicating that the material does not relax from topological constraints (entanglements and nanoparticles) into the terminal regime. Above a specific volume fraction of nanoparticles (Φ _P = 0.05), the flow behaviour of the hybrid materials becomes increasingly elastic, exhibiting wall-slip from the geometry at lower strain values in comparison to the pristine material. In the non-linear viscoelastic regime, Fourier-transformed rheology was used to analyse the raw signals from strain sweep experiments. It was clearly demonstrated the nanoparticle effect by following the second and third harmonic (I _2/1, I _3/1) of the stress response. Comparing the behaviour of the third and second harmonics provided an unambiguous fingerprint for the effect of the nanoparticles.     

Behavior of brittle anisotropic materials with different orientation of mechanical properties at the edge of piercing

The problem of normal interaction between a compact steel isotropic cylindrical projectile and an orthotropic plate at the edge of piercing in the range of impact velocity from 50 m/s to 400 m/s. The obstacle is made of an organoplastic material with some initial orientation of its mechanical properties or the same material whose properties are obtained by a 90° rotation of the initial material about the axis OY. The destruction of obstacles is studied and the efficiency of their protective properties is comparatively analyzed depending on the orientation of the elastic and strength properties of the anisotropic material. The problem is solved numerically by the finite element method in the three-dimensional statement. The behavior of the projectile material is described by an elastoplastic model, while the response of the obstacle anisotropic material is described in the framework of the elastic-brittle model with different tensile and compressive strengths.     

The effect of filler on the mechanical properties of an elastomer at high strain rates

Summary Results are presented which show the effect of filler size and content on the stress-strain curves of an elastomer at high strain rates. An equation is proposed for the prediction of the apparent increase in modulus of an elastomer due to the addition of rigid spherical particles.

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Zusammenfassung Die dargestellten Ergebnisse zeigen den Einfluß von Korngröße und Gehalt des Füllstoffes auf das Spannungsdehnungsdiagramm eines Elastomers unter hohen Dehnungsgeschwindigkeiten. Eine Gleichung für die Voraussagung der anscheinenden Erhöhung des Elastizitätskoffizienten des Elastomers durch den Einschluß von starren kugelförmigen Körnern wird vorgeschlagen.

     

Prediction of in-plane elastic properties of graphene in the framework of first strain gradient theory

Meccanica - In the present study, the in-plane elastic stiffness coefficients of graphene within the framework of first strain gradient theory are calculated on the basis of an accurate molecular...     

广义黏弹组合模型的等效性及其基本性质

对广义黏弹组合模型的等效性及其基本性质进行了研究,结果表明:广义Maxwell模型由于并联结构,其蠕变柔量很难得到. 但提出了求解广义Maxwell模型蠕变柔量的方法,给出了其具体表达式,且在此基础上证明了广义Maxwell模型与Kelvin链的等效性,建立了这两个模型物理参数间的转换关系式. 证明了广义黏弹组合模型的一个基本性质:当将模型的松弛时间谱和延迟时间谱由大到小顺序排列时,松弛时间与延迟时间互不相等,且相互交织,两相邻松弛时间中间有且仅有一个延迟时间,同时,两相邻延迟时间中间有且仅有一个松弛时间;当两者同阶相比时,延迟时间总是大于松弛时间. 这一基本性质明确了使用广义黏弹组合模型来描述现实中某种特定材料的黏弹性行为时,该材料必须具备的基本条件,因此,它可作为这类流变模型在工程应用中的一个实用判据. Wiechert模型和广义中村模型、广义Jeffreys模型和广义N-K模型、Maxwell链和广义Kelvin模型之间的等效性可作为特例.最后,实例验证了所提出的求解广义黏弹组合模型蠕变柔量方法及其基本性质.      

Determination of microcapsule physicochemical,structural, and mechanical properties

Research into the fundamental properties of microcapsules and use of the results to develop a wide variety of products in industries such as printing, fast-moving consumer goods, construction, pharmaceuticals, and agrochemicals is a dynamic and ever-progressing field of study. For microcapsules to be effective in providing protection from harsh environments or delivering large payloads, it is essential to have a good understanding of their properties to enable quality control during formulation, storage, and applications. This review aims to outline the commonly used techniques for determining the physicochemical, structural, and mechanical properties of microcapsules, and highlights the interlinked nature of these three areas with respect to the end-use industrial application. This review provides information on techniques that are well supported in the literature, and also examines microcapsule analytical techniques that will become more prevalent as a result of new technological developments or extensions from other areas of study.     

基于降阶均匀化理论的混凝土双尺度力学性能分析

混凝土材料细观特性对宏观力学性能有着重要影响。为进一步分析混凝土细观特性对宏观力学行为的影响规律,将混凝土材料简化为由骨料、砂浆和界面三相组成,编制了随机凸多面体骨料生成、投放和网格剖分算法,建立可用于有限元计算的满足级配要求的随机细观模型。针对直接使用细观力学模型计算量较大的问题,采用降阶均匀化理论,对混凝土细观胞元模型进行预处理并编制了相应的双尺度计算程序。对不同强度混凝土进行了单轴静态压缩双尺度计算,与实验数据和细观力学模拟结果符合较好。研究表明,降阶均匀化理论在加快求解速度的同时具有较高的精度,可以用于混凝土的多尺度力学性能分析。     

Thermodynamic restrictions,free energies and Saint-Venants principle in the linear theory of viscoelastic materials with voids

This paper examines the thermodynamic restrictions imposed by the second law of thermodynamics upon the relaxation functions in the linear theory of viscoelastic materials with voids. On this basis the existence of a maximal free energy is proved by means of a constructive method. Further, we use such a maximal free energy in order to establish a principle of Saint-Venant type in the dynamics of viscoelastic materials with voids. A uniqueness theorem is proved for finite and infinite bodies and we note that it is free of any kind of a priori assumptions concerning the orders of growth of solutions at infinity.