Characterization of force networks in a dense high-shear system

We detect strong force networks in a dense high-shear system and study their structure and stability in response to variations in the shearing rate. The presence of strong force networks, which usually have a heterogeneous structure, restricts particle movements and can impose non-local mechanisms of momentum transfer. We identify such networks in a dense high-shear system using a community detection algorithm. Moreover, we explain the association between the mechanisms of momentum transfer and the structure, population, strength, and stability of the force networks by tracking the spatial and temporal evolution of the detected networks. In addition, we show that the assumption of a monodisperse assembly of particles leads to an unrealistic enlargement of the force networks, underestimating both the rate of energy dissipation and the rate of mixing.     

Rheology of polydisperse polymers: relationship between intermolecular interactions and molecular weight distribution

An expression of the relaxation function of linear polydisperse polymers is proposed in terms of intermolecular couplings of reptative chains. The relaxation times of each molecular weight are assumed to be shifted according to a tube renewal mechanism accounting for the diffusion of the surrounding chains. The subsequent shift is applied to the relaxation function of each molecular weight obtained from an analytical expression of the complex compliance J ^*(). Therefore the complex shear modulus G ^*() is derived from the overall relaxation function using the probability density accounting for the molecular weight distribution and four species-dependent parameters: a front factor A for zero-shear viscosity, plateau modulus G _N ⁰ , activation energy E and characteristic temperature T . All the main features of the theology of polydisperse polymers are described by the proposed model.     

Development of the “binary interaction” theory for entangled polydisperse linear polymers

Based on the theoretical principles previously described in the literature, the development of the “naïve” binary interaction model is detailed in this paper. The new theory is effectively a sweeping generalization of the “Double Reptation” model. The “switch function” has been shown to be an essential feature of any constraint release model for Doi–Edwards type molecular models that invoke the concept of a discrete slip-link tube and is used in our formulation. Using the assumption of a constant entanglement density, a slip link linear density evolution equation is derived to rigorously count matrix entanglements. This function has no counterpart in the conventional Doi–Edwards theory, or its derivatives, and is absolutely required to properly generalize the “Double Reptation” model so that nonlinear flows can be modeled. The binary interaction polydispersity model is complex mathematically but can be rigorously and justifiably simplified by suppressing the tube coordinate dependence using a boundary layer analysis. The simplification process can be continued to the continuum level to create a hierarchy of approximate binary interaction models, thereby making large-scale numerical simulations of complex flows viable, indeed straightforward.     

Hydrodynamic size distribution of gold nanoparticles controlled by repetition rate during pulsed laser ablation in water

Most investigations on the laser generation and fragmentation of nanoparticles focus on Feret particle size, although the hydrodynamic size of nanoparticles is of great importance, for example in biotechnology for diffusion in living cells, or in engineering, for a tuned rheology of suspensions. In this sense, the formation and fragmentation of gold colloidal nanoparticles using femtosecond laser ablation at variable pulse repetition rates (100-5000 Hz) in deionized water were investigated through their plasmon resonance and hydrodynamic diameter, measured by Dynamic Light Scattering. The increment of the repetition rate does not influence the ablation efficiency, but produces a decrease of the hydrodynamic diameter and blue-shift of the plasmon resonance of the generated gold nanoparticles. Fragmentation, induced by inter-pulse irradiation of the colloids was measured online, showing to be more effective low repetition rates. The pulse repetition rate is shown to be an appropriate laser parameter for hydrodynamic size control of nanoparticles without further influence on the production efficiency.     

多分散性对嵌段共聚物微观相行为影响的耗散粒子动力学模拟研究进展

嵌段共聚物通过微观相分离形成的各种有序结构在纳米印刷、药物输送、太阳能电池模板制备等领域有着广泛的应用.如何实现这些有序结构的有效调控是大家普遍关心的问题.近期的实验及理论研究表明嵌段长度的多分散性对嵌段共聚物的微观相行为有着不可忽视的影响.本文综述了近年来在AB型两嵌段及ABA型三嵌段共聚物的实验及理论研究中的一些主要进展,重点介绍了基于耗散粒子动力学的模拟研究进展.并对多分散性可能带来潜在重要影响的嵌段共聚物相关体系的未来发展方向进行了展望.     

Complex electrooptic research of nano-particle parameters in colloids

A research of some colloids has been carried out by means of dynamic light scattering, electrooptical and magnetooptical techniques. Intensity autocorrelation functions of scattered light have been compared to the relaxation curves of electrooptical effect for colloid particles of different shapes. The results of complex research confirm that the complicated character of light scattering by particles allows us to use the methods of birefringence and dichroism only formally when studying most colloid systems. Very thin nano-disperse structures are an exception to this rule. The investigation of polydispersity of some colloids was carried out by magnetooptical and two electrooptical techniques. Size distribution functions resulted from the different techniques agree. This justifies the suppositions about particle light scattering that are required for the use of the methods.     

聚(γ-苄基-L-谷氨酸)-b-聚(γ-十二烷基-L-谷氨酸)二嵌段共聚物的合成与表征

合成一种分子量分布窄、两嵌段侧链不同的新型聚(γ-苄基-L-谷氨酸)和聚(γ-十二烷基-L-谷氨酸)的二嵌段聚肽(poly(γ-benzyl-L-glutamate)-b-poly(γ-dodecyl-L-glutamate),PBLG-b-PDLG),并研究该嵌段聚肽的分子结构、热性能及液晶性.PBLG-b-PDLG的合成是利用N-羧酸内酸酐(N-carbonyl anhydride,NCA)法,以正己胺为引发剂,在0℃的条件下的氯仿溶液中,先将γ-苄基-L-谷氨酸NCA开环聚合获得了末端带胺基的活性PBLG-NH2沉淀后,再以其为引发剂加入γ-十二烷基-L-谷氨酸的NCA继续反应.氢核磁共振波谱(1H-NMR)和凝胶渗透色谱(GPC)用于表征两嵌段的摩尔比、分子量及其分布,发现嵌段聚肽的分子量分布只有1.07～1.09.由红外光谱(IR)的分析得知PBLG-b-PDLG的二次结构为刚直棒状的α-螺旋结构.利用示差扫描量热分析法(DSC)考察材料的热性质,发现不同组成的PBLG-b-PDLG有不同的热行为,而在100℃附近都出现了微弱的热致相转变.采用偏光显微镜观察PBLG-b-PDLG磁场取向膜的相变行为中发现其相结构不同于一般无规共聚物所呈现的胆甾液晶结构,同时用在PBLG-b-PDLG的二氯乙烷溶致液晶溶液中观察到一般近晶相所呈现的扇形织构.     

High order moment method for polydisperse evaporating sprays with mesh movement: Application to internal combustion engines

Relying on two recent contributions by Massot et al. [SIAM J. Appl. Math. 70 (2010), 3203–3234] and Kah et al. [J. Comput. Phys. 231(2012)], where a Eulerian Multi-Size Moment (EMSM) model for the simulation of polydisperse evaporating sprays has been introduced, we investigate the potential of such an approach for the robust and accurate simulation of the injection of a liquid disperse phase into a gas for automotive engine applications. The original model used a high order moment method in droplet size to resolve polydispersity, with built-in realizability preserving numerical algorithm of high order in space and time, but only dealt with one-way coupling and was restricted to fixed meshes. Extending the approach to internal combustion engine and fuel injection requires solving two major steps forward, while preserving the properties of robustness, accuracy and realizability: 1 – the extension of the method and numerical strategy to two-way coupling with stable integration of potential stiff source terms, 2 – the introduction of a moving geometry and meshes. We therefore present a detailed account on how we have solved these two issues, provide a series of verification of the proposed algorithm, showing its potential in simplified configurations. The method is then implemented in the IFP-C3D unstructured solver for reactive compressible flows in engines and validated through comparisons with a structured fixed mesh solver. It finally proves its potential on a free spray jet injection where it is compared to a Lagrangian approach and its reliability and robustness are assessed, thus making it a good candidate for realistic injection applications.     

纳米颗粒光子相关测定中基线方法对测量结果的影响

从光场自相关函数(Autocorrelation Function,ACF)与光强ACF、衰减率分布函数的关系,推导了基线值对有效直径、分散度的影响.分析了决定基线值取值的两种基线方法,指出基线方法不同,基线值也会不同.对三种纳米颗粒样品的相关实验也证明基线值越大,颗粒的有效直径、分散度越小.分析了自动斜率法的测量结果较稳定的原因.     

Investigation of structure factors in dense colloidal suspensions using frequency domain photon migration: polydisperse systems

Frequency domain photon migration (FDPM) technique was employed to investigate the structure factors of dense, polydisperse colloidal suspensions. The angle-integrated structure factors, [S(q)], extracted from FDPM measurements of scattering properties at volume fractions ranging from 0.05 to 0.4, were compared with the values predicted from the polydisperse hard sphere Percus-Yevick (HSPY) model, as well as decoupling approximation (DA) and local monodisperse approximation (LMA) models that incorporated independently measured particle size information. Results show that the polydisperse HSPY model is the most suitable for accounting for particle interactions which predominantly arise from volume exclusion effects. Furthermore, the influence of size polydispersity upon [S(q)] is most significant at high volume fractions. The static structure factors at small wave vector q, S(0), were also assessed from dual wavelength FDPM measurements by using the small wave number approximation as well as the local monodisperse approximation. The measured S(0) agrees well with the values predicted by the polydisperse HSPY model.