The mechanism of the self-assembly of associating DNA molecules under shear flow: Brownian dynamics simulation

A shear flow induces the assembly of DNAs with the sticky spots. In order to strictly interpret the mechanism of shear-induced DNA assembly, Brownian dynamics simulations with the bead-spring model were carried out for these molecules at various ranges of the Weissenberg numbers (We). We calculate a formation time and analyze the radial distribution function of end beads and the probability distribution of fractional extension at the formation time to understand the mechanism of shear-induced assembly. At low Weissenberg number the formation time, which is defined as an elapsed time until a multimer forms for the first time, decreases rapidly, reaching a plateau at We = 1000. A shear flow changes the radial distribution of end beads, which is almost the same regardless of the Weissenberg number. A shear flow deforms and stretches the molecules and generates different distributions between end beads with a stickly spot. The fractional extension progresses rapidly in shear flow from a Gaussian-like distribution to a uniform distribution. The progress of the distribution of fractional extension increases the possibility of meeting of end beads. In shear flow, the inducement of the assembly mainly results from the progress of the probability distribution of fractional extension. We also calculate properties such as the radius of gyration, stretch, and so on. As the Weissenberg number increases, the radius of gyration at the formation time also increases rapidly, reaching a plateau at We = 1000.     

热致性液晶共聚酯的拉伸流动行为

采用入口收缩流动的实验方法研究了改性PET/ 80PHB液晶共聚酯LCP80的拉伸流动行为 ,考察了拉伸速率、温度等对其拉伸粘度、Trouton比的影响 .实验结果表明 ,LCP80的入口压降值很大 ,其中由拉伸引起的入口压降是主要的 .在该文实验条件下LCP80均表现出拉伸稀化现象 ,并且Trouton比值都远大于 3 .根据流动中液晶织态结构的变化解释了实验现象 ,并对入口收缩流动的实验数据处理方法作了改进 ,比Beery的方法更为合理 ,也具有更广的适用性 .     

Model of processing‐induced microstructure formation in polymeric materials

A general, mechanistic, kinetic model is presented to predict polymer microstructure formation during processing. Applications of the model are presented for three specific cases. The model represents polymer molecules as Kramers chains which may or may not have nucleated. Three forces (hydrodynamic, Brownian, and intermolecular) that act on polymer molecules during processing were considered, which resulted in the presentation of the model as a diffusion equation. The input parameters account for the rheological and thermal history of the polymer melt, the specific type of polymer molecule, and the initial morphology. The solution of the diffusion equation yields a probability distribution function from which the transient and equilibrium morphology can be determined. The three specific cases were chosen to illustrate the versatility of the model and include: the extensional flow‐induced growth of extended chain crystals; the orientation of stiff molecules in solution undergoing shear flow well above the crystallization temperature; and the formation of folded chain vs. extended chain crystals in an extensional flow. Data are available for the first two cases and agree favorably with the model predictions. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2571–2585, 1999     

Shear and extensional rheology of solutions of mixtures of poly(ethylene oxide) and anionic surfactants in ionic environments

Interactions between a high molecular weight poly(ethylene oxide) (PEO) and the anionic surfactant sodium dodecyl benzene sulfonate (SDBS) in aqueous solutions were investigated by shear and extensional rheometry. Results for mixtures between PEO and sodium dodecyl sulfate (SDS) are also presented for comparison purposes. Addition of anionic surfactants to PEO solutions above the critical aggregation concentration (CAC), at which micellar aggregates attach to the polymer chain, results in an increase in shear viscosity due to PEO coil expansion, and a strengthening of interchain interactions. In extensional flows, these interactions result in a decrease of the critical shear rate for the onset of the characteristic extension thickening of the PEO solutions that is due to transient entanglements of polymer molecules. The relaxation times associated with these transient entanglements are not directly proportional to the shear viscosity of the solutions, but rather vary more rapidly with surfactant concentration. In the presence of an electrolyte, coil contraction results in lower shear viscosities and a decrease in the extension thickening effects at surfactant concentrations just beyond the CAC. The relaxation times associated with transient entanglement reach a minimum at the same surfactant concentration as the shear viscosity, which indicates that coil contraction is responsible for the observed effects in both types of flow. However, the increase in extensional-flow entanglement relaxation times is much more abrupt than the decrease in shear viscosity. All these results point to a greater sensitivity of extensional flows on the molecular conformation of PEO/surfactant complexes.     

The Effects of Extensional Flow and Hydrodynamic Interaction on the Nonequilibrium Brownian Dynamics of Dendrimers. A Bead‐Spring Model for Dendrimeric Molecules

We present the effects of incorporating hydrodynamic interactions into the nonequilibrium Brownian dynamics model describing the rheological properties of dendrimers under simple shear flow. The model response to uniaxial extensional flow is considered and compared with the extensional flow behavior of conventional linear molecules. The dendrimers are characterized by low viscosities and display little visco‐elasticity which makes them attractive materials from the stand‐point of energy‐intensive polymer forming processes.     

Combined effect of spin speed and ionic strength on polyelectrolyte spin assembly

Polyelectrolyte spin assembly (PSA) of multilayers is a sequential process featuring adsorption of oppositely charged polyelectrolytes from dilute solutions undergoing spin-coating flow. Here, we report on the dependence of PSA multilayer buildup of poly(sodium 4-styrenesulfonate) and poly(allylamine hydrochloride) on solution ionic strength and spin speed. We observed that at a given spin speed, the PSA coating growth rate (thickness/bilayer) and polymer surface coverage shows a nonmonotonic dependence on salt concentration, first increasing and then decreasing with increasing solution ionic strength. This is argued to be a manifestation of two competing mechanisms responsible for the layer formation. At low salt concentrations, the electrostatic interactions control the multilayer assembly process, while at high salt concentrations it is dominated by shear flow. We explain this nonmonotonic behavior in the framework of a Flory-like theory of multilayer formation from polyelectrolyte solution under shear flow. Additionally, the PSA process led to multilayer coatings with a radial dependence on thickness at lower spin speed in the shear-dominated regime. On increasing spin speed, such radial dependence subsided, eventually leading to uniform coatings by planarization. The surface topography of the multilayered coatings adsorbed at salt concentration less than 0.1 M was flat and featureless for all studied spin speeds. Unique morphological features in the films were formed at salt concentration higher than 0.1 M, the size of which depended on the spin speed and solution ionic strength.     

Micro-Rheological Modeling of Flow-Induced Crystallization in Mixed Shear/Extensional Flows

Flow Induced Crystallization (FIC) is the common term to indicate the acceleration in polymer crystallization kinetics due to the action of flow. When modeling FIC, two major challenges are encountered. On the one hand, the model must be able to produce quantitative reliable results, while correctly describing the coupling between the intrinsic (quiescent) crystallization kinetics and the rheological response of the polymer. On the other hand, the model must be able to describe the complex kinematics taking place in real industrial processes. In this paper, we present the predictions of a recently proposed model for FIC in the case of a mixed flow, where both shear and extensional components are present at the same time. In particular, the effects of the overall flow intensity and of relative weight between shear and extension on the enhancement in nucleation rate are presented and discussed. Some guidelines for future development are also proposed.     

Study on Non-Newtonian Behaviors of Lennard-Jones Fluids via Molecular Dynamics Simulations

Using nonequilibrium molecular dynamics simulations, we study the non-Newtonian rheological behaviors of a monoatomic fluid governed by the Lennard-Jones potential. Both steady Couette and oscillatory shear flows are investigated. Shear thinning and normal stress effects are observed in the steady Couette flow simulations. The radial distribution function is calculated at different shear rates to exhibit the change of the microscopic structure of molecules due to shear. We observe that for a larger shear rate the repulsion between molecules is more powerful while the attraction is weaker, and the above phenomena can also be confirmed by the analyses of the potential energy. By applying an oscillatory shear to the system, several findings are worth mentioning here:First, the phase difference between the shear stress and shear rate increases with the frequency. Second, the real part of complex viscosity first increases and then decreases while the imaginary part tends to increase monotonically, which results in the increase of the proportion of the imaginary part to the real part with the increasing frequency. Third, the ratio of the elastic modulus to the viscous modulus also increases with the frequency. These phenomena all indicate the appearance of viscoelasticity and the domination of elasticity over viscosity at high oscillation frequency for Lennard-Jones fluids.     

剪应力下弱作用势胶体颗粒聚团的特点

探讨了不同剪应力下，具有Ｌｅｎｎａｒｄ－Ｊｏｎｅｓ势的胶体颗粒聚团的结构特性，包括簇团的大小分布，径向分布函数，分形维数和原子配位数。研究表明，在弱作用力下，胶体簇团的分布随剪应力的增加而趋向小簇团一边；径向分布函数曲线随剪应力的增加而降低，胡在近程距离内降低得最多；分形维数随剪应力的增加表现为先增加后减小的趋势，其值随模拟条件的不同而在１．９－２．４之间变动。剪应力“场”对分形维数的大小没有太大     

On the product distribution of degradable polymers

We present a simplified model for calculating the product distribution of degradable polymers which is based on a grand canonical approach. The distribution depends on two physical parameters, the energy of a bond formation and the initial concentration of monomers. Due to its simplicity this model covers the two limiting cases, namely a polymerization process (no degradation) and micellar association (dominant degradation), and allows for straightforward extension to branched chain topologies or flow fields. In shear flow, a flow rate dependent length distribution leads to shear thinning.     

Modeling of interface mobility in the description of flow-induced coalescence in immiscible polymer blends

A new semiempirical equation has been proposed for the rate of approach of highly flattened droplets, applicable in the whole range of the ratios, p, of viscosity of the dispersed droplets and matrix. The equation is utilized to calculate the probability, P _c, that the droplet collision induced by shear or extensional flow is followed by the droplet fusion for systems with Newtonian droplets and Newtonian or viscoelastic matrix. The comparison of the results of these calculations with available experimental data and with the calculation using the trajectory analysis shows that the proposed model of the matrix drainage provides more reasonable results than the broadly applied model of partially mobile interface.     

Computational study of the texture formation in mesophase pitch-based carbon fibres

This paper studies the thermal relaxation phenomena after melt-extrusion of a rigid discotic uniaxial nematic mesophase pitch using mathematical modelling and computer simulation. The Ericksen and Landau-de Gennes continuum theories are used to investigate the structure development and texture formation across mesophase pitch-based carbon fibres. The two-dimensional model captures five types of transverse patterns, which match the commonly observed textures for mesophase pitch-based carbon fibres. They are: random, zig-zagged radial, radial, quasi-onion and onion. These textures represent the various combinations possible from the interplay between structure (i.e. texture) development and cooling during the fibre spinning process. During the thermal relaxation after the cessation of extensional flow the discotic nematic molecules store elastic free energy decays. The distorted nematic molecular profiles reorient to release the stored elastic free energy. The difference in time scales for molecular reorientation and thermal relaxation result in different transverse textures. The rate at which the fibres are cooled is the main factor in controlling the structure development. A slow cooling rate would permit the nematic discotic molecules to reorient to a well-developed (radial or onion) texture. The random texture is a result of rapid quenching. The numerical results are consistent with published experimental observations.     

The dynamic behavior of ethanol and water mixtures inside an Au nanotube molecule filter

Molecular dynamics (MD) simulation was used to investigate the behavior of water and ethanol molecules, which were mixed with five water-ethanol weight fractions (100:0, 0:100, 25:75, 50:50, and 75:25) inside the Au nanotube. To investigate the nano-confinement effect on water and ethanol molecules, the data of both molecules were analyzed by the probability of the number H-bonds per water and ethanol molecule and radial density distribution. Our results reveal that the radial density distributions and the number of H-bonds are significantly influenced by the Au nanotube, and the molecules also display different behavior from those in the bulk environment. In addition, the interaction between water molecules and the Au nanotube is stronger than that between ethanol molecules and the Au nanotube, from the profile of radial density distribution. Finally, both the number of H-bonds per water and per ethanol will be affected by the weight fraction, because the H-bond not only forms between the same material, but also between different materials.     

Effect of the flow field on the rheological behavior of aqueous cetyltrimethylammonium p-toluenesulfonate solutions

It is well-known that solutions of cetyltrimethylammonium p-toluenesulfonate in water exhibit a pronounced shear-thickening phenomenon in a specific concentration range (0.1-0.8%) when they are subjected to simple-shear flows, as a consequence of flow-induced self-assembly of wormlike micelles. This work shows that a strong elongational flow field (opposed-jets flow), applied to the same solutions, does not lead to extension thickening because the extensional flow prevents or destroys micellar association. In flow through a porous medium, a substantial increase in apparent viscosity is observed beyond a critical apparent shear rate, which surpasses increases observed in simple-shear flows. This is explained as the result of a synergistic effect of shear and relatively weak elongation on the solution microstructure.     

The horizontal stretching of test liquid A1

Tests are described in which a polyisobutylene-decalin solution (fluid A1) is stretched horizontally by the method previously used for fluid M1 (D. R. Oliver, J. Non-Newtonian Fluid Mech., 35 (1990) 251). In one type of testing, photographs were taken of the stretching jets and from these the stress, strain and strain rate were found for a series of points moving along the jet. In another type of testing, the speed of the rotating cylinder which pulls the jet was gradually increased and the same quantities were determined for the jet taken as a whole.The extensional viscosity of fluid A1 is of the order of 450 Pa s, corresponding to a Trouton ratio of 450 if the shear viscosity is taken as 1.0 Pa s. However, there are marked variations in extensional viscosity (100–3000 Pa s) depending on both extensional strain rate (1.0–50 s⁻¹) and other test conditions. Furthermore, the fluid is highly shear thinning in character, the shear viscosity falling from 5.0 Pa s at a shear rate of 1.0 s⁻¹ to 0.13 Pa s at a shear rate of 10³ s⁻¹. Thus the Trouton ratio has less meaning than for a fluid of constant viscosity, though it is undoubtedly high.Fluid A1 may also be treated, in extension, as a weak rubber-like solid and evidence is presented of a velocity at which this behaviour commences. the extensional modulus is typically 90 Pa, compared with 10³ Pa for fluid M1 and 10⁶ Pa for a rubber band.The results are discussed and the advantages and disadvantages of the present system pointed out; extensional viscosities below 100 Pa s are difficult to measure in this way. The main advantages are those of experimental simplicity and the production of data which may be examined in several different ways.     

Determination of Interparticle Forces by Colloidal Particle Scattering: A Simulation Study

Preflocculated ferric hydroxide flocs were subjected to either a simple shear flow or a two-dimensional straining flow, and their motion was optically observed. Digital image analysis was applied to extract information on orientation and deformation from the digitized frames. It was found that the simple shear flow led to a rotation of the flocs whose motion can be understood from the behavior of a solid ellipsoid. In the extensional flow, no continuous rotation occurred and flocs were broken apart along the axis of straining. The rupture forces estimated from an ellipsoid model were found to be in the range of 0.1 N/m(2). Copyright 2000 Academic Press.     

Extensional viscosity of aqueous solutions of SDS and PVP measured on the rheometrics RFX

An opposed jets extensional rheometer, the Rheometrics RFX, was used to investigate the extensional flow properties of a series of solutions of polyvinylpyrrolidone (PVP) containing varying amounts of an anionic surfactant, sodium dodecylsulphate (SDS). The measurements were made in comparison with data collected for Newtonian fluids of similar viscosity to the polymeric systems of interest. The instrument was used to make measurements of the extensional viscosity as a function of extension rate, and a study of the effect of the residence time within the flow field was made. PVP has been shown to form a complex in aqueous solution with SDS. This study concentrated on the effect the addition of SDS has on the extensional rheology of semi-dilute solutions of PVP, with a molecular weight of 700 000. It was shown that increasing the amount of SDS increased the extensional thickening characteristics of solutions, as shown by a decrease in the critical extension rate for thickening. An increase in the viscosity of the solvent was shown to have an additional effect. By decreasing the distance between the opposed nozzles it was shown that a decrease in the residence time (and hence total average strain) diminished the extensional thickening character of these solutions.     

Computational study of the texture formation in mesophase pitch‐based carbon fibres

This paper studies the thermal relaxation phenomena after melt‐extrusion of a rigid discotic uniaxial nematic mesophase pitch using mathematical modelling and computer simulation. The Ericksen and Landau–de Gennes continuum theories are used to investigate the structure development and texture formation across mesophase pitch‐based carbon fibres. The two‐dimensional model captures five types of transverse patterns, which match the commonly observed textures for mesophase pitch‐based carbon fibres. They are: random, zig‐zagged radial, radial, quasi‐onion and onion. These textures represent the various combinations possible from the interplay between structure (i.e. texture) development and cooling during the fibre spinning process. During the thermal relaxation after the cessation of extensional flow the discotic nematic molecules store elastic free energy decays. The distorted nematic molecular profiles reorient to release the stored elastic free energy. The difference in time scales for molecular reorientation and thermal relaxation result in different transverse textures. The rate at which the fibres are cooled is the main factor in controlling the structure development. A slow cooling rate would permit the nematic discotic molecules to reorient to a well‐developed (radial or onion) texture. The random texture is a result of rapid quenching. The numerical results are consistent with published experimental observations.     

剪切流场对ABA三嵌段共聚物囊泡形貌的影响

通过同轴圆筒剪切仪和磁子搅拌方式提供的剪切流场,研究了均匀和非均匀流场对ABA两亲性三嵌段共聚物囊泡的影响.研究发现,非均匀流场下囊泡尺寸及其分散度随剪切速率的增加呈现先增大后减小的规律.与搅拌形成的非均匀流场相比,在同轴剪切仪提供的均匀流场下形成的囊泡尺寸更加均匀.结果表明,剪切流场是影响囊泡形貌的重要因素,流场的不均匀性是导致组装体形貌结构多分散性的重要原因之一.