Non-equilibrium molecular dynamics are used in a coarse-grained scale to simulate the simple shear flow of telechelic and non-telechelic associative polymer solutions. The work includes the study of ...inception of flow and stress relaxation in transient flows and the constitutive curve at steady state. The instabilities (plateau stress, stress maxima) and the formation of bands with different velocity gradient and concentration depend on the location of the hydrophobic sites (sticker sites,
SS
) in the chain. Results allow to shed light on the origin of banded flow within the transient state, related to depletion of bridges between aggregates. This situation induces instabilities in the velocity profiles and concentration inhomogeneities along the velocity gradient during the stress overshoot. As the steady state is reached, instabilities disappear for systems where the
SS
are in the middle of the chain. The conformation relaxation occurs during a time scale larger than that of the stress, but this is not sufficient condition for the presence of banded flow in telechelic polymer solutions, as it holds in polymer melts. The difference in concentration among the bands suggests the existence of a shear-gradient concentration coupling mechanism under banded flow conditions, such as spatial non-homogeneities in the network density, induces chain diffusivity in the gradient direction and increasing normal stresses, especially in telechelic chains. The role of normal stresses on the flow-concentration coupling instability is analyzed in detail.
The high flame-retardant loading required for ethylene-vinyl acetate copolymer blends with polyethylene (EVA-PE) employed for insulation and sheathing of electric cables represents a significant ...limitation in processability and final mechanical properties. In this work, melamine triazine (TRZ) and modified bentonite clay have been investigated in combination with aluminum trihydroxide (ATH) for the production of EVA-PE composites with excellent fire safety and improved mechanical properties. Optimized formulations with only 120 parts per hundred resin (phr) of ATH can achieve self-extinguishing behavior according to the UL94 classification (V0 rating), as well as reduced combustion kinetics and smoke production. Mechanical property evaluation shows reduced stiffness and improved elongation at break with respect to commonly employed EVA-PE/ATH composites. The reduction in filler content also provides improved processability and cost reductions. The results presented here allow for a viable and halogen-free strategy for the preparation of high performing EVA-PE composites.
This is the first on a series of articles that deal with nonlinear dynamical systems under oscillatory input that may exhibit harmonic and non-harmonic frequencies and possibly complex behavior in ...the form of chaos. Frequency response techniques of nonlinear dynamical systems are usually analyzed with numerical methods because, most of the time, analytical solutions turn out to be difficult, if not impossible, since they are based on infinite series of trigonometric functions. The analytic matrix method reported here is a direct one that speeds up the solution processing compared to traditional series solution methods. In this method, we work with the invariant submanifold of the problem, and we propose a series solution that is equivalent to the harmonic balance series solution. However, the recursive relation obtained for the coefficients in our analytical method simplifies traditional approaches to obtain the solution with the harmonic balance series method. This method can be applied to nonlinear dynamic systems under oscillatory input to find the analog of a usual Bode plot where regions of small and medium amplitude oscillatory input are well described. We found that the identification of such regions requires both the amplitude as well as the frequency to be properly specified. In the second paper of the series, the method to solve problems in the field of large amplitudes will be addressed.
This work is the second in a series of articles that deal with analytical solutions of nonlinear dynamical systems under oscillatory input that may exhibit harmonic frequencies. Frequency response ...techniques of nonlinear dynamical systems are usually analyzed with numerical methods, because in most cases analytical solutions such as the harmonic balance series solution turn out to be difficult, if not impossible, as they are based on an infinite series of trigonometric functions with harmonic frequencies. The key contribution of the analytic matrix methods reported in the present series of articles is to work with the invariant submanifold of the problem and to propose the solution as infinite power series of the oscillatory input; this procedure is a direct one that speeds up the computations compared to traditional series solution methods. The method reported in the first contribution of this series allows for the computation of the analytical solution only for small and medium amplitudes of the oscillatory input, because these series may diverge when large amplitudes are applied. Therefore, the analytic matrix method reported here, which is a reconfiguration of the method proposed in the first contribution in this series, allows the solving of problems in the regime of large-amplitude oscillations where the contributions of the high order harmonics affect the amplitudes of the low order harmonics, leading to amplitude- and frequency-dependent coefficients for the infinite series of trigonometric function expansion.
Blood is a rheologically complex suspension, in which the soluble fraction contains proteins, total cholesterol and triglycerides. The blood rheological behavior is strongly affected by the ...concentration of these components. This work evaluates the total cholesterol effect on the rheological behavior of
Wistar
rat blood by means of an in vitro study. Twenty-one rats were divided into 3 groups, each one had an assigned diet with different fat content: reference group (RG) with 3%, medium fat content group (MG) with 4.5% and high-fat content group (HG) with 6.5%; in the latter group, mixed-vegetable fat was added. From each group, intraocular representative blood samples were taken with time lapse of 15 days between each sampling followed by biochemical and hemo-rheological tests. The first analysis detected changes in total cholesterol levels, attributed to the rat metabolism, formation of adipose tissue and competition for food. The second test consisted in steady simple-shear and linear oscillatory flow. The linear viscoelastic spectra reveal that the viscous modulus is larger than the elastic modulus (
G
ʺ >
G′
), with simple-shear viscosity exhibiting shear-thinning behavior. An important finding is a
pseudo-solid-
like behavior at low frequencies (1 rad/s) akin to the presence of yield stresses in the high-fat content group after 30 days, revealing strong interactions between total cholesterol levels and blood cells. The hemo-rheological tests represent a promising alternative to identify pathologies present in the blood (total cholesterol, digestive problems, and diabetes).
In this work, the effect of inclusion of nanographite particles in a polypropylene (PP) matrix is studied. Nanographite particles were obtained through ultrasound exfoliation from graphite upon using ...a water-based hydrophobically modified alkali-swellable emulsion (HASE) associative polymer as a surfactant. Results indicate that exfoliation renders particle size distribution ranging from 3 to 3000 nanometers. Nanographite was blended with PP through two extrusion processes: twin screw and single screw, the latter includes the coupling to a static-mixer head, to generate extensional flows. Concurrently, ultrasonic waves are applied to the molten flow through ultrasonic transducers attached to the mixing head, which induces high particle dispersion and good particle distribution in the polymer matrix. It was found that at HASE concentration of 5% by weight and sonication time of 14 days (period of the exfoliation process), optimum tensile properties of the compound were achieved. Also, with respect to the PP matrix, the rate of thermal degradation decreased from 2.1 (PP) to 1.9 (% °C−1), melt temperature ranged from 442°C (PP) to 396°C, and melt index decreased from 7.4 (PP) to 6.2 (g/10 min). Raman spectroscopy confirmed the exfoliation process, rendering sizes ranged from graphite particles of few graphene layers to micron-sized particles. Rheological measurements of the compounds revealed that the extrusion-ultrasound process influences the viscosity, storage, and loss moduli. The dispersion and distribution of nanoparticles improved the electromagnetic radiation shield (approximately 35%). The dielectric constant changed from 2.21 (pristine PP) to 9.02 for the compounds, which enables a good level of electrostatic charge dissipation.
A Buckley–Leverett analysis with capillary pressure to model the oil displacement in fractal porous media is herein presented. The effective permeability for a non-Newtonian micellar fluid is ...calculated by a constitutive equation used to describe the rheological properties of a displacement fluid. The main assumption of this model involves a bundle of tortuous capillaries with a size distribution and tortuosity that follow fractal laws. The BMP model predicts two asymptotic (Newtonian) regions at low and high shear and a power-law region between the two Newtonian regions corresponding to a stress plateau. Both the stress at the wall and the fluidity are calculated using an imposed pressure gradient in order to determine the mobility of the solution. We analyze different mobility ratios to describe the behavior of the so-called self-destructive surfactants. Initially, the viscosity of the displacing fluid (micellar solution) is high; however, interactions with the porous media lead to a breakage process and degradation of the surfactant, producing low viscosity. This process is simulated by varying the applied pressure gradient. The resulting equation is of the reaction–diffusion type with various time scales; a shock profile develops in the convective time scale, as in the traditional Buckley-Leverett analysis, while at longer times diffusion effects begin to affect the profile. Predictions include shock profiles and compressive waves. These results may find application when selecting surfactants for enhanced oil recovery processes in oilfields.
Important public health problems worldwide such as obesity, diabetes, hyperlipidemia and coronary diseases are quite common. These problems arise from numerous factors, such as hyper-caloric diets, ...sedentary habits and other epigenetic factors. With respect to Mexico, the population reference values of total cholesterol in plasma are around 200 mg/dL. However, a large proportion has higher levels than this reference value. In this work, we analyze the rheological properties of human blood obtained from 20 donors, as a function of cholesterol and triglyceride levels, upon a protocol previously approved by the health authorities. Samples with high and low cholesterol and triglyceride levels were selected and analyzed by simple-continuous and linear-oscillatory shear flow. Rheometric properties were measured and related to the structure and composition of human blood. In addition, rheometric data were modeled by using several constitutive equations: Bautista-Manero-Puig (BMP) and the multimodal Maxwell equations to predict the flow behavior of human blood. Finally, a comparison was made among various models, namely, the BMP, Carreau and Quemada equations for simple shear rate flow. An important relationship was found between cholesterol, triglycerides and the structure of human blood. Results show that blood with high cholesterol levels (400 mg/dL) has flow properties fully different (higher viscosity and a more pseudo-plastic behavior) than blood with lower levels of cholesterol (tendency to Newtonian behavior or viscosity plateau at low shear rates).
In this work, the rheological behavior of complex fluids is analyzed with a model based on the classical transient network formulation, in which the description of nonlinear viscoelasticity and ...time-dependent phenomena considers spatial and temporal variations of the entanglement density in the flow region. The entropic law of the segments that join entanglement points of macromolecules (or dispersed phase) is modeled with a Warner spring law with variable extensibility. The structure modification is described with a function that is dependent of a kinetic process that involves the formation of a more entangled microstate on one extreme, and a flow-induced degradation of the transient network with variable entanglement density on the other extreme (disentangled microstate). Predictions of the model under simple shear, inception of shear flow, stress relaxation, interrupted shear, and shear-thickening are compared with those of other models and with experiments.
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