Wide Range Of Volume Fractions Research Articles

Micropolar effects on the effective shear viscosity of nanofluids

The modified size-dependent Einstein's and Brinkman's solutions are established for the effective shear viscosity of rigid particle suspensions taking into account the micropolar effects in the base fluid. Solutions are obtained based on the homogenization approach and allow us to take into account the influence of the particle size. Two non-classical parameters arise in the considered micropolar solutions: the length scale parameter and the coupling (micropolarity) number of the base fluid. The solutions developed are validated using tests performed with polydimethylsiloxane based TiO2 nanofluids as well as other published data on the size-dependent shear viscosity of different nanofluids. Good agreement between the predictions and the experimental data is established across a wide range of volume fractions and size of nanoparticles. The possibility for unique identification (at given temperature) of the micropolar parameters of the base fluids is shown. Temperature-dependent values of non-classical rotational and spin viscosities of polydimethylsiloxane, ethylene glycol, and water are evaluated.

Large strain micromechanics of thermoplastic elastomers with random microstructures

Thermoplastic polyurethanes (TPU) are block copolymeric materials composed of plastomeric “hard” and elastomeric “soft” domains, by which they exhibit highly resilient yet dissipative large deformation features depending on volume fractions and microstructures of the two distinct domains. Here, we develop a new methodology to explore the microscopic deformation mechanisms in TPU materials with highly disordered microstructures. We propose new micromechanical models for randomly dispersed (or occluded) as well as randomly continuous hard domains (and mixtures of both dispersed and continuous hard domains), each within a continuous soft structure as widely found in representative TPU materials over a wide range of volume fractions, vhard = 26.9% to 52.2%. The micromechanical modeling results are compared to experimental data on the macroscopic large strain behaviors reported previously (Cho et al., 2017). We explore the role of the dispersed vs. continuous nature of the geometric features of the random microstructures on shape recovery and energy dissipation at the microstructural level in these phase-separated copolymeric materials.

Dynamical behavior and transport coefficients of the pseudo hard-sphere fluid

In this work, we employ a recent approach to characterize the hard-sphere (HS) fluid by means of a continuous interaction potential, commonly referred to as pseudo hard-sphere potential, in order to determine HS transport coefficients as a function of the volume fraction for the three-dimensional mono disperse fluid. Using equilibrium molecular dynamics simulations, we determine time-dependent velocity, shear stress, and energy flux autocorrelation functions in order to use them within the Green–Kubo framework to compute the self-diffusion, shear viscosity, and thermal conductivity coefficients, respectively. Results are discussed as a function of the volume fraction and were compared to theoretical and simulations results previously reported by other authors. The main purpose of this work is twofold: first, testing the continuous approach of the HS fluid for the computation of dynamic properties and second, performing a systematic determination of aforementioned transport coefficients to analyze them as a function of fluid volume fraction. Furthermore, our results are used to provide a practical correction to the Chapman–Enskog equations for the HS self-diffusion, shear viscosity, and thermal conductivity predictions in a wide range of volume fractions.

Thermal conductivity of hydraulic oil-GO/Fe3O4/TiO2 ternary hybrid nanofluid: Experimental study, RSM analysis, and development of optimized GPR model

In the present paper, the thermal conductivity (TC) of a hydraulic oil-based nanofluid in the presence of ternary nano-additives, graphene oxide (GO), iron oxide (Fe3O4), and titanium dioxide (TiO2), is analyzed in a wide range of volume fractions (VFs), temperatures, and mixing ratios (MRs). The stability of ternary hybrid nanofluids (THNFs) and size distribution of nanomaterial is obtained through zeta potential and dynamic light scattering (DLS) tests. Zeta potential and DLS tests indicated the remarkable stability of the samples with the GO(2): Fe3O4(1): TiO2(1) MR. Analysis of the measurements revealed that the enlargement in temperature and VFs improved the TC of THNFs for all MRs (1:1:1, 2:1:1, 1:2:1, 1:1:2). The highest TC enhancement is observed at the highest temperature (65 °C) and VF (1%), which for the MRs of 1:1:1, 1:1:2, 1:2:1, and 2:1:1 equal to 36.04%, 26.28%, 25.95%, and 33.86%, respectively. Furthermore, considering the average TC enhancement in the presence of nano-additives for various temperatures, MRs of GO(1): Fe3O4(1): TiO2(1) and GO(1): Fe3O4(1): TiO2(2) indicated the best and worst efficiency with 30.46% and 22.01%, respectively. The RSM method is applied to provide a simple and efficient formula-based model to describe the TC of THNFs in terms of input variables. In addition, a novel genetic algorithm-based optimization of training/structure parameters of Gaussian process regression (GPR) as a leading machine learning algorithm is developed, which provided thoroughly precise outcomes (Rtest2=0.9994 and Rtrain2=0.9998) for the prediction of TC of THNFs. The sensitivity analysis for the present THNFs revealed that the TC sensitivity is maximized at the highest VF and temperature.

Concentration Scaling on Linear Viscoelastic Properties of Cellular Suspensions and Effects of Equilibrium Phase Behavior.

Concentration scaling on linear viscoelastic properties of cellular suspensions has been studied by rheometric characterisation of Phormidium suspensions and human blood in a wide range of volume fraction under small amplitude oscillatory shear experiments. The rheometric characterisation results are analysed by the time-concentration superposition (TCS) principle and show a power law scaling of characteristic relaxation time, plateau modulus and the zero-shear viscosity over the concentration ranges studied. The results show that the concentration effect of Phormidium suspensions on their elasticity is much stronger than that of human blood due to its strong cellular interactions and a high aspect ratio. For human blood, no obvious phase transition could be observed over the range of hematocrits studied here and with respect to a high-frequency dynamic regime, only one concentration scaling exponent could be identified. For Phormidium suspensions with respect to a low-frequency dynamic regime, three concentration scaling exponents in the volume fraction Region I (0.36≤ϕ/ϕref≤0.46), Region II (0.59≤ϕ/ϕref≤2.89) and Region III (3.11≤ϕ/ϕref≤3.44) are identified. The image observation shows that the network formation of Phormidium suspensions occurs as the volume fraction is increased from Region I to Region II; the sol-gel transition takes place from Region II to Region III. In combination with analysis of other nanoscale suspensions and liquid crystalline polymer solutions reported in the literature, it is revealed that such a power law concentration scaling exponent depends on colloidal or molecular interactions mediated with solvent and is sensitive to the equilibrium phase behaviour of complex fluids. The TCS principle is an unambiguous tool to give a quantitative estimation.

Ultrasonic Monitoring of the Water Content in Concentrated Water-Petroleum Emulsions Using the Slope of the Phase Spectrum.

This work proposes the slope of the phase spectrum as a signal processing parameter for the ultrasonic monitoring of the water content of water-in-crude oil emulsions. Experimental measurements, with water volume fractions from 0 to 0.48 and test temperatures of 20 °C, 25 °C, and 30 °C, were carried out using ultrasonic measurement devices operating in transmission–reception and backscattering modes. The results show the phase slope depends on the water volume fraction and, to a lesser extent, on the size of the emulsion droplets, leading to a stable behavior over time. Conversely, the behavior of the phase slope as a function of the volume fraction is monotonic with low dispersion. Fitting a power function to the experimental data provides calibration curves that can be used to determine the water content with percentage relative error up to 70% for a water volume fraction of 0.06, but less than 10% for water volume fractions greater than 0.06. Furthermore, the methodology works over a wide range of volume fractions.

Parametric design and evaluation of TPMS-like cellular solids

Triply periodic minimal surfaces (TPMS) present a starting point for designing infinitely many novel cellular solids, creating the need for a structured approach to their design and evaluation. In this study the design space is formalised using a set of input parameters and evaluation metrics to provide a structured and reproducible approach to designing a TPMS-like cellular solids. Interactions between the design parameters and evaluation metrics are identified using a full-factorial design of experiments. From this the most impactful design parameters are identified as the TPMS equation, type of structure, and volume fraction. It was found that the gyroid-surface structure provided the most flexibility, with near-isotropic mechanical properties across a wide range of volume fractions, allowing the creation of reduced mass/stiffness designs without introducing anisotropy or variation in Poisson’s ratio. The volume fraction at which the connectivity of a structure changes, “pinch-off limits” are presented for six of the most commonly investigated TPMS-like cellular solids. Additionally, the effects of common transformations are presented including: examples of how phase-shift and/or rotation can be used to modify the features which intersect the free-boundary of a design; dimensional analysis which shows the practical effects changing unit cell size; and analysis showing the effect of rotating a cellular structure on both the mechanical properties and surface orientation. These computational studies are supported with a physical case study which demonstrates how several design parameters may be modified to change the manufacturability of a TPMS-like cellular solid.

A Multiscale Approach for the Numerical Simulation of Turbulent Flows with Droplets.

A multiscale approach for the detailed simulation of water droplets dispersed in a turbulent airflow is presented. The multiscale procedure combines a novel representative volume element (RVE) with the Pseudo Direct Numerical Simulation (P-DNS) method. The solution at the coarse-scale relies on a synthetic model, constructed using precomputed offline RVE simulations and an alternating digital tree, to characterize the non-linear dynamic response at the fine-scale. A set of numerical experiments for a wide range of volume fractions, particle distribution sizes, and external shear forces in the RVE are carried out. Quantitative results of the statistically stationary turbulent state are obtained, and the turbulence modulation phenomenon due to the presence of droplets is discussed. The developed synthetic model is then employed to solve global scale simulations of flows with airborne droplets via the P-DNS method. Improved predictions are obtained for flow conditions where turbulence modulation is noticeable.

Modeling the temperature dependent ultimate tensile strength of fiber/polymer composites considering fiber agglomeration

A temperature dependent ultimate tensile strength (TDUTS) model of polymer composites reinforced with fibers (FPCs) for a wide range of volume fractions was developed according to the Force-Heat Equivalence Energy Density Principle and combined with the Halpin-Tsai model. This model involved the quantitative evolution of thermo-physical performance of fiber, polymer with temperature, as well as the effects of fiber agglomeration, volume fraction, orientation and geometry. The predictive model was verified by comparing the predictions with the available experimental results of both short and unidirectional fiber/polymer composites for a wide range of temperature and volume fractions. To illustrate the effect of fiber agglomeration on the composite TDUTS, influencing factor analysis was also made. Compared with other classical models, this model predicts more accurate and only basic and easily accessible material parameters are required. This study demonstrates theoretical support to estimate the UTS of FPCs versus volume fractions at elevated temperatures, further provides opinion about the composite preparation and performance improvement.

Sheared Amorphous Packings Display Two Separate Particle Transport Mechanisms.

Shearing granular materials induces nonaffine displacements. Such nonaffine displacements have been studied extensively, and are known to correlate with plasticity and other mechanical features of amorphous packings. A well known example is shear transformation zones as captured by the local deviation from affine deformation, D_{min}^{2}, and their relevance to failure and stress fluctuations. We analyze sheared frictional athermal disc packings and show that there exists at least one additional mesoscopic transport mechanism that superimposes itself on top of local diffusive motion. We evidence this second transport mechanism in a homogeneous system via a diffusion tensor analysis and show that the trace of the diffusion tensor equals the classic D_{min}^{2} when this second mesoscopic transport is corrected for. The new transport mechanism is consistently observed over a wide range of volume fractions and even for particles with different friction coefficients and is consistently observed also upon shear reversal, hinting at its relevance for memory effects.

Collagen fiber interweaving is central to sclera stiffness

The mechanical properties of the microstructural components of sclera are central to eye physiology and pathology. Because these parameters are extremely difficult to measure directly, they are often estimated using inverse-modeling matching deformations of macroscopic samples measured experimentally. Although studies of sclera microstructure show collagen fiber interweaving, current models do not account for this interweaving or the resulting fiber-fiber interactions, which might affect parameter estimates. Our goal was to test the hypothesis that constitutive parameters estimated using inverse modeling differ if models account for fiber interweaving and interactions. We developed models with non-interweaving or interweaving fibers over a wide range of volume fractions (36–91%). For each model, we estimated fiber stiffness using inverse modeling matching biaxial experimental data of human sclera. We found that interweaving increased the estimated fiber stiffness. When the collagen volume fraction was 64% or less, the stiffness of interweaving fibers was about 1.25 times that of non-interweaving fibers. For higher volume fractions, the ratio increased substantially, reaching 1.88 for a collagen volume fraction of 91%. Simulating a model (interweaving/non-interweaving) using the fiber stiffness estimated from the other model produced substantially different behavior, far from that observed experimentally. These results show that estimating microstructural component mechanical properties is highly sensitive to the assumed interwoven/non-interwoven architecture. Moreover, the results suggest that interweaving plays an important role in determining the structural stiffness of sclera, and potentially of other soft tissues in which the collagen fibers interweave. Statement of SignificanceThe collagen fibers of sclera are interwoven, but numerical models do not account for this interweaving or the resulting fiber-fiber interactions. To determine if interweaving matters, we examined the differences in the constitutive model parameters estimated using inverse modeling between models with interweaving and non-interweaving fibers. We found that the estimated stiffness of the interweaving fibers was up to 1.88 times that of non-interweaving fibers, and that the estimate increased with collagen volume fraction. Our results suggest that fiber interweaving is a fundamental characteristic of connective tissues, additional to anisotropy, density and orientation. Better characterization of interweaving, and of its mechanical effects is likely central to understanding microstructure and biomechanics of sclera and other soft tissues.

Rheological and flow birefringence studies of rod-shaped pigment nanoparticle dispersions

We study rheological and rheo-optical properties of suspensions of anisometric pigment particles in a non-polar fluid. Different rheological regimes from the dilute regime to an orientationally arrested gel state were characterized and compared with existing theoretical models. We demonstrate the intricate flow behaviour in a wide range of volume fractions. A unique combination of the optical properties of the particles results in a giant rheo-optical effect: an unprecedentedly large shear stress-induced birefringence was found in the isotropic range, exhibiting a sharp pre-transitional behaviour.

The inhibition of concentrated active baths.

Passive tracers in the active bath express fascinating behaviors. However, most studies are restricted to dilute active baths. Here, we use 2D simulation of suspensions consisting of active Brownian particles and a passive disk-shaped tracer to investigate tracers' diffusive behaviors in a wide range of volume fractions. Due to the competition between the thermal noise and collisions with active particles, tracers express a first transition from the normal diffusion to the superdiffusion at a short time scale and recur to normal diffusion at a long time scale. At a low volume fraction, infrequent active collisions retard the first transition of smaller tracers. At a high volume fraction, active particles with high activity aggregating around tracers induce a bimodal probability distribution function of tracer displacements during superdiffusion. Considering the enhancement of diffusion, the non-dimensional enhanced diffusivity increases asymptotically with the Peclet number. The asymptotic line gives an upper limit of non-dimensional enhanced diffusivity of tracers. Cases with lower enhanced diffusion have a high volume fraction and a low active velocity that indicates the inhibition of concentrated active baths. With the high negentropic work of these cases, the inhibition is explained as the change of the configuration of active baths for introducing tracers.

Microstructure and mechanical behavior of Al–TiAl3 composites containing high content uniform dispersion of TiAl3 particles

The microstructure, mechanical behavior and deformation mechanisms of Al–TiAl3 composites containing a wide range of volume fraction of TiAl3 (Vp: 24.6–89.0%) with uniform distribution were investigated. Microhardness and strength of the composites increase with increasing Vp of TiAl3 particles, reaching ~11.0 times and ~8.0 times those of the matrix respectively. While the strength of the composites can be correlated well to the comprehensive contributions of various strengthening mechanisms of the matrix for low Vp, that for high Vp was found to fit to direct contributions from both the matrix and particle following the law of mixture. Besides the deformation of the matrix, evident plastic deformation of TiAl3 particles by dislocation activities as well as twinning was observed in the composites upon loading at room temperature. Two mechanisms for crack initiation inside the TiAl3 particle were identified, namely by intersection of twins and by interaction between dislocations and twins. Microcracks tend to propagate along the twin boundaries in TiAl3 particles and were found to be suspended at the particle/matrix interface. The unique deformation behavior of TiAl3 particles in the composites can be attributed to the uniform distribution of TiAl3 particles that ensures a continuous network of surrounding matrix even at very high Vp.

Carbon-Fiber Enriched Cement-Based Composites for Better Sustainability.

Damage caused by global warming is rapidly increasing, and its adverse effects become more evident with each passing day. Although it is known that the use of alternative binder materials in concrete would decrease this negative effect, reluctance to use such new composites continues. Waste plays a vital role in sustainability studies. In this study, pure cement paste was prepared and enriched with carbon fiber. This study also investigated the wide range of volume fraction of carbon fiber in cement-based composites. Two different types of industrial waste, i.e., marble dust and bottom ash, were chosen and mixed with cement and four different (0.3%, 0.75%, 1.5%, and 2.5%) carbon fiber volume fractions. Based on physical, mechanical, and durability tests at 7, 28, and 56 days of curing, the composites were resistant to sulfate and seawater attacks. The 0.75% carbon fiber addition seems to be an optimum volume percentage, beyond which both physical and mechanical properties were adversely affected. The composites with 0.75% carbon fiber reached 48.4 and 47.2 MPa at 56 days of curing for marble dust and bottom ash mixture groups, respectively.

Excess entropy scaling for soft particle glasses

The transport properties of soft particle glasses, such as dynamic viscosity, normal stress coefficients, and shear-induced diffusivity of its particles, are determined by the microstructure of the suspension under flow. A thermodynamic measure of the microstructure is the excess entropy, which we show here accurately correlates the transport properties of soft particle glasses onto master curves across a wide range of volume fractions, suspending fluid viscosities, particle moduli, and shear rates. The excess entropy for soft particle glasses is approximated with the two-body excess entropy computed from the pair distribution function extracted from dynamic simulations. The shear viscosity and normal stress functions diverge and the diffusivity vanishes at a critical excess entropy, corresponding to the yield stress of the suspension. An effective temperature is computed and is found to vary linearly with the shear stress and the elastic energy of the sheared soft particle glass. From this, an equation of state is derived relating the excess entropy to the shear stress. Consequently, three of the four transport properties are determined from the measurement of just one. Finally, a single master curve of particle diffusivity versus excess entropy is presented that unifies observations for both equilibrium and nonequilibrium suspensions.

FEM analysis of the elastic behavior of composites and nanocomposites with arbitrarily oriented reinforcements

This work is aimed at studying the effect of different morphological features on the stiffness of discontinuous fiber reinforced composites and nanocomposites by means of FEM analysis. The morphological features include not only volume fraction and aspect ratio of reinforcement, but also its orientation. The FEM approach, based on continuum mechanics, required the definition of a proper representative volume element, in which reinforcement is randomly dispersed in the simulation domain. The developed model has been used in order to calculate the stiffness matrix of unidirectional discontinuous glass fiber reinforced composite (DGFRC) as well as carbon nanotubes reinforced nanocomposites (CNTRNC). Comparison with closed form analytical models showed that the Halpin-Tsai (HT) model is able to predict the evolution of the elastic constants over a wide range of volume fractions and aspect ratio. The Tandon-Weng (TW) model, in contrast, shows a poor agreement with the simulation results.On the other hand, the developed FEM model also allowed accounting for reinforcement orientation. The calculated elastic properties are in excellent good agreement with the corresponding values calculated according to the classical lamination theory, indicating the accuracy of the developed FEM model for estimation of stiffness of composites and nanocomposites, also accounting for reinforcement orientation.

Novel computational approaches to retention modeling in dual hydrophilic interactions/reversed phase chromatography

The mixed-mode chromatographic behavior was estimated for imidazoline and serotonin receptor ligands, and their related compounds on dual hydrophilic/reversed phase stationary phase. The Box-Cox transformation was used to obtain the most suitable mathematical equations which describe the mixed-mode retention. Optimal equations were found for the optimization parameter (λ): λ = -1, λ = -0.5, λ = 0, λ = 0.5, and λ = 1. The proposed equations show satisfactory characteristics compared to standard multimodal and quadratic approaches.For a wide range of volume fractions of the mobile phase modifier, crossing between hydrophilic and reversed phase interactions (the turning point) was defined in terms of the minimal retention and the minimum value of the volume fraction of the aqueous eluent in the mobile phase. The cubic spline interpolation was used as a reference method for estimation of the turning point. It was found out that the newly proposed equations can be used as alternative mathematical forms for the description of the dual retention mechanism and for the evaluation of the turning point.Three new experimental descriptors of the mixed-mode retention were proposed. Two descriptors quantitatively characterize hydrophilic (log kH) and reversed phase (log kR) interactions, while the third one (log kA) refers to the average retention for the whole HILIC/RP range. It was established that the main factors which control dual nature of the mixed-mode retention are lipophilicity, dipol-dipol, van der Waals and hydrogen bonding interactions.It was concluded that the newly proposed estimations of the retention data reliably characterize the mixed-mode chromatographic behavior.

Conduction paths of backbones for thermal enhancement of nanocomposite phase change materials

Thermal energy storage (TES) based on phase change materials (PCMs) has become a research hot spot due to its high energy storage density and maintained operating temperature during the phase change. However, as PCM has a poor thermal conductivity that can be as low as 0.2~0.5 W/m·K, the charging/discharging processes of PCM modules are significantly restrained, which severely affects the application of the TES technology in industrial sectors. This study concerns the improvement of the effective thermal conductivity of composite PCM formed by adding nanoparticles with high thermal conductivity into different PCMs. A theoretical model is established to reveal the intrinsic mechanism for the promotion of thermal conductivity of composite PCM consisting of nanoparticles. The results show that aggregation and interfacial thermal resistance are the main reasons for the change of the thermal conductivity. By forming effective conduction paths composed of backbones in the composite PCM, the average thermal conductivity can be improved significantly, which can be as high as 10~50 W/m·K with a wide range of volume fraction of the additives.

X-ray radiography of viscous resuspension

We use X-ray imaging to study viscous resuspension. In a Taylor-Couette geometry, we shear an initially settled layer of spherical glass particles immersed in a Newtonian fluid and measure the local volume fraction profiles. In this configuration, the steady-state profiles are simply related to the normal viscosity defined in the framework of the suspension balance model. These experiments allow us to examine this fundamental quantity over a wide range of volume fractions, in particular, in the semidilute regime where experimental data are sorely lacking. Our measurements strongly suggest that the particle stress is quadratic with respect to the volume fraction in the dilute limit. Strikingly, they also reveal a nonlinear dependence on the Shields number, in contrast with previous theoretical and experimental results. This likely points to shear-thinning particle stresses and to a non-Coulomb or velocity-weakening friction between the particles, as also evidenced from shear reversal experiments.