Fixed Volume Fraction Research Articles

Numerical study of filament suspensions at finite inertia

We present a numerical study on the rheology of semi-dilute and concentrated filament suspensions of different bending stiffness and Reynolds number, with the immersed boundary method used to couple the fluid and solid. The filaments are considered as one-dimensional inextensible slender bodies with fixed aspect ratio, obeying the Euler–Bernoulli beam equation. To understand the global suspension behaviour we relate it to the filament microstructure, deformation and elastic energy and examine the stress budget to quantify the effect of the elastic contribution. At fixed volume fraction, the viscosity of the suspension reduces when decreasing the bending rigidity and grows when increasing the Reynolds number. The change in the relative viscosity is stronger at finite inertia, although still in the laminar flow regime, as considered here. Moreover, we find the first normal stress difference to be positive as in polymeric fluids, and to increase with the Reynolds number; its value has a peak for an intermediate value of the filament bending stiffness. The peak value is found to be proportional to the Reynolds number, moving towards more rigid suspensions at larger inertia. Moreover, the viscosity increases when increasing the filament volume fraction, and the rate of increase of the filament stress with the bending rigidity is stronger at higher Reynolds numbers and reduces with the volume fraction. We show that this behaviour is associated with the formation of a more ordered structure in the flow, where filaments tend to be more aligned and move as a compact aggregate, thus reducing the filament–filament interactions despite their volume fraction increases.

Investigation on mechanical properties of hybrid graphene and beryl reinforced aluminum 7075 composites

The emerging technologies and trends of present generation requires downsizing the unwieldy structures to light weight structures on one hand and integration of varied properties on other hand to meet the application demands. In the present investigation an attempt is made to familiarize and best possibilities of reinforcing agent in aluminum 7075 matrix with naturally occurring beryl (Be) and graphene (Gr) to develop a new hybrid composite material. A stir casting process was used to fabricate with fixed volume fraction of 6wt% weight beryl and various volume fractions of 0.5wt%, 1wt%, 1.5wt% and 2wt% of graphene. The properties such as tensile strength, compression strength, and hardness of hybrid composites were examined. The crystallite size and morphology of the graphene and beryl particles were analyzed with X-ray diffraction (XRD) and scanning electron microscopy (SEM) respectively. It was observed that ultimate tensile strength and compression strength of the hybrid composite increased with increasing reinforcement volume fraction as compared to specimen without reinforcement additions.

Brownian Dynamics Simulation of Colloidal Gels as Matrix for Controlled Release Application

The form of crystallization colloidal gels is important as matrix for controlled release application. In this work, we use the Brownian Dynamics simulation to study the formation of gels by varying the inverse Debye length. We choose a fixed volume fraction, ϕ = 0.1 and a fixed quenched temperature at room temperature, while the inverse Debye length, κ, is varied. To ensures that the simulations cover the fluid-phase region down to the unstable phase region above the critical coagulation concentration, the inverse Debye length is varied between κ = 120σ−1 to 250σ−1. It shows that at the inverse Debye length κ = 250σ−1 the gel forms by colloidal particles that can support the active ingredient by forming long range network.

Microstructure, Tensile Properties and Hardness Behavior of Al7075 Matrix Composites Reinforced With Graphene Nanoplatelets And Beryl Fabricated by Stir Casting Method

In this research, an effort is made to familiarize and best potentials of the reinforcing agent in aluminum 7075 matrices with naturally occurring Beryl (Be) and Graphene (Gr) to develop a new hybrid composite material. A stir casting technique was adopted to synthesize the hybrid nanocomposites. GNPS were added in volume fractions of 0.5wt%, 1wt%, 1.5wt%, and 2wt% and with a fixed volume fraction of 6 wt.% of Beryl. As cast hybrid composites were microstructurally characterized with scanning electron microscopy and X-ray diffraction. Microstructure study through scanning electron microscope demonstrated that the homogeneous distribution reinforcement Beryl and GNPs into the Al7075 matrix. Brinell hardness and tensile strength of synthesized materials were investigated. The hybrid Al7075-Beryl-GNPs composites showed better mechanical properties compared with base Al7075 matrix material. The as-cast Al7075-6wt.% Beryl-2wt.%GNPs showed 49.41% improvement in hardness and 77.09% enhancement in ultimate tensile strength over Al7075 alloy.

Devising Carbon Nanotube, Green Tea, and Polyaniline Based Nanocomposite plus Investigating Its Rheological together with Bactericidal Efficacies.

Subsequently, engines are designed to operate at low viscosity engine oils. Low viscosity oils take less power from engines, bring down the internal drag, cut the fuel consumption, and ultimately improve the engine’s efficiency. Considering this focus, an approach has been made to formulate a multiwalled carbon nanotube based green tea and polyaniline nanocomposite, that is, GT/MWCNT/PANI, and incorporate it in engine oil (base fluid). The objective was to reduce the viscosity of engine oil by examining the effects of the constant shear rate and varying shear rates on the viscosity of Castrol class 15W-40 engine oil. The investigation was performed at a constant temperature of 25 °C for a fixed volume fraction of 0.1% GT/MWCNT/PANI in engine oil on the experimental setup rheometer from Anton Paar Series. Primordial findings revealed that, at a constant shear rate of 100 s–1, engine oil viscosity was lowered from 0.221000 to 0.001402 Pa·s, that is, 99% reduction in viscosity of the engine oil, after incorporating the GT/MWCNT/PANI nanocomposite. Furthermore, a new correlation has been proposed considering the experimental and theoretical models with an average percentage error of 0.040%. Also, at varying shear rates, up to 90 s–1, the shear viscosity of nanofluid decreases significantly, leading to shear-thinning behavior of the nanofluid, while at a shear rate of >90 s–1, it shows Newtonian behavior. Besides, the ternary nanocomposite with 0.2 wt % GT/MWCNT/PANI also showed significant bactericidal effects with the zones of inhibition of 19, 18, and 15 mm against Gram-negative (Pseudomonas aeruginosa, Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria, respectively, as measured using the well diffusion method.

Proposing an optimal tree-like design of highly conductive material configuration with unequal branches for maximum cooling a heat generating piece

Due to the concerns regarding the space and cost of high thermal conductivity materials, researching for a better design of high thermal conductivity pathways embedded into a heat generating piece is important. Constructal design theory is applied and a numerical optimization procedure is carried out to find a new geometric pattern of tree-like highly conductive material pathways with unequal branches. The optimization purpose is the minimization of the peak temperature in the heat generating piece, by varying the lengths of the branches and the initial distance from the middle plane. The constraint is the fixed volume fraction occupied by the high thermal conductivity insert. Heat conduction equations are solved by a finite element method (FEM). The numerical code develops in MATLAB software and use unstructured triangular elements for meshing. Because of the multiple optimization variables existed in this work, the “Pattern Search” algorithm is used. The results indicate that the performance of the tree-like configuration with unequal branches is better than the one with equal branches for the same number of branches, the same volume fraction of the highly conductive material and the same conductive ratio. It is also shown that the new proposed configurations of highly thermal conductive pathways, significantly surpass the latest configurations presented in the literature. Depending on the volume fraction of highly conductive material and thermal conductivity ratio, the performance of the proposed configurations is observed to be 2–61% better than the performance of the best result existed in previous works.

Surface waves with negative phase velocity supported by temperature-dependent hyperbolic materials

A numerical investigation was undertaken to elucidate the propagation of electromagnetic surface waves guided by the planar interface of two temperature-sensitive materials. One partnering material was chosen to be isotropic and the other to be anisotropic. Both partnering materials were engineered composite materials, based on the temperature-sensitive semiconductor InSb. At low temperatures the anisotropic partnering material is a non-hyperbolic uniaxial material; as the temperature is raised this material becomes a hyperbolic uniaxial material. At low temperatures, a solitary Dyakonov wave propagates along any specific direction in a range of directions parallel to the planar interface. At high temperatures, up to three different surface waves can propagate in certain directions parallel to the planar interface; one of these surface waves propagates with negative phase velocity (NPV). At a fixed temperature, the range of directions for NPV propagation decreases uniformly in extent as the volume fraction of InSb in the isotropic partnering material decreases. At a fixed volume fraction of InSb in the isotropic partnering material, the angular range for NPV propagation varies substantially as the temperature varies.

Absorption characteristics of nanoparticles with sharp edges for a direct-absorption solar collector

Plasmonic nanofluids has been reported beneficial for enhancing absorption of the solar energy in a direct-absorption solar collector (DASC). In order to overcome the shortage of narrow absorption band associated with the localized surface plasmon, two strategies can be adopted. One is to blend nanoparticles with different absorption peaks, and the other is to develop nanoparticles capable of exhibiting multiple absorption peaks at different wavelengths. In this study, the latter strategy is explored systematically by investigating the absorption efficiency of metallic nanoparticles with sharp edges. The results show that the Ag nanoparticles with sharp edges can induce multiple absorption peaks due to both localized surface plasmon resonance and lightning rod effect. We also show that the sharper edges (i.e., with either smaller radius of curvature or smaller edge angle) can greatly enhance the lightning rod effect. In addition, the study of SiO2-core/Ag-shell suggests that the core/shell configuration is beneficial for further broadening the absorption band compared to the Ag nanoparticle. Further investigation shows that the solar-weighted absorption coefficient of a DASC using the four-edge nanoparticle is 35% and 20% point higher than the nanosphere and the nanorod respectively with a fixed volume fraction of 10−6.

Numerical modeling of the porosity influence on the elastic properties of sintered materials

The effect of structural porosity on the elastic properties of sintered materials was studied using the new multi-pore unit cell numerical model - MPUC. Comparison between proposed MPUC model and previously adopted two-phase unit cell - FCC numerical model as well as available experimental data in literature, was done by comparing obtained values for modulus of elasticity - E, shear modulus - G and Bulk modulus - K. Results obtained by proposed MPUC model are in excellent agreement with available experimental data in literature. It was confirmed that material porosity regarding pores? size (volume fraction) has noticeable influence on elastic properties of sintered material. Less porosity in the material microstructure generally leads to noticeable higher values of E, G and K. For fixed volume fraction, shape of pores has no significant influence on elastic characteristics.

Estimation Model for Effective Thermal Conductivity of Reinforced Concrete Containing Multiple Round Rebars

The objective of this research is to estimate the effective thermal conductivity model for the reinforced concrete containing round rebars. The thermal network concept and Fourier’s law are used to develop a mathematical model to calculate the effective thermal conductivity (keff) of reinforced concrete with different numbers of round rebars oriented either normal or parallel to the heat-transfer direction, and different volume fractions of steel. Model predictions generally agree well with results of numerical simulations using the finite-volume method (FVM). FVM results suggest that at fixed volume fractions of steel, the effective thermal conductivity decreases as the number of round rebars increases if the rebars are in the normal orientation but increases as the number of round rebars increases if they are in the parallel orientation. This research can be used in practical conditions to predict the effective thermal conductivity of reinforced concrete containing round rebars accurately.

Unsteady Radiative Heat Transfer Model of a Ceria Particle Suspension Undergoing Solar Thermochemical Reduction

Unsteady radiative heat transfer is analyzed numerically in a directly irradiated plane-parallel medium containing a suspension of ceria particles undergoing nonstoichiometric thermal reduction. The micrometer-sized ceria particles are assumed homogenous, nongray, absorbing, emitting, and anisotropically scattering, whereas the overall medium is of nonuniform temperature and composition. The unsteady mass and energy conservation equations are solved using the finite volume method and the Shampine–Gordon time integration scheme. Radiative transport is modeled using the energy-portioning Monte Carlo ray-tracing method with radiative properties obtained from the Mie theory. Increasing the particle volume fraction and decreasing the particle diameter both increase the optical thickness of the particle suspension, resulting in increasing peak temperature and nonstoichiometry at steady state. For -diam particles under 1000 suns irradiation, the peak temperature at steady state ranges from 1855 K for a particle volume fraction of to 2092 K for ; the temperature nonuniformity ranges from 9 to 622 K. For a fixed volume fraction of , decreasing the particle diameter from 20 to increases the peak temperature at steady state from 1734 to 2162 K; the temperature nonuniformity increases from 9 to 61 K.

Computational modeling of fracture in encapsulation-based self-healing concrete using cohesive elements

Fracture behavior of encapsulation-based self-healing concrete (SHC) is investigated numerically. In this paper, we study the influence of capsules including its volume ratio and core-shell thickness on the load carrying capacity and fracture likelihood of the capsules. In order to randomly create the mesoscale structure of self-healing concrete, an efficient packing algorithm is employed. For a given number of circular capsules with particular diameter and shell thickness, the aggregates are generated from prescribed distributions of their size and volume fraction. The capsules are made of Poly Methyl Methacrylate and potential cracks are represented by pre-inserted cohesive elements with tension and shear softening laws along all element boundaries including the interfaces between different phases. The effects of the volume fraction of capsules and core-shell thickness ratio on the load carrying capacity and fracture probability of the capsules are analyzed. It is found that the load-carrying capacity of self-healing concrete decreases as the capsules volume fraction increases. The capsule core-shell thickness ratio has no significant influence on the specimen strength but a very significant impact on the breakage of capsule shell. Given a fixed volume fraction of capsules, the higher core-shell thickness ratio, the higher is the number of capsules which fracture. Assuming the core-shell ratio thickness is fixed, increasing the volume fraction of capsules will increase the probability of capsules which break.

Novel Nanofluids Based on Magnetite Nanoclusters and Investigation on Their Cluster Size-Dependent Thermal Conductivity

To probe the effect of particle size on thermal conductivity (k) enhancement in nanofluids, especially in a very large particle size range, we study the cluster size-dependent k in novel nanofluids containing magnetite (Fe3O4) nanoclusters. The Fe3O4 nanoclusters in the size range of 115 to 530 nm were synthesized by a facile and cost-effective solvothermal approach. The structural, surface, and magnetic characteristics of Fe3O4 nanoclusters were investigated by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and vibrating sample magnetometer (VSM). Thermal conductivity studies in diethylene glycol (DEG)-based Fe3O4 cluster nanofluids showed an enhancement in k with an increase in nanocluster size. With a fixed volume fraction (ϕ) = 0.0193, the k enhancement was about 5.3% and 12.6%, respectively, for nanofluids having cluster size of 115 and 530 nm. The observed increase in nanofluid k with increase...

Local Stress–Strain State of Tribocomposites with Spherical Microcapsules Filled with Liquid Lubricant

A method for predicting the local elastic characteristics of multicomponent matrix-assisted tribocomposites with spherical microcapsules filled with a liquid lubricant is proposed. These characteristics are defined by the operators (tensors) of the stress and strain concentration. This method is based on a generalized singular approximation of the theory of random fields and allows taking into consideration the inherent size of microcapsules (the ratio of the shell thickness to the radius of the liquid filler) and the volume fraction of the tribocomposite components. The local elastic characteristics are numerically modelled for the composites based on phenylon with disperse inclusions of alkali-free glass and microcapsules, which are spherical kapton shells filled with glycerin. The dependences of the local elastic properties of the tribocomposites on the inherent size of the microcapsules at fixed volume fractions of the heterogeneity elements are studied.

A study of MRI gradient echo signals from discrete magnetic particles with considerations of several parameters in simulations

Modeling MRI signal behaviors in the presence of discrete magnetic particles is important, as magnetic particles appear in nanoparticle labeled cells, contrast agents, and other biological forms of iron. Currently, many models that take into account the discrete particle nature in a system have been used to predict magnitude signal decays in the form of R2* or R2′ from one single voxel. Little work has been done for predicting phase signals. In addition, most calculations of phase signals rely on the assumption that a system containing discrete particles behaves as a continuous medium. In this work, numerical simulations are used to investigate MRI magnitude and phase signals from discrete particles, without diffusion effects. Factors such as particle size, number density, susceptibility, volume fraction, particle arrangements for their randomness, and field of view have been considered in simulations. The results are compared to either a ground truth model, theoretical work based on continuous mediums, or previous literature. Suitable parameters used to model particles in several voxels that lead to acceptable magnetic field distributions around particle surfaces and accurate MR signals are identified. The phase values as a function of echo time from a central voxel filled by particles can be significantly different from those of a continuous cubic medium. However, a completely random distribution of particles can lead to an R2′ value which agrees with the prediction from the static dephasing theory. A sphere with a radius of at least 4 grid points used in simulations is found to be acceptable to generate MR signals equivalent from a larger sphere. Increasing number of particles with a fixed volume fraction in simulations reduces the resulting variance in the phase behavior, and converges to almost the same phase value for different particle numbers at each echo time. The variance of phase values is also reduced when increasing the number of particles in a fixed voxel. These results indicate that MRI signals from voxels containing discrete particles, even with a sufficient number of particles per voxel, cannot be properly modeled by a continuous medium with an equivalent susceptibility value in the voxel.

Study of dynamical heterogeneities in colloidal nanoclay suspensions approaching dynamical arrest

The dynamics of aqueous Laponite clay suspensions slow down with increasing sample waiting time (tw). This behavior, and the material fragility that results, closely resemble the dynamical slowdown in fragile supercooled liquids with decreasing temperature, and are typically ascribed to the increasing sizes of distinct dynamical heterogeneities in the sample. In this article, we characterize the dynamical heterogeneities in Laponite suspensions by invoking the three-point dynamic susceptibility formalism. The average time-dependent two-point intensity autocorrelation and its sensitivity to tw are probed in dynamic light scattering experiments. Distributions of relaxation time scales, deduced from the Kohlrausch-Williams-Watts equation, are seen to widen with increasing tw. The calculated three-point dynamic susceptibility of Laponite suspensions exhibits a peak, with the peak height increasing with evolving tw at fixed volume fraction or with increasing volume fraction at fixed tw, thereby signifying the slowdown of the sample dynamics. The number of dynamically correlated particles, calculated from the peak-height, is seen to initially increase rapidly with increasing tw, before eventually slowing down close to the non-ergodic transition point. This observation is in agreement with published reports on supercooled liquids and hard sphere colloidal suspensions and offers a unique insight into the colloidal glass transition of Laponite suspensions.

Crystal perfection by strain engineering: The case of Fe/V (001)

We study the effect of bilayer thickness at fixed volume fraction on the structural quality of Fe/V (001) superlattices. We find that such artificial metallic superlattices can be manufactured with excellent crystal quality and layering up to at least 50Å in repeat distance (Λ=LFe+LV). For an intended fixed ratio of the constituents: LFe/LV= 1/7, out-of-plane coherence lengths comparable to the thicknesses of the samples were obtained. We evaluate the strain in- and out-of-plane of both layers as a function of the bilayer thickness and comment on the growth using the framework of linear elasticity theory. We interpret the stability of the superlattice against crystal degradation due to the alternating compressive and tensile strain, yielding close to ideal lattice matching to the substrate.

Electrical impedance response for physical simulations of composites with conductive fiber-bridged insulating cracks

Physical simulations of cement-based composites containing “fiber-bridged cracks” were prepared using insulating right cylindrical plastic discs at a fixed volume fraction and each disc was pierced through by different numbers and/or different lengths of perpendicular steel fibers. All the discs were z-axis aligned, with the axis of rotation of the disc parallel to the z-axis of the sample, and placed at random positions within the composites. The conductivity of the composites were measured by AC-impedance spectroscopy (AC-IS) to quantitatively separate the effects of the conductive fibers and insulating cracks on the composite conductivity when they coexist in a large number. Using a known orientation of both inclusions, in the form of “fiber-bridged cracks,” will serve to verify the accuracy and the applicability of using each of their separated effects to predict each of their orientations. Compared to the low-frequency resistance of the plain matrix, the insulating cracks increased both the low-frequency and high-frequency resistances of the composites. The effect of the bridging conductive fibers, on the other hand, could be observed only through the high-frequency resistance, that was lower than the low-frequency resistance of the composites. Based on the intrinsic conductivity approach, the important parameters of the cracks and the fibers that determine the magnitude of change in the resistance are their aspect ratios, volume fractions, and their orientations with respect to the direction of measurement. Good agreements for the separated effects of each inclusion were obtained from the comparison of the measured AC-IS responses and the theoretical calculations. The understanding of the resistance change in the fiber-bridging area helps enable the use of AC-IS as a health-monitoring technique during the early stage of mechanical damage of the composites. It should be noted that although cement paste is used as the matrix material in the present work, the AC-IS approach established is equally applicable to other composites with moderately conductive matrix and within the dilute regime to avoid the percolation behavior of the inclusions.

Size-Dependent Elasticity of Nanoporous Materials Predicted by Surface Energy Density-Based Theory

The size effect of nanoporous materials is generally believed to be caused by the large ratio of surface area to volume, so that it is also called surface effect. Based on a recently developed elastic theory, in which the surface effect of nanomaterials is characterized by the surface energy density, combined with two micromechanical models of composite materials, the surface effect of nanoporous materials is investigated. Closed-form solutions of both the effective bulk modulus and the effective shear one of nanoporous materials are achieved, which are related to the surface energy density of corresponding bulk materials and the surface relaxation parameter of nanomaterials, rather than the surface elastic constants in previous theories. An important finding is that the enhancement of mechanical properties of nanoporous materials mainly results from the compressive strain induced by nanovoid's surface relaxation. With a fixed volume fraction of nanovoids, the smaller the void size, the harder the nanoporous material will be. The results in this paper should give some insights for the design of nanodevices with advanced porous materials or structures.

Effects of silica aerogel particle sizes on the thermal–mechanical properties of silica aerogel – unsaturated polyester composites

ABSTRACTSilica aerogels with a surface area as high as 773 m2 g−1 and a density of 0.077 g cm−3 were produced from rice husk via sol–gel process and ambient pressure drying. A particulate composite material was prepared by adding silica aerogel particles of three different particle sizes (powder, granules and bead) to unsaturated polyester resin with a fixed volume fraction of 30%. Thermogravimetric and thermal conductivity studies revealed that silica aerogel composites were having higher thermal stability and thermal insulation than the neat resin. It was suggested that the preservation of aerogel pores from resin intrusion is important for better thermal properties. Larger silica aerogel particles have more porous area (unwetted region) which results in a lower degradation rate and lower thermal conductivity of the base polymer. However, the addition of silica aerogel into resin has reduced the tensile modulus of the polymer matrix where smaller particle size displayed higher toughness than those with bigger particle size.