Effect Of Particle Volume Fraction Research Articles

Molecular dynamics simulation study of self-diffusion for penetrable-sphere model fluids

Molecular dynamics simulations are carried out to investigate the diffusion behavior of penetrable-sphere model fluids characterized by a finite energy barrier ϵ. The self-diffusion coefficient is evaluated from the time-dependent velocity autocorrelation function and mean-square displacement. Detailed insights into the cluster formation for penetrable spheres are gained from the Enskog factor, the effective particle volume fraction, the mean free path, and the collision frequency for both the soft-type penetrable and the hard-type reflective collisions. The simulation data are compared to theoretical predictions from the Boltzmann kinetic equation and from a simple extension to finite ϵ of the Enskog prediction for impenetrable hard spheres (ϵ→∞). A reasonable agreement between theoretical and simulation results is found in the cases of ϵ∗ ≡ ϵ/k(B)T=0.2, 0.5, and 1.0. However, for dense systems (packing fraction ϕ>0.6) with a highly repulsive energy barrier (ϵ∗ = 3.0), a poorer agreement was observed due to metastable static effects of clustering formation and dynamic effects of correlated collision processes among these cluster-forming particles.

Effects of particle volume fraction on distortion of particle-arrayed structure during immobilization of colloidal crystals formed by poly(methyl methacrylate)-grafted silica in acetonitrile

Changes of particle array structure with particle volume fraction during immobilization of colloidal crystals, formed by poly(methyl methacrylate)-grafted silica in acetonitrile, were investigated. Immobilization of colloidal crystals formed in acetonitrile was carried out by two-step photo-radical copolymerization of methyl methacrylate and ethylene dimethacrylate to make organogel, followed by solidification after exchanging the solvent with methyl methacrylate. Crystallite size in colloidal crystals formed in acetonitrile was mostly unchanged with particle volume fraction in the range of 0.11–0.18, while the size and number of single crystals decreased during gelation. Disordering in particle array in immobilized colloidal crystals in gel and poly(methyl methacrylate) matrix was observed to decrease with increasing particle volume fraction less than 0.18 due to strong electrostatic repulsion between particles.

Thermal properties of particulate TIMs in squeeze flow

Direct numerical simulations based on a thermal Lattice–Boltzmann method are utilized to compute the effective thermal conductivity of particulate thermal interface materials (TIMs). By simulating the squeezing process, we obtain the particle distribution in a situation similar to application. Therefore, there is no need to calculate the average thermal characteristics from several pre-defined random distributions. The effects of particle volume fraction, particle size, and particle to matrix thermal conductivity ratio on the thermal performance are investigated. The results for the effective thermal conductivity are in agreement with the existing semi-analytical and experimental results reported in the literature.

Small-Amplitude Oscillatory Shear Magnetorheology of Inverse Ferrofluids

A comprehensive investigation is performed on highly monodisperse silica-based inverse ferrofluids under small-amplitude oscillatory shear in the presence of external magnetic fields up to 1 T. The effect of particle volume fraction and continuous medium Newtonian viscosity is thoroughly investigated. Experimental results for storage modulus are used to validate existing micromechanical magnetorheological models assuming different particle-level field-induced structures.

Effect of Temperature on High Shear-Induced Gelation of Charge-Stabilized Colloids without Adding Electrolytes

We demonstrated previously (Wu, H.; Zaccone, A.; Tsoutsoura, A.; Lattuada, M.; Morbidelli, M. Langmuir 2009, 25, 4715) that, for a colloid stabilized by charges from both polymer chain-end groups and adsorbed sulfonate surfactants, when the surfactant surface density reaches a certain critical value, the shear-induced gelation becomes unachievable at room temperature, even at an extremely large Peclet number, Pe = 4.6 x 10(4). This is due to the presence of the short-range, repulsive hydration force generated by the adsorbed surfactant. In this work, we investigate how such hydration force affects the shear-induced gelation at higher temperatures, in the range between 303 and 338 K. It is found that a colloidal system, which does not gel at room temperature in a microchannel at a fixed Pe = 3.7 x 10(4), does gel when temperature increases to a certain value. The critical initial particle volume fraction for the gelation to occur decreases as temperature increases. These results indicate that the effect of the hydration force on the gelation decreases as temperature increases. Moreover, we have observed that at the criticality only part of the primary particles is converted to the gel network and the effective particle volume fraction forming the gel network does not change significantly with temperature. The effective particle volume fraction is also independent of the surfactant surface coverage. Since the effective particle volume fraction corresponds to space filling requirement of a standing gel network, which is mainly related to the clusters structure, this result indicates that at a given shear rate the cluster structure does not change significantly with the surfactant surface coverage. On the other hand, since the cluster morphology is a strong function of the shear rate, we have observed that when the Peclet number is lowered from Pe = 3.7 x 10(4) to 1.7 x 10(4), the effective particle volume fraction reduces from 0.19 to 0.12 at 313 K.

The effect of viscous dissipation on laminar nanofluid flow in a microchannel heat sink

The present article focuses on analytical and numerical study on the effect of viscous dissipation when nanofluid is used as the coolant in a microchannel heat sink (MCHS). The nanofluid is made from CuO nanoparticles and water. To analyse the MCHS, a modified Darcy equation for the fluid and two-equation model for heat transfer between fluid and solid sections are employed in porous media approach. In addition, to deal with nanofluid heat transfer, a model based on the Brownian motion of nanoparticles is used. The model evaluates the thermal conductivity of nanofluid considering the thermal boundary resistance, nanoparticle diameter, volume fraction, and the fluid temperature. At first, the effects of particle volume fraction on temperature distribution and overall heat transfer coefficient are investigated with and without considering viscous dissipation. After that, the influence of different channel aspect ratios and porosities is studied. The results show that for nanofluid flow in microchannels, the viscous dissipation can be neglected for low volume fractions and aspect ratios only. Finally, the effect of porosity and Brinkman number on the overall Nusselt number is studied, where asymptotic behaviour of the Nusselt number is observed and discussed from the energy balance point of view.

Investigation of a nanofluid-cooled microchannel heat sink using Fin and porous media approaches

The present paper focuses on analytical and numerical study on using nanofluids as coolant of a microchannel heat sink. The nanofluid studied in this paper is made from copper oxide (CuO) and water. Two common analytical approaches are used: the Fin model and the porous media approach. The Fin model is based on the assumption of uniform fluid temperature in the direction normal to the fluid flow which causes inaccuracy in the predictions of this approach. On the other hand, modified Darcy equation for the fluid and two-equation model for heat transfer between fluid and solid sections are employed in porous media approach. In addition, to deal with nanofluid heat transfer, a model based on Brownian-motion of nanoparticles is used. The model evaluates the thermal conductivity of nanofluid considering the thermal boundary resistance, nanoparticle diameter, volume fraction and the fluid temperature. This model is included in heat transfer equations of the two approaches, and the velocity profile is obtained analytically considering the transport properties of nanofluids. Firstly, the effects of particle volume fraction and Brownian–Reynolds number on temperature distribution and overall heat transfer coefficient are investigated. After that, the influence of different channel aspect ratios and porosities are studied. Both approaches are considered and compared in details. Furthermore, an optimum aspect ratio is found to minimize the friction factor in different Reynolds numbers.

New temperature dependent thermal conductivity data for water-based nanofluids

This paper presents effective thermal conductivity measurements of alumina/water and copper oxide/water nanofluids. The effects of particle volume fraction, temperature and particle size were investigated. Readings at ambient temperature as well as over a relatively large temperature range were made for various particle volume fractions up to 9%. Results clearly show the predicted overall effect of an increase in the effective thermal conductivity with an increase in particle volume fraction and with a decrease in particle size. Furthermore, the relative increase in thermal conductivity was found to be more important at higher temperatures. Obtained results compare favorably with certain data sets and theoretical models found in current literature.

Effects of particle volume fraction and ambient turbulence on drag force on a sphere

Effects of particle volume fraction and ambient turbulence on drag force on a sphere

Estimation of size distribution in concentrated particle systems from light scattering measurements

We consider the estimation of the particle size distribution (PSD) of concentrated particle systems with low contrast between particles and solvent from light scattering (LS) measurements. We follow Pedersen's monodisperse approximation, in which an effective particle volume fraction is used to take into account interference between particles. This parameter must be determined simultaneously with the PSD to characterize the system. To solve the inverse problem, we first assumed that the PSD is described by a log-normal function. We applied the Levenberg–Marquardt algorithm for the non-linear parameter estimation. We also approached the free-form case following a scheme by which Tikhonov's regularization method is used to estimate the PSD, simultaneously with a non-linear least-squares optimization used to estimate the effective parameter. The solutions obtained from the analysis of simulated measurements showed that a priori knowledge of the distribution form improves the quality of the estimation, but the problem can be solved properly when actual PSD form is not known.

Floating of SiC particles in a Zn–Al filler metal

Floating of SiC particles in a Zn–Al filler metal was investigated as functions of isothermal holding time, holding temperature, particle volume fraction and size. The floating distance increased with isothermal holding time until a stabilized condition was reached with all particles floating on the top of the melt. The floating distance increased significantly with increasing holding temperature and particle size and decreased with increasing particle volume fraction. The measured floating rates deviated significantly from those predicted by the modified Stokes’ law at relatively low holding temperatures, but the correspondence turned better with increasing holding temperature. Although the effect of particle volume fraction had been taken into account in the modified Stokes’ law, the correspondence was not good between the measured and the calculated values at various particle volume fractions. The measured floating rates which increased with particle size were much lower than the calculated values, particularly for large particle sizes.

Effect of Al<SUB>2</SUB>O<SUB>3</SUB> Particles on the Microstructure and Sliding Wear of 7075 Al Alloy Manufactured by Squeeze Casting Method

Aluminum (Al) alloy 7075 reinforced with Al2O3 particles was prepared using the stir casting method. The microstructure of the cast composites showed some degree of porosity and sites of Al2O3 particle clustering, especially at high-volume fractions of Al2O3 particles. Different squeeze pressures (25 and 50 MPa) were applied to the cast composite during solidification to reduce porosity and particle clusters. Microstructure examinations of the squeeze cast composites showed remarkable grain refining compared with that of the matrix alloy. As the volume fraction of particles and applied squeeze pressure increased, the hardness linearly increased. This increase was related to the modified structure and the decrease in the porosity. The effect of particle volume fraction and squeeze pressure on the dry-sliding wear of the composites was studied. Experiments were performed at 10, 30, and 50 N with a sliding speed of 1 m/s using a pin-on-ring apparatus. Increasing the particle volume fraction and squeeze pressure improved the wear resistance of the composite compared with that of the monolithic alloy, because the Al2O3 particles acted as load-bearing constituents. Also, these results can be attributed to the fact that the application of squeeze pressure during solidification led to a reduction in the porosity, and an increase in the solidification rate, leading to a finer structure. Moreover, the application of squeeze pressure improved the interface strength between the matrix and Al2O3 particles by elimination of the porosity at the interface, thereby providing better mechanical locking.

Modelling of binder removal from a (fibre+powder) composite pre-form

A diffusion-controlled ‘shrinking core’ model was established to simulate the removal process of organic binder in a fibre+powder composite pre-form of plate geometry with binder removal essentially in one direction. The influences of compact shrinkage and fibre orientation are manifested via the effective particle volume fraction, the monomer diffusion coefficient, and hence polymer decomposition rate. Two shrinkage modes and three types of fibre orientation were considered. Simulation results were in agreement with experimental ones. It is shown that compact shrinkage leads to slower binder removal. The effect of fibres depends on their aligned orientation to the removal direction and a higher fibre volume fraction has greater effect on the burnout kinetics.

Damage Formulation of Random Particulate Composites by Combined Micromechanics and Damage Mechanics

Damage evolution of random particulate composites is formulated by the combination of two distinct theories, micromechanics and damage mechanics. Damage mechanics is applied to filler particles to describe interfacial debonding, which in turn incorporated into Mori-Tanaka's mean field theory for stiffness prediction of damaged composites. The model uses an energy balance concept to account for nonlinear behavior due to particle debonding. With this model, the effects of particle volume fraction, size distribution and interfacial strength on damage development of composites are analytically investigated. Emphasis is also placed on the effect of partial debonding which plays a dominant role for damage induced anisotropy. It is found that the model can reproduce not only stress-strain relationship but also variation in Poisson's ratio observed in damage process.

Toughening of a brittle thermosetting polymer: Effects of reinforcement particle size and volume fraction

Micron- and nanometer-sized aluminum particles were used as reinforcements to enhance the fracture toughness of a highly-crosslinked, nominally brittle, thermosetting unsaturated polyester resin. Both particle size and particle volume fraction were systematically varied to investigate their effects on the fracture behavior and the fracture toughness. It was observed that, in general, the overall fracture toughness increased monotonically with the volume fraction of aluminum particles, for a given particle size, provided particle dispersion and deagglomeration was maintained. The fracture toughness of the composite was also strongly influenced by the size of the reinforcement particles. Smaller particles led to a greater increase in fracture toughness for a given particle volume fraction. Scanning electron microscopy of the fracture surfaces was employed to establish crack front trapping as the primary extrinsic toughening mechanism. Finally, the effects of particle volume fraction and size on the tensile properties of the polyester-aluminum composite were also investigated. The measured elastic modulus was in accordance with the rule-of-mixtures. Meanwhile, the tensile strength was slightly reduced upon the inclusion of aluminum particles in the polyester matrix.

Effect of Particle Volume Fraction on the Gelation Behavior of the Temperature Induced Forming (TIF) Aqueous Alumina Suspensions

Stable aqueous Al2O3 suspensions that can be flocculated by increasing temperature were prepared using tri‐ammonium citrate and poly(acrylic acid) (PAA). Effects of particle volume fraction on the rheological properties of Al2O3 suspensions were investigated. The modified Dougherty–Krieger equations were used to describe the volume fraction dependence of relative viscosity for both the PAA‐addition and PAA‐free suspensions. An equation using percolation model was proposed to analyze the temperature and volume fraction dependence of the relative viscosity of the suspensions. The calculated results were in good agreement with the experimental data.

Damage to rigid-particle-filled polymer under cyclic tension

The effect of particle volume fraction, strain rate and interfacial bonding strength on the damage to a glass bead-filled high density polyethylene composite was studied experimentally by means of cyclic tension tests. Although the volume fraction of the glass beads varies from 5% to 25%, the materials studied are always sensitive to the strain rate, reflected by an increase of the modulus with strain rates changing from 10−3 s−1 to 10−5 s−1. The damage evolution is a function of the applied far-field strain, but is also strongly influenced by the particle volume fraction, strain rate and interfacial bonding strength. Strong interfacial adhesion can postpone the initiation of damage and cause a lower level of damage than that occuring with weak adhesion under a given strain. For the studied materials with interfacial debonding taking place, the higher the volume fraction of the glass beads, the greater the number of microcracks formed at the interface. The effect of strain rate on the damage may be related to the effect of loading time, ie low strain rate favours damage development. The residual strain is a function of far-field strain and also depends on the strain rate and residual deformation of microvoids formed at two poles of the particle. © 2001 Society of Chemical Industry

Rheological Modeling of Microgel Suspensions Involving Solid−Liquid Transition

The rheological behavior of suspensions containing cross-linked polymer particles (microgels) was studied. Under flow, these particles behave as soft spheres due to the shear-induced deformability of their external polymeric layer and the suspensions are strongly shear-thinning. At concentrations where particles are densely packed, the suspensions present a solidlike behavior involving elasticity and yield stress. The rheological modeling of such suspensions was based on the concept of the effective volume fraction of particles, which depends on hydrodynamic forces. In this sense, microgel particles were assumed to have a simple core−shell configuration and to interact through a repulsive polymer−polymer potential. The viscosity equation resulting from the modeling allows: (a) appropriate correlation of the experimental data shear stress versus shear rate in steady shear flow and (b) accounting for the softness of particles, which changes with concentration and shear stress. Also the predictions of the m...

Model filled rubber. II. Particle composition dependence of suspension rheology

Monodisperse size colloidal particles varying in chemical composition were synthesized by emulsifier-free emulsion polymerization. Using a stress-controlled rheometer, the rheological behavior of colloidal suspensions in a low molecular weight liquid polysulfide was investigated. All suspensions exhibited shear thinning behavior. The shear viscosity, dynamic moduli, and yield stress increased as interactions between particles and matrix increased. The rheological properties associated with network buildup in the suspensions were sensitively monitored by a kinetic recovery experiment. We propose that interfacial interactions by polar and hydrogen bonding between particles and matrix strongly promote affinity of matrix polymer to the filler particles, resulting in adsorption or entanglement of polymer chains on the filler surface. A network structure was formed consisting of particles with an immobilized polymer layer on the particle surface with each particle floc acting as a temporary physical crosslinking site. As the interfacial interaction increases, the adsorbed layer thickness on the filler particles, hence, the effective particle volume fraction, increases. As a result, the rheological properties were enhanced in the order PS < PMMA < PSVP. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 815–824, 1999

Model filled rubber. II. Particle composition dependence of suspension rheology

Monodisperse size colloidal particles varying in chemical composition were synthesized by emulsifier-free emulsion polymerization. Using a stress-controlled rheometer, the rheological behavior of colloidal suspensions in a low molecular weight liquid polysulfide was investigated. All suspensions exhibited shear thinning behavior. The shear viscosity, dynamic moduli, and yield stress increased as interactions between particles and matrix increased. The rheological properties associated with network buildup in the suspensions were sensitively monitored by a kinetic recovery experiment. We propose that interfacial interactions by polar and hydrogen bonding between particles and matrix strongly promote affinity of matrix polymer to the filler particles, resulting in adsorption or entanglement of polymer chains on the filler surface. A network structure was formed consisting of particles with an immobilized polymer layer on the particle surface with each particle floc acting as a temporary physical crosslinking site. As the interfacial interaction increases, the adsorbed layer thickness on the filler particles, hence, the effective particle volume fraction, increases. As a result, the rheological properties were enhanced in the order PS < PMMA < PSVP. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 815–824, 1999