Further Increase In Shear Rate Research Articles

Effect of particle concentration and temperature on the thermo-physical properties of ethylene glycol–water mixture based boron nitride nanofluids

Present article focusses on the thermal and rheological characteristics of ethylene glycol–water mixture (volume = 60/40) based boron nitride (h-BN) nanofluids measured at different volume concentrations (0.5–2 vol% h-BN) between temperatures 30–60 °C. X-ray diffraction and TEM analysis have confirmed the hexagonal structure of h-BN nanoparticles and the size range of the nanoparticles is within 90–170 nm. To optimize the ultrasonication time, the thermal conductivity of h-BN nanofluids has been monitored after each 30 min of sonication until a maximum thermal conductivity increase is achieved. The thermal conductivity of h-BN nanofluids shows an increasing trend with respect to particle concentration. Also, the thermal conductivity enhancement exhibits a temperature independent nature. The viscosity studies carried out over a shear rate of 0.612–122 s−1 revealed an increasing trend with the increasing concentration of h-BN loading. For all the volume concentrations, at lower shear rates, the viscosity initially decreased, displaying a non-Newtonian nature, and with a further increase in shear rate, the viscosity stays constant exhibiting a Newtonian nature. Based on the experimental outcomes, a correlation is introduced. The correlation showed a strong agreement with the current results, with an R2 value of 0.99.

The Chain Distribution Tensor: Linking Nonlinear Rheology and Chain Anisotropy in Transient Polymers.

Transient polymer networks are ubiquitous in natural and engineered materials and contain cross-links that can reversibly break and re-form. The dynamic nature of these bonds allows for interesting mechanical behavior, some of which include nonlinear rheological phenomena such as shear thickening and shear thinning. Specifically, physically cross-linked networks with reversible bonds are typically observed to have viscosities that depend nonlinearly on shear rate and can be characterized by three flow regimes. In slow shear, they behave like Newtonian fluids with a constant viscosity. With further increase in shear rate, the viscosity increases nonlinearly to subsequently reach a maximum value at the critical shear rate. At this point, network fracture occurs followed by a reduction in viscosity (shear-thinning) with a further increase in shear rate. The underlying mechanism of shear thickening in this process is still unclear with debates between a conversion of intra-chain to inter-chain cross-linking and nonlinear chain stretch under high tension. In this paper, we provide a new framework to describe the nonlinear rheology of transient polymer networks with the so-called chain distribution tensor using recent advances from the transient network theory. This tensor contains quantitatively and statistical information of the chain alignment and possible anisotropy that affect network behavior and mechanics. We investigate shear thickening as a primary result of non-Gaussian chain behavior and derive a relationship for the nonlinear viscosity in terms of the non-dimensional Weissenberg number. We further address the criterion for network fracture at the critical shear rate by introducing a critical chain force when bond dissociation is suddenly accelerated. Finally, we discuss the role of cross-linker density on viscosity using a “sticky” reptation mechanism in the context of previous studies on metallo-supramolecular networks with reversible cross-linkers.

Effect of the micro-flocculation stage on the flocculation/sedimentation process: The role of shear rate

Dynamic analysis on the variation of particle size distribution (PSD) and the fractal characteristics of PSD (Df) were investigated to better understand the continuous procedure of the floc growth and optimize the control of flocculation process. It was found that the flocculation process could be divided into three stages, i.e., the micro-flocculation stage, the growth stage and the steady (or breakage) stage. As the stage which is crucial to the morphology of micro-flocs (the building blocks of large flocs), the micro-flocculation stage plays an important role on flocculation/sedimentation process. The results showed that an increase in shear rate (11s−1<G<30s−1) during the micro-flocculation stage contributed to micro-flocs with larger size and more compact structure. As shear rate further increased (30s−1<G<55s−1), the micro-floc average size gently decreased from 13.61μm to 10.91μm, whereas two-dimension fractal dimension of micro-flocs gradually increased from 1.85 to 1.89. This indicated that further increase of shear rate during the micro-flocculation was incline to the formation of smaller micro-flocs with more compact structure. According to the results of final floc properties, the moderate shear rate (G=30s−1) benefited to the micro-floc formation to form final flocs with desired properties, further improved the treatment efficiency in the whole process. Based on the kinetics in the micro-flocculation stage, a conceptual model was proposed to describe the micro-floc growth under different shear rates, further revealed the reason for the different properties of final flocs under various shear rate during the micro-flocculation stage. Combining the results with model, it was concluded that shear rate during the micro-flocculation stage mainly affected final flocs by the domination of micro-floc structure. This research gives indications both for theoretical and actual works to improve the efficiency in the solid/liquid process.

Morphological and rheological responses to the transient and steady shear flow for a phase-separated polybutadiene/polyisoprene blend

The morphological and rheological responses to the transient and steady shear flow for a phase-separated polybutadiene (PB)/low vinyl content polyisoprene (LPI) blend have been investigated. Under steady shear flow where the applied shear rate is not too large, the steady sheared structures become increasingly anisotropic and interconnected with an “en route” to the formation of string phases as shear rate increases. After that, the further increase of shear rate leads to a blurred domain interface. These shear-induced complex structures in turn affect the rheological response greatly and both the shear thinning and shear thickening were observed in the steady shear behavior of the phase-separated PB/LPI blend. Under transient shear flow, the time (or strain) dependence of viscosity and morphology after a shear rate jump were extensively studied in order to obtain the insight into the steady state formation and found to be mainly determined by the final shear rates. Depending on whether the transient string phases which were formed by the transient shear flow can be stabilized and with clear domain interface, three kinds of transient shear viscosity changes have been observed. Some of the observations are quite different from the model immiscible blend and believed to be closely related to the significant shear-induced mixing effect happened in the PB/LPI blend.

Erythrocyte Concentration Distribution in Sheathed Microfluidic Flows

The combined effect of dissociative and associative forces on erythrocytes flowing in direct, layered contact with cell-free "sheath" flows in microfluidic systems was studied to aid in the design of blood purification devices. A blood stream flowed in direct contact between two fluids that were cell free. The layered flow created a sharp transverse concentration gradient of erythrocytes that promoted lateral movement of cells. This movement was perceived as both dispersive (arising from particle collisions or shear-enhanced movement) and associative (arising from rouleau formation and shear-induced migration, which limited the extent of movement and maintained a higher hematocrit in the central layer). The concentration changes of erythrocytes in the blood and sheath streams were measured to determine the flux of cells between streams. At flows with low wall shear rates, the flux of erythrocytes into the sheath streams was small. When shear was increased, cell flux away from the central layer increased. There was an intermediate shear rate that maximized cellular flux away from the center stream, and a further increase in shear rate showed no change in cell flux. Changing the transverse position of erythrocytes entering the system strongly affects their distribution in the exiting stream even for long fluid residence times. The results clarify microfluidic device design and elucidate how shear and hematocrit affect erythrocyte movement in blood.

Viscoelasticity and wearability of hyaluronate solutions

This work systematically studied the viscoelastic properties of hyaluronic acid (HA) solution, a major component of synovial fluid, under various testing conditions. The optimum relaxation time of HA solution is around 4.5 s at pH 6.8, in the absence of salt at room temperature, indicating that synovial fluid is viscous when the shear rate is less than 1/4.5 s −1 but elastic when the shear rate exceeds this critical value. HA viscosity declines markedly as the following factors are increased in their order, salt concentration > pH level > temperature, demonstrating that these factors weaken the intermolecular attraction among HA molecules. The non-thixotropic behavior of the HA solution suggests that the breakdown and recovery of the HA structure proceed through the same intermediate states, reconfirming the strong performance of HA as a main component of synovial fluid. The wear results reveal that when the shear rate exceeds a critical value of around 20 s −1, the drop in viscosity leveled out independent of any further increase in shear rate. However, in a broad range of wear rates (20–300 s −1), the HA viscosity sustains for wearing time less than 20 min but declines without leveling off as the wear duration increases thereafter. Finally, experimental results verify that bovine albumin (BA), in HA solution, acts as both hydrodynamic and boundary lubricant, substantially improving the wearability of HA.

Non-linear Viscoelastic Behavior of PVC Plastisol

Abstract Rheology of PVC plastisol is usually conducted with steady state flow. It is inadequate, because plastisol is viscoelastic, so that deformational experiment is also required. In the viscoelastic measurements, when either frequency or amplitude is increased, dynamic viscosity decreases first (pseudo-plastic), reaching a minimum and then, increases (dilatant). It is because the shear rate during the oscillation is a product of frequency and amplitude. The pseudo-plastic behavior is explained with stress-induced phase separation into an immobilized layer of packed particles and mobile phase of reduced particle concentration. The viscosity minimum corresponds to the yield point of the immobilized layer. The yielding obeys Coulomb's criterion. After yielding the immobilized layer dilates with the further increase of shear rate. Two different modes of fracture are observed; one without yielding occurs at rather low shear stress, slightly higher than the cohesive strength of the immobilized layer. After yielding and dilation, the fracture occurs when shear stress becomes higher than normal stress. Before yielding the deformation of the immobilized layer is very small and accompanied with small fraction of particles moving over the neighboring particles. After yielding, deformation involves entire particles of the immobilized layer.

Structural change of polydomain in the liquid crystalline polymers by weak shear flow

Steady-state shear stress (τ12) and first normal stress difference (N1) of liquid crystalline polymers at low shear rates were examined by using a mesoscopic constitutive equation set including the idea of initial domain size. For the applicability to the weak shear flow at low shear rates, a Hinch-Leal closure approximation was adopted in the calculation of the constitutive equation set. The steady-state rheological behaviors predicted by adopting the Hinch-Leal approximation were compared with those by the Doi simple decoupling approximation. It could be predicted from the plot of N1 versus τ12 that smaller domains distributed isotropically at a quiescent state might maintain the isotropic domain distribution even at the imposition of moderate shear rate, and then could be changed to the ordered (or partially elongated) domain phase by a further increase of shear rate. Such change of the polydomain structure with the increase in shear rate could be proved more precisely by the transient rheological behaviors of N1 and τ12 after the start-up of shear flow.

Self-diffusion in equilibrium and sheared liquid mixtures by molecular dynamics

The mass and shear rate, γ, dependence of the self-diffusion tensor, D, of isotopically substituted Lennard-Jones (LJ) and Weeks-Chandler-Anderson liquids are determined here, for the first time, by non-equilibrium molecular dynamics using the Gaussian thermostatted SLLOD algorithm. The mass and composition dependence of the self-diffusion coefficients of solute and solvent are characterized as a function of mass ratio of the two species and composition at zero shear rate. In a LJ mixture containing 10% of impurity atoms, each of the three diagonal elements D αα of the self-diffusion tensor D, increases initially with shear rate, reaches a maximum (which depends on the mass of the solute particle, m A), and then decreases with a further increase in shear rate. D(m A, γ) can be represented well by a polynomial, up to second order in the reduced shear rate γ*. The individual D αα diverge as the shear rate increases, with diffusion along the flow direction always being the largest. To help interpret the tren...

A Consideration on the Behavior of Capillary Flow Fracture on the Basis of Distribution and Vibration of Pressure in Reservoir of Polymer Melt

The objective of the present study is to confirm the hypothesis presented by one of the authors that the initiating mechanism of melt flow fracture is affected by the behaviors of distribution and vibration of pressure in shear flow field of high density polyethylene melts. It was recognized that the pressure at reservoir wall and in the reservoir, which was measured in rectangular direction with the flow direction, varied with the positions of measurement points i.e., the pressure distribution curves were similar to the profile of shear rate distribution curves under normal flow region or low shear rate region. The mean pressure in a pressure distribution curve increases with the increase of shear rate. The pressure vibrations occurred at particular measurement points, which were presented in the previous paper as fracture region at the critical condition of normal flow to spiralling flow under moderate shear rate. At these positions of measurement points the vibration phenomena generally initiated near the capillary inlet under low shear rate, and moved to the reservoir wall from the reservoir axis with the increase of shear rate and ultimately, disappeared with further increase of shear rate. The larger melt index of high density polyethylene, the larger pressure vibration becomes. Both flow rate excess induced by the melt flow fracture and the effects of L/D of capillaries upon flow rate excess can be explained by the hypothesis utilizing the particular fracture surface in shear flow field.

Application of the Hydrodynamic-Structural Theory of Non-Newtonian Flow to Suspensions which Exhibit Moderate Shear Thickening with Particular Reference to “Dilatant” Vinyl Plastisols

Abstract A general equation, derived in previous work, fits flow data for moderately shear thickening dispersions of poly (vinyl chloride) particles in an oil. The theory on which the equation is based predicts that the number of structural links in a disperse system can increase with increasing shear rate without shear thickening, provided n∞/n0&amp;lt;2, where n∞ and n0 are the number of links per cubic centimeter at infinite shear rate and zero shear rate, respectively. This could account for the unexplained occurrence of “volume dilatancy” without “rheological dilatancy” previously reported in the literature. When n∞/n0&amp;lt;2, viscosity increases with increasing shear rate to a maximum value and then the viscosity decreases with further increase in shear rate. This effect is explained as a competition between increase in the number of links with increasing shear rate and decrease in link lifetime with increasing shear rate. Aging of vinyl plastisols is examined and found to involve changes in both hydrodynamic and structural viscosity which can be explained by absorption of oil into the porous poly (vinyl chloride) particles.

Application of the hydrodynamic-structural theory of non-Newtonian flow to suspensions which exhibit moderate shear thickening with particular reference to “dilatant” vinyl plastisols

A general equation, derived in previous work, fits flow data for moderately shear thickening dispersions of polyvinyl chloride particles in an oil. The theory on which the equation is based predicts that the number of structural links in a disperse system can increase with increasing shear rate without shear thickening, provided n ∞/ n 0 < 2, where n ∞ and n 0 are the number of links per cubic centimeter at infinite shear rate and zero shear rate, respectively. This could account for the unexplained occurrence of “volume dilatancy” without “rheological dilatancy” previously reported in the literature. When n ∞/ n 0 > 2 the viscosity increases with increasing shear rate to a maximum value and then the viscosity decreases with further increase in shear rate. This effect is explained as a competition between the increase in the number of links with increasing shear rate and the decrease in link lifetime with increasing shear rate. The aging of vinyl plastisols is examined and found to involve changes in both the hydrodynamic and structural viscosity which can be explained by absorption of oil into the porous polyvinyl chloride particles.