Large Amplitude Oscillatory Flow Research Articles

Magnetorheological Payne effect in bidisperse MR fluids containing Fe nanorods and Fe3O4 nanospheres: A dynamic rheological study

The spherical Fe3O4 with 300 nm in diameter was synthesized by typical thermal decomposition of Fe (III) organometallic precursor in polyol and polyacrylic acid. Fe-nanorods were prepared by reducing Fe (III) nitrate in presence of polyol-hydrazine-CTAB. Morphology and magnetic characterization of the nanoparticles were performed by FESEM, XRD and VSM studies. We performed detailed non-linear magnetorheological properties of three MR fluids (10 vol-%) containing isotropic Fe3O4 and anisotropic Fe-nanorods under both small and large amplitude oscillatory flow. The MR samples demonstrated strong magnetorheological Payne effect i.e. rapid stress relaxation under increasing deformation and uniform magnetic field beyond linear viscoelastic region (LVR), which was not studied in detail before in case of bidisperse MR fluids. We have also shown that stress softening was more pronounced for MR fluids with higher anisotropic contents, in contrast to isotropic MR fluid. The onset strains for LVR to non-linear region transition for anisotropic fluids were much lower than that of isotropic spherical nanoparticle containing fluid. The stronger MR response for nanorod-containing MR fluids can be explained in terms of enhanced field-induced structuration.

Dynamics of red blood cells in oscillating shear flow

We present a three-dimensional computational study of fully deformable red blood cells of the biconcave resting shape subject to sinusoidally oscillating shear flow. A comprehensive analysis of the cell dynamics and deformation response is considered over a wide range of flow frequency, shear rate amplitude and viscosity ratio. We observe that the cell exhibits either a periodic motion or a chaotic motion. In the periodic motion, the cell reverses its orientation either by passing through the flow direction (horizontal axis) or by passing through the flow gradient (vertical axis). The chaotic dynamics is characterized by a non-periodic sequence of horizontal and vertical reversals. The study provides the first conclusive evidence of the chaotic dynamics of fully deformable cells in oscillating flow using a deterministic numerical model without the introduction of any stochastic noise. In certain regimes of the periodic motion, the initial conditions are completely forgotten and the cells become entrained in the same sequence of horizontal reversals. We show that chaos is only possible in certain frequency bands when the cell membrane can rotate by a certain amount, allowing the cells to swing near the maximum shear rate. As such, the bifurcation between the horizontal and vertical attractors in phase space always occurs via a swinging inflection. While the reversal sequence evolves in an unpredictable way in the chaotic regime, we find a novel result that there exists a critical inclination angle at the instant of flow reversal which determines whether a vertical or horizontal reversal takes place, and is independent of the flow frequency. The chaotic dynamics, however, occurs at a viscosity ratio less than the physiological values. We further show that the cell shape in oscillatory shear at large amplitude exhibits a remarkable departure from the biconcave shape, and that the deformation is significantly greater than that in steady shear flow. A large compression of the cells occurs during the reversals which leads to over/undershoots in the deformation parameter. We show that due to the large deformation experienced by the cells, the regions of chaos in parameter space diminish and eventually disappear at high shear rate, in contradiction to the prediction of reduced-order models. While the findings bolster support for reduced-order models at low shear rate, they also underscore the important role that the cell deformation plays in large-amplitude oscillatory flows.

Large amplitude oscillatory microrheology

We study the motion of a colloidal particle as it is driven by an oscillating external force of arbitrary amplitude and frequency through a colloidal dispersion. Large amplitude oscillatory flows (LAOFs) are examined predominantly from a phenomenological perspective in which experimental measurements inform constitutive models. Here, we investigate a LAOF from a microstructural perspective by connecting motion of the probe particle to the material response while making no assumptions a priori about how stress relaxes in the material. The suspension exerts nonconservative, hydrodynamic forces on the probe, while distortions in the particle configuration exert conservative forces: Brownian and interparticle forces, for example. The relative importance of each of these contributions to particle motion evolves with the degree of displacement from equilibrium. When the force on the probe is weak, the linear microviscoelasticity of the suspension is probed [see, e.g., Khair and Brady, J. Rheol. 49, 1449–1481 (2005)]. When oscillation rate is slow, the steady microrheology is probed [see, e.g., Squires and Brady, Phys. Fluids 17, 073101 (2005); Khair and Brady, J. Fluid Mech. 557, 73–117 (2006)]. This article develops a micromechanical model that recovers these limiting cases and then uses the same model to reveal the microrheology of colloidal dispersions deformed by a probe driven with arbitrary force amplitude and frequency. A chief result of this work is the discovery of a regime in which the resistance to motion of the probe particle is on average weaker than the resistance the probe experiences when deformed by high frequency oscillation. This hypoviscous effect arises when the reciprocating motion of the probe particle opens a channel free of other particles which is thus less resistive to probe motion. This effect is most apparent under the conditions of strong forces, rapid oscillation, and large extent of deformation.

Characterization of yield stress and slip behaviour of skin/hair care gels using steady flow and LAOS measurements and their correlation with sensorial attributes

Gels made with three different polymers widely used as rheology modifiers in cosmetic formulations (cross-linked poly(acrylic acid), cross-linked poly(maleic acid-alt-methyl vinyl ether) copolymer and cross-linked poly(acrylic acid-co-vinyl pyrrolidone) copolymer) were characterized by rheological and sensory evaluation methods to determine the relationship between sensorial perception and corresponding rheological parameters. Both conventional rheological characterization methods and a more recent method, Fourier Transform Rheology with Large Amplitude Oscillatory Flow data (LAOS), were utilized to characterize the material with and without wall slip. Sensorial analyses were implemented in vivo to evaluate the perceived ease of initial and rub-out spreadability, cushion, pick-up and slipperiness attributes of the gels. Results were statistically analysed by both variance (ANOVA) and principle component analysis (PCA). Sensorial panel testing characteristics discriminated the three materials, and PCA analyses revealed that sensory attributes could be well predicted by rheological methods. Rheological experiments, without wall slip, revealed that gel strength in the linear viscoelastic region (LVR) and yield stress of these materials are similar, but exhibit significantly different wall slip and thixotropy behaviour in the low shear rate region under wall slip conditions. Above the critical shear rate, which corresponds to the yield stress, all tested materials did not slip and behaved as conventional, shear thinning polymeric fluids. In particular, the rheological parameters and sensorial perception of the 1% cross-linked vinyl pyrrolidone/acrylic acid copolymer were significantly affected by wall slip and/or thixotropy-related shear banding phenomena.

S0501-2-4 マイクロチャネル内の渦を伴う縮小流におけるDNA高分子の伸張変形(複雑流体の流動現象(2))

Flow behaviors of polymer solution in oscillatory flow have been investigated with a micro abrupt contraction channel. Fluorescently-stained DNA polymer solutions have been utilized to observe the flow patterns with fluorescent microscope. In conversing flow periods in the micro oscillatory flow, corner vortexes were generated and grew to a maximum size, but no separation or no vortex was observed in diverging flow period. In the cases of large amplitude oscillatory flow, the vortexes were generated in the first half accelerating flow from flow reversal and grew even in the later half decelerating flow. When the amplitude was small, the vortexes were generated in the latter decelerating flow. It is considered that these results meant vortex generation was affected not only instantaneous condition but also flow history. The maximum size of the vortexes was comparable to the amplitude of the oscillatory flow.

Evaluation of a transient network model for telechelic associative polymers

A number of models are available to describe the rheological behaviour of associative polymers. Model evaluation has most often been limited to the linear properties and to a qualitative prediction of the characteristic shear thickening at intermediate shear rates. Several other nonlinear features of associative behaviour have, however, been identified. These include failure of the Cox–Merz analogy and an unusual response in nonlinear stress relaxation, large amplitude oscillatory flow and in superposition flows. Here, experimental results for such experiments are compared with the predictions of a transient network model, i.e. the Vaccaro–Marrucci model [J. Non-Newtonian Fluid Mech. 92 (2000) 261], which has only a single adjustable parameter to describe nonlinear behaviour. It is shown that the various nonlinearities are well predicted qualitatively. The only deviation is a substantial overprediction of the critical strain or strain rate for the onset of these nonlinearities. The effect of changing the value of the model parameters and of relaxing the model assumptions on the predictions is investigated. The results suggest that the presence of a small number of highly stretched chains could be responsible for the observed deviation from the experimental data.

Large amplitude oscillatory shear and Fourier-transform rheology for a high-density polyethylene: Experiments and numerical simulation

Rheological characterization of a high-density polyethylene is performed by means of measurements of the storage and loss moduli with a new viscometric device. Shear viscosity is deduced from both the Cox–Merz rule and capillary measurements. Subsequently, oscillatory experiments are performed in the nonlinear regime, with deformation amplitudes as high as 10. A multimode Giesekus model is selected to describe the rheological behavior of the melt, the parameters of which are successively identified on the basis of the linear properties and the shear viscosity. This model is subsequently used for the numerical simulation of large amplitude oscillatory flows. Fourier-transform rheology is applied to both experimental measurements and simulation results. A comparison is made on the basis of the value of the harmonics, the maximum amplitude of the stress signals, and the phase shift occurring between the applied deformation and the stress output signal. Good quantitative agreement is found between experiments and calculations, demonstrating the relevance of the present approach.

Structure and rheology during shear-induced crystallization of a latex suspension.

Microstructure and rheology of a concentrated sterically stabilized colloidal suspension undergoing flow-induced ordering was studied by combined small-angle x-ray scattering and rheometry. This system is known to form bundlelike structures at high stress values in continuous shear flow. Under large amplitude oscillatory flow, hexagonal close-packed crystalline domains are formed within 1 sec of the inception of shear. In the intermediate range of frequencies and amplitudes, a nearly perfect hexagonally close-packed layer structure was observed after the cessation of flow. Lower frequencies or stress amplitudes resulted in polycrystals and, on the other hand, high frequencies or stress amplitudes led to partial melting of the layered structure. During the oscillatory flow, the intensity of the Bragg peaks showed pronounced oscillations.

Shear thickening in steady and superposition flows effect of particle interaction forces

The phenomenon of shear thickening has been studied in sterically stabilized colloidal suspensions. Temperature has been used to vary the solvent quality of the suspending medium for the grafted polymer in the stabilizer layer. A decrease in temperature causes the polymer coat to shrink. This results in a lower viscosity in the shear thinning region but also in a lower critical shear rate and shear stress at the onset of shear thickening. The critical shear stress increases with increasing particle volume fraction, at least for the volume fractions studied here. In large amplitude oscillatory flow, strain hardening for both storage and loss moduli sets in at the same peak shear rate that causes shear thickening in steady state flow. Parallel superposition also has been used to probe the effect of the stabilizer layer. The frequency dependence of the superposition moduli changes drastically once shear thickening sets in. Surprisingly, this produces a simple and constant shape for the moduli-frequency curves with relaxation times that become nearly independent of the shear rate.

Oscillatory squeezing flow of a biological material

Large-amplitude oscillatory squeezing flow data are reported for a complex biological material, which is highly shear-thinning in oscillatory shear flow. This soft tissue has a linear viscoelastic limit at a strain of approximately 0.2%. The oscillatory squeezing flow data at large strain are analyzed using two constitutive models: a bi-viscosity Newtonian model, and a non-linear Maxwell model. It is found that although both models may have the same response in shape, the later matches with our non-linear experimental data better. It is also concluded that the non-linear response of the material in large amplitude oscillatory flow is mainly due to the shear thinning of the material.

Steady and Oscillatory Shear Flow Alignment Dynamics in a Lamellar Diblock Copolymer Solution

Shear flow alignment in a symmetric polystyrene−polyisoprene (PS−PI) diblock copolymer solution in dioctyl phthalate has been studied using flow birefringence and rheology. In large amplitude oscillatory shear, we find behavior similar to that previously reported in lamellar PS−PI melts. At high reduced frequency, parallel alignment of the layers is preferred, confirmed with birefringence measurements using oblique light paths. At low reduced frequency, perpendicular alignment is favored. In steady shear flow experiments well below the ODT, a similar behavior pattern is found: perpendicular alignment results at low shear rates, and parallel alignment is found at high shear rates. These are the first results demonstrating flipping by a change in steady shear flow conditions. As temperature is increased, the degree of perpendicular alignment obtained at low reduced shear rates decreases, unlike the behavior in large amplitude oscillatory flow, where high degrees of perpendicular alignment are achieved at l...

220 ダクト内対流場に重なる音響流の可視化

The phenomena described in this paper are concerned with air-column oscillation induced by acoustic standing wave in a closed duct driven by a sound source. When the driving frequency is in the vicinity of the resonance frequency of the air-column, a large amplitude oscillatory flow is produced in the duct. According as an increase of the amplitude, steady pressure distribution, steady streaming and thermoacoustic effect appear in addition to the oscillatory fluid motion in the duct. The steady streaming, which is called Rayleigh's acoustic streaming caused by frictional dissipation in the Stokes layer, results in circulating flows within each segment of 1/4 wave-length of the standing wave. When a strong acoustic standing wave is superimposed on the field of unstable convection generating in a horizontal duct by heating of duct floor, the coupling of acoustic streaming on the convection is capable of transforming it.This paper describes experimental investigations on the coupling of the acoustic streaming with the natural convection developed in a rectangular duct. The results obtained show that the coupling of the sound field on the unstable convection not only transforms it into a stable convection with cellular structure characterized by the wave length of the sound, but also the velocity of convection current increases.

Effect of shear on subsequent rheological transients in polymeric liquid crystals

AbstractThe transient rheological behaviour of a lyotropic polymeric liquid crystal is investigated. The system consists of a 12% by weight solution of poly(γ‐benzyl‐L‐glutamate) in m‐cresol. The structural changes caused by steady state shearing and by large amplitude oscillatory flow are measured. This is done by following the linear moduli upon cessation of flow. The stress transients during a stepwise increase in shear rate are used for the same purpose. Flow always increases the subsequent moduli in the system used, but the mode and the intensity of the flow have a complex effect. The variable domain structure seems to be responsible for most of the measured behaviour. The data put specific constraints on the detailed mechanisms that can contribute to the global transient stress.