Effects Of Shear-thinning Viscosity Research Articles

Effects of shear-thinning viscosity and viscoelastic stresses on flagellated bacteria motility

The behavior of flagellated bacteria swimming in non-Newtonian media remains an area with contradictory and conflicting results. We report on the behavior of wild-type and smooth-swimming E. coli in Newtonian, shear thinning and viscoelastic media, measuring their trajectories and swimming speed using a three dimensional real-time tracking microscope. We conclude that the speed enhancement in Methocel solution at higher concentration is due to shear-thinning and an analytical model is used to support our experimental result. We argue that shear-induced normal stresses reduce the wobbling behavior during cell swimming but do not significantly affect swimming speed. However, the normal stresses play an important role in decreasing the flagellar bundling time which changes the swimming speed distribution. A dimensionless number, the "Strangulation number" (Str) is proposed and used to characterize this effect.

Heat transfer and flow of nanofluids: Effect of shear thinning viscosity

In this paper, we study the heat transfer and flow characteristics of a pressure-driven flow of nanofluids between two plates. For modelling the motion of nanoparticles, a concentration convection–diffusion equation is applied, where the effect of the Brownian motion and thermophoretic diffusion are included. We consider that the shear viscosity and the thermal conductivity of the nanofluids depend on the concentration of the nanoparticles. Furthermore, the viscosity is also a function of shear rate, which has been widely observed by many experiments. The flow is assumed to be fully developed and steady state. The effect of the dimensionless parameters related to shear thinning viscosity, thermal diffusivity, particle transport flux, etc., are studied numerically.

Coarse Particles in Homogeneous Non-Newtonian Slurries: Combined Effects of Shear-Thinning Viscosity and Fluid Yield Stress on Drag and Heat Transfer from Hemispherical Particles

In this work, the momentum and energy equations have been solved numerically for predicting the hydrodynamic drag and heat transfer coefficient for a hemispherical particle submerged in a flow stream of yield-pseudoplastic fluids in order to elucidate the combined effects of shear-thinning viscosity and fluid yield stress. In this case, the momentum transfer aspects are influenced by the values of the Reynolds number (0.1 ≤ Re ≤ 100), Bingham number(0 ≤ Bn ≤ 100), shear-thinning index (0.2 ≤ n ≤ 1) and the orientation of the hemisphere. Similarly, the corresponding heat transfer results show additional dependence on the Prandtl number (0.7 ≤ Pr ≤ 100) and the type of thermal (isothermal or isoflux) boundary condition specified on the surface of the heated hemisphere. The numerical results are discussed in terms of the size and shape of the fluid-like yielded regions, wake lengths, hydrodynamic drag and heat transfer coefficients as functions of the preceding dimensionless parameters. Finally, the present values of the drag coefficient and Nusselt number have been fitted using simple expressions thereby enabling the interpolation of the present results for the intermediate values of the parameters and/or their prediction in a new application.

Chemorheological study of phosphorylated flame retardant epoxies

The aim of this work is to study the chemorheology of the phosphorylated novolac epoxy used in flame retardants. The rheological properties are measured by a parallel plate viscometer and are combined with the kinetics of the curing reaction to explore the conversion dependent character of the viscosity. It was found that dynamic viscosity is a unique function of the effective shear rate over a wide range of frequencies and strains after being corrected for temperature. The effective shear rate dependence of the viscosity is described with a power law with an exponent that depends on the conversion. The effect of temperature is described with an Arrhenius type equation with conversion dependent parameters. Differential scanning calorimetry was applied to determine the kinetic equation that is used, in combination with the thermal history, to obtain the conversion during the rheological measurements. Time–cure–temperature modelling was applied to study a variety of factors that had an influence on the chemorheology. It was also found that the Cox–Merz relation can be utilised to correlate the dynamic viscosity data to the shear viscosity data. The Castro–Macosko model is proposed when considering the combination effects of shear thinning viscosity and reaction kinetics.

Effect of non-newtonian viscosity for surfactant solutions on vortex characteristics in a swirling pipe flow

Effects of non-Newtonian viscosity for surfactant solution on the vortex characteristics and drag-reducing rate in a swirling pipe flow are investigated by pressure drop measurements, velocity profile measurements and viscosity measurements. Non-Newtonian viscosity is represented by power-law model (τ=kDn). Surfactant solution used has shear-thinning viscosity with n<1.0. The swirling flow in this study has decay of swirl and vortex-type change from Rankin’s combined vortex to forced vortex. It is shown that the effect of shear-thinning viscosity on the decay of swirl intensity is different by vortex category and the critical swirl number with the vortex-type change depends on shear-thinning viscosity.

Interfacial stability and deformation of two stratified power law fluids in plane poiseuille flow Part I. Stability analysis

The interface stability of two stratified power law fluids in plane Poiseuille flow is considered. Stability diagrams are given in terms of viscosity and depth ratios as a function of power law indices. The work of previous investigators for two Newtonian and second-order fluids has been generalized and presented in a similar fashion. It is found that dependence of viscosity on shear rate has a pronounced effect on the interfacial stability regime in a more drastic manner than of an effective viscosity change. Furthermore, it is found that for power law fluids the effect of shear thinning viscosity in the less viscous fluid mainly affects the stability region for viscosity ratios less than unity, while for viscosity ratios greater than unity shear thinning in both layers affects the stability regime, particularly at small depth ratios.

Numerical Simulation of the Exit Flow for the Viscoelastic Fluids

The nature of viscoelastic flow near the exit of a nozzle is dependent die swell phenomena and various polymer processing operations such as fiber spinning, sheet forming, blow molding, etc. Although many calculations have been performed by various investigators, the local values of stresses have seldom been compared experimentally and numerically. In the present paper, the exit flow is analyzed by the finite element method. The constitutive equations used in the simulations are the Phan Thien-Tanner (PTT) type model of a viscoelastic fluid with shear thinning viscosity, the Oldroyd-B model of a viscoelastic fluid with constant viscosity, the power-law model of a pure viscous fluid with the shear thinning viscosity, and the Newtonian model. The stress distributions and swelling ratios are compared among these models, and the effects of shear thinning viscosity and elasticity on the exit flow are considered. It is found that swelling ratio decreases as the shear thinning effect of viscosity becomes stronger, and increases with the elastic effect. The swelling mechanism can be explained mostly from the viewpoint of stress. The calculated results of stress distributions by the PTT type model are compared with the experimental values measured by means of the flow birefringence technique. The stress distributions calculated are found to be in agreement with the experimental values, but the estimation of swelling ratio is insufficient.