Large Shear Rates Research Articles

Analytical and Computational Studies of Correlations of Hydrodynamic Fluctuations in Shear Flow

We study correlations of hydrodynamic fluctuations in shear flow analytically and also by dissipative particle dynamics~(DPD) simulations. The hydrodynamic equations are linearized around the macroscopic velocity field and then solved by a perturbation method in Fourier-transformed space. The autocorrelation functions~(ACFs) from the analytical method are compared with results obtained from DPD simulations under the same shear-flow conditions. Upto a moderate shear rate, various ACFs from the two approaches agree with each other well. At large shear rates, discrepancies between the two methods are observed, hence revealing strong additional coupling between different fluctuating variables, which is not considered in the analytical approach. In addition, the results at low and moderate shear rates can serve as benchmarks for developing multiscale algorithms for coupling of heterogeneous solvers, such as a hybrid simulation of molecular dynamics and fluctuating hydrodynamics solver, where thermal fluctuations are indispensable.

Dynamic Response of a Red Blood Cell in Shear Flow

Three-dimensional simulation of a red blood cell deformation in a shear flow isperformed using immersed boundary lattice Boltzmann method for the fluid flow simulation, as well asfinite element method for membrane deformation. Immersed boundary method has been used to modelinteraction between fluid and membrane of the red blood cell. Red blood cell is modeled as a biconcavediscoid capsule containing fluid with an elastic membrane. Computations are performed at relativelysmall and large shear rates in order to study the dynamic behavior of red blood cell, especially tumblingand swinging modes of its motion. A rigid-body-like motion with the constant-amplitude oscillationof deformation parameter and continuous rotation is observed for red blood cell at its tumbling mode.However, at a relatively large shear rate, red blood cell follows a periodic gradual deformation andelongation with a final ellipsoidal shape. The effect of different initial orientations of red blood cell isalso investigated in the present paper. Results show that the dynamic response of red blood cell is notsensitive to this parameter.

Fullerene-water nanofluid confined in graphene nanochannel

The flow behaviors and boundary slip of the fullerene-water nanofluids (NFs) confined in graphene nanochannels are first investigated by using classical molecular dynamics simulations. The influences of the shear rate in Couette model, the driving force in Poiseuille model, the volume fraction, and the charge magnitude on the motion behaviors and the boundary slip are explored with considering the dynamics and the accumulation of the fullerene within the NFs. The results show that the boundary slip velocity increases almost linearly with the shear rate below a threshold of the shear rate while it increases sharply above the threshold. The relatively large driving force in Poiseuille model and the large shear rate in Couette model can reduce the accumulation of the fullerenes. The increase in the volume fraction of the fullerene in NFs can enhance the shear viscosity, and interestingly, it can increase the boundary slip velocity of the NFs in graphene channels. As the charge magnitude of the graphene channel increases, the boundary slip of fullerene NFs first increases to a threshold and then decreases slightly. The findings may be helpful to the design and fabrication of the low dimensional carbon materials-based nano-apparatus.

Non-stationary flows of asymptotically Newtonian fluids

We study nonlinear parabolic Stokes systems with an asymptotically linear structure. This refers to the slow flow of a non-Newtonian fluid with Newtonian behavior for large shear rates. We show that the symmetric gradient of the velocity field is locally bounded in space-time.

Shear-induced laning transition in a confined colloidal film.

Using Brownian dynamics simulations, we investigate a dense system of charged colloids exposed to shear flow in a confined (slit-pore) geometry. The equilibrium system at zero flow consists of three well-pronounced layers with a squarelike crystalline in-plane structure. We demonstrate that, for sufficiently large shear rates, the middle layer separates into two sublayers where the particles organize into moving lanes with opposite velocities. The formation of this "microlaned" state results in a destruction of the applied shear profile; it also has a strong impact on the structure of the system, and on its rheology as measured by the elements of the stress tensor. At higher shear rates, we observe a disordered state and finally a recrystallization reminiscent of the behavior of bilayer films. We also discuss the system size dependence and the robustness of the microlaned state against variations of the slit-pore width. In fact, for a pore width allowing for four layers, we observe a similar shear-induced state in which the system splits into two domains with opposite velocities.

Thixotropy and shear thinning of lubricated contacts with confined membranes.

We have modeled the nonlinear dynamics and the rheological behavior of a system under shear containing a membrane confined between two attractive walls. The presence of the membrane induces additional tangential forces on the walls that always increase the global friction. At low shear rates, the membrane exhibits chaotic dynamics with slow coarsening leading to thixotropy, i.e. to a slow decrease of the membrane-induced tangential forces on the walls. At intermediate shear rates, the membrane profile presents stationary periodic patterns. At higher shear rates, membrane dynamics are governed by a nonlinear evolution equation which is similar to the Kuramoto-Sivashinski equation, but with a sixth-order stabilizing term. The membrane experiences chaotic dynamics without coarsening. As a consequence of the nonlinear dynamics of the membrane at intermediate and large shear rates, the system exhibits shear thinning.

Kinetic theory of discontinuous shear thickening

A simple kinetic theory to exhibit a discontinuous shear thickening (DST) is proposed. The model includes the collision integral and the friction from environment as well as a thermostat term characterized by $T_{\rm ex}$. The viscosity of this model is proportional to $\dot\gamma^2$ for large shear rate $\dot\gamma$, while it is Newtonian for low $\dot\gamma$. The emergence of the DST is enhanced for lower density and lower nonzero $T_{\rm ex}$.

Multiple shear bands in granular materials

The transition regime from quasi-static to dynamic shear regime is studied by discrete element method (DEM) and by means of a mesoscopic model. We show that at moderate shear rate multiple shear bands appear which eventually appear as a continuous shear profile for large shear rates. The model and the DEM simulations both show a minimum in the shear stress and a maximum for the number of simultaneous shear bands, however the position of the two do not coincide. The scaling analysis of the kinetic energy indicates that the peak in the latter is the relevant parameter. We also show how the long range order present in the quasi-static regime is gradually destroyed by the presence of multiple shear bands.

Surface strain rates and crevassing of Campbell Glacier Tongue in East Antarctica analysed by tide-corrected DInSAR

ABSTRACTThis research presents the measurement of surface strain rates over Campbell Glacier Tongue (CGT) in East Antarctica from tide-corrected ice velocity derived by removing the vertical tidal deflection from the 14 COSMO-SkyMed 1-day differential interferometric synthetic aperture radar images obtained in 2011. The hinge zone of CGT shows large longitudinal, transverse, shear, and vertical strain rates, especially in a heavy crevassing band, while the freely floating zone shows very small strain rates. This represents that the surface deformation by ice flow in the hinge zone is significantly larger than that in the freely floating zone. The orientations of crevasses in the hinge zone agree reasonably well with the directions orthogonal to the axes of the most tensile strain rates calculated from the tide-corrected ice velocity, which demonstrates that the crevassing is attributed to the gravitational ice flow rather than to the tidal flexure.

Nanofluidic flow assisted assembly of dispersed plasmonic nanostructures into shallow nanochannel sensors

The authors present a method for assembling plasmonic nanostructures into already-sealed shallow nanochannel-based nanofluidic sensor structures. This method is termed as nanofluidic-flow-assisted-assembly (NFAA). NFAA utilizes nanofluidic flows with large shear rate and stress to deposit high-areal-density, well-dispersed plasmonic nanoparticles (NPs) into shallow nanochannel sensing areas. In particular, in a NFAA process, the nano/microfluidic structures are first patterned into a Si or SiO2 substrate and permanently sealed with fused quartz coverslips using plasma sealing. Afterward, a colloidal solution of plasmonic NPs is driven into the shallow nanochannel structures. In the shallow nanochannel areas, the large shear rate and stress of the nanofluidic colloidal solution flow results in the deposition of well-dispersed NPs and effectively prevents undesirable aggregation of NPs. Using NFAA, the authors have demonstrated the deposition of well-dispersed Au NPs with various areal densities (102–104 μm−2) into shallow nanochannels. The light absorbance peak of NFAA-coated Au NPs exhibits the narrower full-width-at-half-maximum than that of the Au NPs directly deposited from a colloidal solution, further indicating that NFAA can result in the higher degree of dispersion of high-density NPs. The authors also demonstrated that the additional nanoscale anchoring structures prepatterned in a shallow nanochannel, in combination with NFAA processes, can enable selective deposition of functional nanoparticles around designated locations. This work provides a nanofabrication scheme for introducing functional nanostructures into already-sealed nanofluidic structures. This method could be further generalized to enable integration of various electrically/optically active nanoscale components into permanently sealed nano/microfluidic structures and therefore address the incompatibility among the fabrication routes of these device structures.

The effect of damping function on extrudate swell

The extrudate swell of an industrial grade high molecular weight high-density polyethylene (HDPE) in capillary dies is studied using the integral K-BKZ constitutive model. The non-linear viscoelastic flow properties of the polymer resin are studied for a broad range of large step shear strains and high shear rates using the cone partitioned plate (CPP) geometry of the stress/strain controlled rotational rheometer. This allowed the determination of the rheological parameters accurately, in particular the damping function, which is proven to be the most important in simulating transient flows such as extrudate swell. A series of simulations performed using the integral K-BKZ Wagner model with different values of the Wagner exponent n, ranging from n=0.15–0.5, demonstrates that the extrudate swell predictions are extremely sensitive to the Wagner damping function exponent. Using the correct n-value resulted in extrudate swell predictions that are in excellent agreement with experimental measurements.

A thermodynamic study of shear banding in polymer solutions

Although shear banding is a ubiquitous phenomenon observed in soft materials, the mechanisms that give rise to shear-band formation are not always the same. In this work, we develop a new two-fluid model for semi-dilute entangled polymer solutions using the generalized bracket approach of nonequilibrium thermodynamics. The model is based on the hypothesis that the direct coupling between polymer stress and concentration is the driving mechanism of steady shear-band formation. To obtain smooth banded profiles in the two-fluid framework, a new stress-diffusive term is added to the time evolution equation for the conformation tensor. The advantage of the new model is that the differential velocity is treated as a state variable. This allows a straightforward implementation of the additional boundary conditions arising from the derivative diffusive terms with respect to this new state variable. To capture the overshoot of the shear stress during the start of a simple shear flow, we utilize a nonlinear Giesekus relaxation. Moreover, we include an additional relaxation term that resembles the term used in the Rouse linear entangled polymer model to account for convective constraint release and chain stretch to generate the upturn of the flow curve at large shear rates. Numerical calculations performed for cylindrical Couette flow confirm the independency of the solution from the deformation history and initial conditions. Furthermore, we find that stress-induced migration is the responsible diffusive term for steady-state shear banding. Because of its simplicity, the new model is an ideal candidate for the use in the simulation of more complex flows.

Solution of the complete Curtiss-Bird model for polymeric liquids subjected to simple shear flow.

The complete kinetic theory model for concentrated polymer solutions and melts proposed by Curtiss and Bird is solved for shear flow: (a) analytically by providing a solution for the single-link (or configurational) distribution function as a real basis spherical harmonics expansion and then calculating the materials functions in shear flow up to second order in the dimensionless shear rate and, (b) numerically via the execution of Brownian dynamics simulations. These two methods are actually complementary to each other as the former is accurate only for small dimensionless shear rates where the latter produces results with increasingly large uncertainties. The analytical expansions of the material functions with respect to the dimensionless shear rate reduce to those of the extensively studied, simplified Curtiss-Bird model when ε' = 0, and to the rigid rod when ε' = 1. It is known that the power-law behavior at high shear rates is very different for these two extremal cases. We employ Brownian dynamics simulation to not only recover the limiting cases but to find a gradual variation of the power-law behaviors at large dimensionless shear rates upon varying ε'. The fact that experimental data are usually located between these two extremes strongly advocates the significance of studying the solution of the Curtiss-Bird model. This is exemplified in this work by comparing the solution of this model with available rheological data for semiflexible biological systems that are clearly not captured by the original Doi-Edwards or simplified Curtiss-Bird models.

Changes in rheology of self-consolidating concrete induced by pumping

Abstract Pumping is an easy and flexible process to place concrete inside a formwork. Many studies have recently been performed to understand and optimize the pumping process and identify the main differences between pumping of conventional vibrated concrete and self-consolidating concrete (SCC). However, due to pumping, changes in fresh concrete properties and the air-void system have been noticed. This paper describes the consequences of pumping on the fresh properties of SCC by means of two experimental pumping campaigns. In many cases, the concrete undergoes a large shear rate in the pipe, thus (re-)dispersing cement particles. This is expected to be the main cause of the observed decrease in plastic viscosity, V-Funnel flow time and pumping pressure resulting from increased flow rate or pumping time. The changes in yield stress or slump flow are anticipated to be influenced by the same phenomenon, but the final outcome is assumed to depend on the availability of residual superplasticizer in the mixing water. Pumping can cause a stable SCC to become segregating if both the yield stress and plastic viscosity decrease, or it can provoke a significant loss in filling ability, passing ability and self-consolidation of the concrete if the yield stress increases dramatically.

Merging fluid and solid granular behavior.

Simple homogeneous shear flows of frictionless, deformable particles are studied by particle simulations at large shear rates and for differently soft, deformable particles. Particle stiffness sets a time-scale that can be used to scale the physical quantities; thus the dimensionless shear rate, i.e. the inertial number I (inversely proportional to pressure), can alternatively be expressed as inversely proportional to the square root of particle stiffness. Asymptotic scaling relations for the field variables pressure, shear stress and granular temperature are inferred from simulations in both fluid and solid regimes, corresponding to unjammed and jammed conditions. Then the limit cases are merged to unique constitutive relations that cover also the transition zone in the proximity of jamming. By exploiting the diverging behavior of the scaling laws at the jamming density, we arrive at continuous and differentiable phenomenological constitutive relations for stresses and granular temperature as functions of the volume fraction, shear rate, particle stiffness and distance from jamming. In contrast to steady shear flows of hard particles the (shear) stress ratio μ does not collapse as a function of the inertial number, indicating the need for an additional control parameter. In the range of particle stiffnesses investigated, in the solid regime, only pressure is rate independent, whereas shear stress exhibits a slight shear rate- and stiffness-dependency.

Lifetime of Flow-Induced Precursors in Isotactic Polypropylene

Brief intervals of strong flow stretch chains in a semicrystalline polymer melt to form flow-induced precursors, which accelerate crystallization kinetics and transform the morphology. Using commercial isotactic polypropylene, the persistence and lifetime of flow-induced precursors were investigated, focusing on the effects of specific work and annealing time and temperature. Precursors were formed by shearing polypropylene in a rotational rheometer and then quenching to a desired temperature. The crystallization time and crystallization temperature of the sheared samples were investigated using novel rheology and DSC experiments. For sufficiently large shear rates, the appearance of flow-induced precursors is controlled by the applied work. A qualitative change in many properties related to precursors takes place at a critical work value Wc, about 7 MPa for our iPP material. To erase the persistent “memory” of flow-induced nuclei, samples must be annealed for a very long time below 210 °C (>3000 min) or ...

Flow of a generalised Newtonian fluid due to a rotating disk

The boundary-layer flow due to a rotating disk is considered for a number of generalised Newtonian fluid models. In the limit of large Reynolds number the flow inside the three-dimensional boundary-layer is determined via a similarity solution. Results for power-law and Bingham plastic fluids agree with previous investigations. We present solutions for fluids that adhere to the Carreau viscosity model. It is well known that unlike the power-law and Bingham models the Carreau model is applicable for vanishingly small, and infinitely large shear rates, as such we suggest these results provide a more accurate description of non-Newtonian rotating disk flow.

Spatiotemporal dynamics of multiple shear-banding events for viscoelastic micellar fluids in cone-plate shearing flows

We characterize the transient response of semi-dilute wormlike micellar solutions under an imposed steady shear flow in a cone-plate geometry. By combining conventional rheometry with 2-D Particle Image Velocimetry (PIV), we can simultaneously correlate the temporal stress response with time-resolved velocimetric measurements. By imposing a well defined shear history protocol, consisting of a stepped shear flow sweep, we explore both the linear and nonlinear responses of two surfactant solutions: cetylpiridinium chloride (CPyCl) and sodium salicylate (NaSal) mixtures at concentrations of [66:40]mM and [100:60]mM, respectively. The transient stress signal of the more dilute solution relaxes to its equilibrium value very fast and the corresponding velocity profiles remain linear, even in the strongly shear-thinning regime. The more concentrated solution also exhibits linear velocity profiles at small shear rates. At large enough shear rates, typically larger than the inverse of the relaxation time of the fluid, the flow field reorganizes giving rise to strongly shear-banded velocity profiles. These are composed of an odd number of shear bands with low-shear-rate bands adjacent to both gap boundaries. In the non-linear regime long transients (much longer than the relaxation time of the fluid) are observed in the transient stress response before the fluid reaches a final, fully-developed state. The temporal evolution in the shear stress can be correlated with the spatiotemporal dynamics of the multiple shear-banded structure measured using RheoPIV. In particular our experiments show the onset of elastic instabilities in the flow which are characterized by the presence of multiple shear bands that evolve and rearrange in time resulting in a slow increase in the average torque acting on the rotating fixture.

Suction–shear–Coriolis instability in a flow between parallel plates

AbstractA rotating fluid flow between differentially translating parallel plates, which induce uniform suction and injection, is studied as a canonical model of swirling flow where suction, shear and Coriolis effects compete. This relatively simple modelling yields several reduced equations that are valid for asymptotically large suction, shear and/or rotation rates. The linear stability problems derived from the full Navier–Stokes and reduced problems are numerically solved and compared. In addition to Taylor-vortex modes, transverse-roll-type instabilities are found in Rayleigh-stable and -unstable parameter regions when weak suction is applied. These instabilities, separated by the so-called Rayleigh line, are characterised by vortices attached to the suction wall. Another type of instability, which exists beyond the Rayleigh line and shows inviscid motion in the fluid core, is found when suction is sufficiently strong. The relation of this instability to the stability results by Gallet, Doering & Spiegel (Phys. Fluids, vol. 22, 2010, 034105) is discussed. Our nonlinear analyses indicate subcritical and supercritical bifurcations of finite-amplitude solutions for the near-wall and fluid-core instabilities, respectively.

Do column frits contribute to the on-column, flow-induced degradation of macromolecules?

Flow-induced, on-column degradation is a major hindrance to the accurate characterization of ultra-high molar mass macromolecules and colloids. This degradation is a direct result of the large shear rates which are generated within the column, which cause chain scission to occur both in the interstitial medium and, it has been postulated, at the packing particle pore boundary. An additional putative source of degradation has been the column frits, though little experimental evidence exists to either support or refute this claim. To this effect, the present experiments examine the role of the frits in the degradation of high molar mass macromolecules. Two narrow dispersity polystyrene standards, the molar mass of which differs by a factor of two, were analyzed on three different size-exclusion chromatography (SEC) columns, each with frits of different pore size, at various flow rates. In the smallest pore size column, which also contained the smallest frits and which was packed with the smallest diameter particles, the larger standard was forced to degrade by increasing the flow rate of the mobile phase. During the course of the latter portion of the study, the inlet and the outlet frits were removed from the column, in stepwise fashion. It was concluded that neither frit played any appreciable role in the degradation. Results of our studies were applied to explain previously observed degradation in ultra-high pressure liquid chromatography of polymers. The general conclusion arrived at herein is that the column frits are likely to have a secondary role (as compared to interstitial and pore boundary stresses), or no role at all, in polymer degradation for cases where the frit radius is larger than or equal to the hydraulic radius rcof the column.