Showing papers on "Shear stress published in 2010"
TL;DR: It is demonstrated from stress-induced Raman bands shifts that stress can be transferred from a polymer matrix to a graphene monolayer in a model nanocomposite.
Abstract: It is demonstrated from stress-induced Raman bands shifts that stress can be transferred from a polymer matrix to a graphene monolayer (see image) in a model nanocomposite. It is shown further that the behavior can be modeled using continuum mechanics and that the interface between the graphene and the polymer breaks down at a shear stress of the order of 2 MPa.
578 citations
TL;DR: The shear stress of flowing blood on the surfaces of endothelial cells that provide the barrier to transport of solutes and water between blood and the underlying tissue modulates the permeability to solute and the hydraulic conductivity.
Abstract: The shear stress of flowing blood on the surfaces of endothelial cells that provide the barrier to transport of solutes and water between blood and the underlying tissue modulates the permeability to solutes and the hydraulic conductivity. This review begins with a discussion of transport pathways across the endothelium and then considers the experimental evidence from both in vivo and in vitro studies that shows an influence of shear stress on endothelial transport properties after both acute (minutes to hours) and chronic (hours to days) changes in shear stress. Next, the effects of shear stress on individual transport pathways (tight junctions, adherens junctions, vesicles and leaky junctions) are described, and this information is integrated with the transport experiments to suggest mechanisms controlling both acute and chronic responses of transport properties to shear stress. The review ends with a summary of future research challenges.
314 citations
TL;DR: In this paper, the authors exploit the concept of strain-induced band-structure engineering in graphene through the calculation of its electronic properties under uniaxial, shear, and combined uniaxonial-shear deformations.
Abstract: We exploit the concept of strain-induced band-structure engineering in graphene through the calculation of its electronic properties under uniaxial, shear, and combined uniaxial-shear deformations. We show that by combining shear deformations to uniaxial strains it is possible modulate the graphene energy-gap value from zero up to 0.9 eV. Interestingly enough, the use of a shear component allows for a gap opening at moderate absolute deformation, safely smaller than the graphene failure strain.
297 citations
TL;DR: High-resolution measurements of the forces between two atomically smooth solid surfaces across a film of 1-ethyl-3-methylimidazolium ethylsulfate ionic liquid reveal low friction coefficients which are 1-2 orders of magnitude smaller than for analogous films of non-polar molecular liquids, including standard hydrocarbon lubricants.
Abstract: We report high-resolution measurements of the forces between two atomically smooth solid surfaces across a film of 1-ethyl-3-methylimidazolium ethylsulfate ionic liquid, for film thickness down to a single ion diameter. For films thinner than ∼2 nm oscillatory structural forces are observed as the surface separation decreases and pairs of ion layers are squeezed out of the film. Strikingly, measurements of the shear stress of the ionic liquid film reveal low friction coefficients which are 1–2 orders of magnitude smaller than for analogous films of non-polar molecular liquids, including standard hydrocarbon lubricants, up to ca. 1 MPa pressure. We attribute this to the geometric and charge characteristics of the ionic liquid: the irregular shapes of the ions lead to a low shear stress, while the strong coulombic interactions between the ions and the charged confining surfaces lead to a robust film which is maintained between the shearing surfaces when pressure is applied across the film.
288 citations
TL;DR: The ratio of shear stress to normal stress can locally far exceed the static-friction coefficient without precipitating slip and different modes of rupture selected by the system correspond to distinct regimes of the local stress ratio.
Abstract: The way in which a frictional interface fails is critical to our fundamental understanding of failure processes in fields ranging from engineering to the study of earthquakes. Frictional motion is initiated by rupture fronts that propagate within the thin interface that separates two sheared bodies. By measuring the shear and normal stresses along the interface, together with the subsequent rapid real-contact-area dynamics, we find that the ratio of shear stress to normal stress can locally far exceed the static-friction coefficient without precipitating slip. Moreover, different modes of rupture selected by the system correspond to distinct regimes of the local stress ratio. These results indicate the key role of nonuniformity to frictional stability and dynamics with implications for the prediction, selection, and arrest of different modes of earthquakes.
262 citations
TL;DR: It is demonstrated that flexible wall modeling plays an important role in accurate prediction of patient-specific hemodynamics in vascular fluid–structure interaction modeling when compared to the rigid arterial wall assumption.
Abstract: A computational vascular fluid-structure interaction framework for the simulation of patient-specific cerebral aneurysm configurations is presented. A new approach for the computation of the blood vessel tissue prestress is also described. Simulations of four patient-specific models are carried out, and quantities of hemodynamic interest such as wall shear stress and wall tension are studied to examine the relevance of fluid-structure interaction modeling when compared to the rigid arterial wall assumption. We demonstrate that flexible wall modeling plays an important role in accurate prediction of patient-specific hemodynamics. Discussion of the clinical relevance of our methods and results is provided.
224 citations
TL;DR: In this paper, the effects of friction modeling at the tool-chip-workpiece interfaces on chip formation process in predicting forces, temperatures and other field variables such as normal stress and shear stress on the tool by using advanced finite element (FE) simulation techniques.
207 citations
TL;DR: The rheological function of N cycles and force chains is elucidated, suggesting that the three-force cycles are called upon for reinforcements to ward off failure via shear banding in the lead up to failure.
Abstract: We examine the coevolution of $N$ cycles and force chains as part of a broader study which is designed to quantitatively characterize the role of the laterally supporting contact network to the evolution of force chains. Here, we elucidate the rheological function of these coexisting structures, especially in the lead up to failure. In analogy to force chains, we introduce the concept of force cycles: $N$ cycles whose contacts each bear above average force. We examine their evolution around force chains in a discrete element simulation of a dense granular material under quasistatic biaxial loading. Three-force cycles are shown to be stabilizing structures that inhibit relative particle rotations and provide strong lateral support to force chains. These exhibit distinct behavior from other cycles. Their population decreases rapidly during the initial stages of the strain-hardening regime---a trend that is suddenly interrupted and reversed upon commencement of force chain buckling prior to peak shear stress. Results suggest that the three-force cycles are called upon for reinforcements to ward off failure via shear banding. Ultimately though, the resistance to buckling proves futile; buckling wins under the combined effects of dilatation and increasing compressive load. The sudden increase in three-force cycles may thus be viewed as an indicator of imminent failure via shear bands.
198 citations
TL;DR: In this article, a mobilized dilation angle model considering the influence of both confining stress and plastic shear strain is proposed to predict the volumetric-axial strain relationships, and the predictions are found to be in good agreement with experimental results.
195 citations
TL;DR: In this article, the mechanical properties of graphite in the forms of single graphene layer and graphite flakes (containing several graphene layers) were investigated using molecular dynamics (MD) simulation, respectively, by applying axial tensile stress and in-plane shear stress on the simulation box through the modified NPT ensemble.
186 citations
TL;DR: In this article, a numerical model capable of studying the dynamic failure process of rock under coupled static geo-stress and dynamic disturbance is proposed, and it is implemented into the Rock Failure Process Analysis (RFPA), a general finite element package to analyze the damage and failure of engineering materials such as rock and concrete.
TL;DR: In this article, microstructural evolution associated with the shear banding in nano-scale twin/matrix (T/M) lamellae of a Cu-Al alloy processed by means of dynamic plastic deformation was investigated using transmission electron microscopy (TEM) and high-resolution TEM.
TL;DR: In this paper, the authors present a unified continuum mechanics representation of the mechanisms commonly involved in post-seismic transients such as viscoelasticity, fault creep and poroelasticness.
Abstract: We present a unified continuum mechanics representation of the mechanisms believed to be commonly involved in post-seismic transients such as viscoelasticity, fault creep and poroelasticity. The time-dependent relaxation that follows an earthquake, or any other static stress perturbation, is considered in a framework of a generalized viscoelastoplastic rheology whereby some inelastic strain relaxes a physical quantity in the material. The relaxed quantity is the deviatoric stress in case of viscoelastic relaxation, the shear stress in case of creep on a fault plane and the trace of the stress tensor in case of poroelastic rebound. In this framework, the instantaneous velocity field satisfies the linear inhomogeneous Navier's equation with sources parametrized as equivalent body forces and surface tractions. We evaluate the velocity field using the Fourier-domain Green's function for an elastic half-space with surface buoyancy boundary condition. The accuracy of the proposed method is demonstrated by comparisons with finite-element simulations of viscoelastic relaxation following strike-slip and dip-slip ruptures for linear and power-law rheologies. We also present comparisons with analytic solutions for afterslip driven by coseismic stress changes. Finally, we demonstrate that the proposed method can be used to model time-dependent poroelastic rebound by adopting a viscoelastic rheology with bulk viscosity and work hardening. The proposed method allows one to model post-seismic transients that involve multiple mechanisms (afterslip, poroelastic rebound, ductile flow) with an account for the effects of gravity, non-linear rheologies and arbitrary spatial variations in inelastic properties of rocks (e.g. the effective viscosity, rate-and-state frictional parameters and poroelastic properties).
TL;DR: The simulations reveal that planar crack propagation is linearly unstable against helical deformations of the crack front, which evolve nonlinearly into a segmented array of finger-shaped daughter cracks, and a theoretical estimate for this scale is derived using a new propagation law for curved cracks in three dimensions.
Abstract: Evolution of fractures in materials can vary substantially depending on what conditions the material is in. In particular, if only tension loading is applied, the crack generally moves in a plane and the fracture surface is smooth. On the other hand, the addition of a shear stress orthogonal to this plane of crack propagation generates an instability that results in three-dimensional helical crack propagation, atomically rough surfaces and a characteristic fracture pattern that resembles the shape of a series of lances. Antonio Pons and Alain Karma explore the causes of this instability, performing extensive numerical simulations, to understand and predict this behaviour, which is important in a number of engineering and geological materials. They uncover a new law that governs crack propagation in space for materials subject to general stress conditions, as well as a delicate balance of forces acting to stabilize or destabilize the crack motion. The addition of shear orthogonal to the tension-loading plane of crack propagation generates an instability that results in three-dimensional helical crack propagation, atomically rough surfaces and a fracture pattern resembling a series of lance shapes. Here numerical simulations reveal a new law that governs crack propagation in space for materials subject to general stress conditions. Planar crack propagation under pure tension loading (mode I) is generally stable. However, it becomes universally unstable with the superposition of a shear stress parallel to the crack front (mode III). Under this mixed-mode (I + III) loading configuration, an initially flat parent crack segments into an array of daughter cracks that rotate towards a direction of maximum tensile stress1. This segmentation produces stepped fracture surfaces with characteristic ‘lance-shaped’ markings observed in a wide range of engineering2,3,4,5,6,7 and geological materials1,8. The origin of this instability remains poorly understood and a theory with which to predict the surface roughness scale is lacking. Here we perform large-scale simulations of mixed-mode I + III brittle fracture using a continuum phase-field method9,10,11 that describes the complete three-dimensional crack-front evolution. The simulations reveal that planar crack propagation is linearly unstable against helical deformations of the crack front, which evolve nonlinearly into a segmented array of finger-shaped daughter cracks. Furthermore, during their evolution, facets gradually coarsen owing to the growth competition of daughter cracks in striking analogy with the coarsening of finger patterns observed in nonequilibrium growth phenomena12,13,14. We show that the dynamically preferred unstable wavelength is governed by the balance of the destabilizing effect of far-field stresses and the stabilizing effect of cohesive forces on the process zone scale, and we derive a theoretical estimate for this scale using a new propagation law for curved cracks in three dimensions. The rotation angles of coarsened facets are also compared to theoretical predictions and available experimental data.
TL;DR: In this paper, direct numerical simulation is used to investigate the drag reducing performance of super-hydrophobic surfaces in turbulent channel flow, and the simulation results suggest that the mean velocities near the super hydrophobic wall continue to scale with the wall shear stress and the log layer is still present, but both are offset by a slip velocity that is primarily dependent on the microfeature spacing.
Abstract: These surfaces have been shown to provide drag reduction in laminar and turbulent flows. In this work, direct numerical simulation is used to investigate the drag reducing performance of superhydrophobic surfaces in turbulent channel flow. Slip velocities, wall shear stresses, and Reynolds stresses are determined for a variety of superhydrophobic surface microfeature geometry configurations at friction Reynolds numbers of Re180, Re395, and Re590. This work provides evidence that superhydrophobic surfaces are capable of reducing drag in turbulent flow situations by manipulating the laminar sublayer. For the largest microfeature spacing, an average slip velocity over 80% of the bulk velocity is obtained, and the wall shear stress reduction is found to be greater than 50%. The simulation results suggest that the mean velocity profile near the superhydrophobic wall continues to scale with the wall shear stress and the log layer is still present, but both are offset by a slip velocity that is primarily dependent on the microfeature spacing. © 2010 American Institute of Physics. doi:10.1063/1.3432514
TL;DR: In this article, the authors proposed an extrusion model of the higher Himalayan Shear Zone (HHSZ) in the Sutlej section, which predicts the shear strain after ductile extrusion to be maximum at the boundaries of the HHSZ, which crudely matches with existing data.
Abstract: The Higher Himalayan Shear Zone (HHSZ) in the Sutlej section reveals (1) top-to-SW ductile shearing, (2) top-to-NE ductile shearing in the upper- and the lower strands of the South Tibetan Detachment System (STDSU, STDSL), and (3) top-to-SW brittle shearing corroborated by trapezoid-shaped minerals in micro-scale. In the proposed extrusion model of the HHSZ, the E1-phase during 25–19 Ma is marked by simple shearing of the upper sub-channel defined by the upper strand of the Main Central Thrust (MCTU) and the top of STDSU as the lower- and the upper boundaries, respectively. Subsequently, the E2a-pulse during 15–14 Ma was characterized by simple shear, pure shear, and channel flow of the entire HHSZ. Finally, the E2b-pulse during 14–12 Ma observed simple shearing and channel flow of the lower sub-channel defined by the lower strand of the Main Central Thrust (MCTL) and the top of the STDSL as the lower- and the upper boundaries, respectively. The model explains the constraints of thicknesses of the STDSU and the STDSL along with spatially variable extrusion rate and the inverted metamorphism of the HHSZ. The model predicts (1) shear strain after ductile extrusion to be maximum at the boundaries of the HHSZ, which crudely matches with the existing data. The other speculations that cannot be checked are (2) uniform shear strain from the MCTU to the top of the HHSZ in the E1-phase; (3) fastest rates of extrusion of the lower boundaries of the STDSU and the STDSL during the E2a- and E2b-pulses, respectively; and (4) variable thickness of the STDSL and rare absence of the STDSU. Non-parabolic shear fabrics of the HHSZ possibly indicate heterogeneous strain. The top-to-SW brittle shearing around 12 Ma augmented the ductile extruded rocks to arrive a shallower depth. The brittle–ductile extension leading to boudinage possibly did not enhance the extrusion.
TL;DR: In this paper, a new methodology for 3D simulations of earthquake sequences is presented that accounts not only for inertial effects during seismic events but also for shear-induced temperature variations on the fault and the associated evolution of pore fluid pressure.
Abstract: [1] A new methodology for three-dimensional (3-D) simulations of earthquake sequences is presented that accounts not only for inertial effects during seismic events but also for shear-induced temperature variations on the fault and the associated evolution of pore fluid pressure. In particular, the methodology allows to capture thermal pressurization (TP) due to frictional heating in a shear zone. One-dimensional (1-D) diffusion of heat and pore fluids in the fault-normal direction is incorporated using a spectral method, which is unconditionally stable, accurate with affordable computational resources, and highly suitable to earthquake sequence calculations that use variable time steps. The approach is used to investigate the effect of heterogeneous hydraulic properties by considering a fault model with two regions of different hydraulic diffusivities and hence different potential for TP. We find that the region of more efficient TP produces larger slip in model-spanning events. The slip deficit in the other region is filled with more frequent smaller events, creating spatiotemporal complexity of large events on the fault. Interestingly, the area of maximum slip in model-spanning events is not associated with the maximum temperature increase because of stronger dynamic weakening in that area. The region of more efficient TP has lower interseismic shear stress, which discourages rupture nucleation there, contrary to what was concluded in prior studies. Seismic events nucleate in the region of less efficient TP where interseismic shear stress is higher. In our model, hypocenters of large events do not occur in areas of large slip or large stress drop.
TL;DR: In this article, a two-phase model of ductile extrusion of the Higher Himalayan Shear Zone (HHSZ) was proposed, where a top-to-SW simple shearing during 22−18 Ma was followed by a combination of topto-NE simple shear and channel flow at 18−16 Ma, and the channel flow component ceased around 16 Ma, the extruding HHSZ entered the brittle regime but the top to SW shearing continued until perturbed by faults and fractures.
Abstract: Thin-section studies of the Zanskar Shear Zone (ZSZ) rocks reveal a top-to-SW and subsequent primary and secondary top-to-NE ductile shearing; brittle–ductile and brittle extensions; top-to-SW brittle shear, steep normal faulting and fracturing. In the proposed two-phase model of ductile extrusion of the Higher Himalayan Shear Zone (HHSZ), a top-to-SW simple shearing during 22–18 Ma was followed by a combination of top-to-SW simple shear and channel flow at 18–16 Ma. The second phase simulates a thin ZSZ characterized by a top-to-NE shearing. The channel flow component ceased around 16 Ma, the extruding HHSZ entered the brittle regime but the top-to-SW shearing continued until perturbed by faults and fractures. Variation in the extrusion parameters led to variable thickness of the ZSZ. Shear strain after the extrusion is presumably maximum at the boundaries of the HHSZ and falls towards the base of the ZSZ, which crudely matches with the existing data. The other predictions: (1) spatially uniform shear strain after the first stage, (2) fastest extrusion rate at the base of the ZSZ, and (3) a lack of continuation of the ZSZ along the Himalayan trend are not possible to validate due to paucity of suitable data. Non-parabolic shear fabrics of the ZSZ indicate their heterogeneous deformation.
Journal Article•
TL;DR: In this article, the influence of temperature, concentration, shearing time, and storage time on the rheological properties of chitosan solubilized in weakly acid solutions was examined.
Abstract: This study examined the influence of temperature, concentration, shearing time, and storage time on the rheological properties, i.e. the dynamic viscosity and shear stress, as a function of shear rate of chitosan solubilized in weakly acid solutions. The results showed that shear thinning behavior (pseudoplastic non-Newtonian behavior) was pronounced at temperatures from 20 to 50 °C, but was more remarkable at lower temperature. In addition, the activation energy value derived from ln h vs. 1/T data was found to be 20.86 kJ mol-1. When the effect of concentration was studied, the shear thinning behavior was pronounced at all concentrations. The effect of shearing time on the dynamic viscosity and shear stress of chitosan solutions did not show any significant differences at all shearing times applied in this study. Furthermore, an increase in viscosity was obtained with extending the period of storage to 3 months, after which a drop in viscosity was recorded. Thermal properties of chitosan films were also investigated by employing thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) in this work. Results of TGA revealed that the decomposition of the chitosan film proceeds in 2 distinct stages, which was confirmed with DSC curves.
TL;DR: In this article, the emergence of shear thickening in dense suspensions of non-Brownian particles was studied using magnetic resonance imaging with macroscopic rheometry experiments, and it was observed that the material is heterogeneous, and that the local rheology presents a continuous transition at low shear rate from a viscous to a shear-thickening, Bagnoldian, behavior with shear stresses proportional to the shear rates squared.
Abstract: We study the emergence of shear thickening in dense suspensions of non-Brownian particles. We combine local velocity and concentration measurements using magnetic resonance imaging with macroscopic rheometry experiments. In steady state, we observe that the material is heterogeneous, and we find that the local rheology presents a continuous transition at low shear rate from a viscous to a shear thickening, Bagnoldian, behavior with shear stresses proportional to the shear rate squared, as predicted by a scaling analysis. We show that the heterogeneity results from an unexpectedly fast migration of grains, which we attribute to the emergence of the Bagnoldian rheology. The migration process is observed to be accompanied by macroscopic transient discontinuous shear thickening, which is consequently not an intrinsic property of granular suspensions.
TL;DR: The magnitude of shear rates in idealized severely stenotic coronary arteries (≥75% by diameter) are quantified using computational fluid dynamics to characterize the shear environment that may exist during atherothrombosis.
TL;DR: In this paper, a simple kinetic model is presented for the shear rheology of a dilute suspension of particles swimming at low Reynolds number, and the configuration of the suspension is characterized by the particle orientation distribution, which satisfies a Fokker-Planck equation including the effects of the external shear flow, rotary diffusion, and particle tumbling.
Abstract: A simple kinetic model is presented for the shear rheology of a dilute suspension of particles swimming at low Reynolds number. If interparticle hydrodynamic interactions are neglected, the configuration of the suspension is characterized by the particle orientation distribution, which satisfies a Fokker-Planck equation including the effects of the external shear flow, rotary diffusion, and particle tumbling. The orientation distribution then determines the leading-order term in the particle extra stress in the suspension, which can be evaluated based on the classic theory of Hinch and Leal (J Fluid Mech 52(4):683–712, 1972), and involves an additional contribution arising from the permanent force dipole exerted by the particles as they propel themselves through the fluid. Numerical solutions of the steady-state Fokker-Planck equation were obtained using a spectral method, and results are reported for the shear viscosity and normal stress difference coefficients in terms of flow strength, rotary diffusivity, and correlation time for tumbling. It is found that the rheology is characterized by much stronger normal stress differences than for passive suspensions, and that tail-actuated swimmers result in a strong decrease in the effective shear viscosity of the fluid.
TL;DR: In this paper, a comprehensive experimental and numerical study is presented, aiming at a more judicious use of hat-shaped specimens and a fundamental understanding of the obtained results, and the relation between the global measured response and the local material behaviour is studied.
TL;DR: The results show that platelets in vitro exposed beyond a threshold of high-shear stress are primed for subsequent activation under normal cardiovascular circulation conditions, and they do not recover from the initial high- shear insult.
Abstract: Individuals with mechanical heart valve implants are plagued by flow-induced thromboembolic complications, which are undoubtedly caused by platelet activation Flow fields in or around the affected regions involve brief exposure to pathologically high-shear stresses on the order of 100 to 1000 dyne/cm2 Although high shear is known to activate platelets directly, their subsequent behavior is not known We hypothesize that the post-high-shear activation behavior of platelets is particularly relevant in understanding the increased thrombotic risk associated with blood-recirculating prosthetic cardiovascular devices Purified platelets were exposed to brief (5–40 s) periods of high-shear stress, and then exposed to longer periods (15–60 min) of low shear Their activation state was measured using a prothrombinase-based assay Platelets briefly exposed to an initial high-shear stress (eg, 60 dyne/cm2 for 40 s) activate a little, but this study shows that they are now sensitized, and when exposed to subsequent low shear stress, they activate at least 20-fold faster than platelets not initially exposed to high shear The results show that platelets in vitro exposed beyond a threshold of high-shear stress are primed for subsequent activation under normal cardiovascular circulation conditions, and they do not recover from the initial high-shear insult
TL;DR: In this paper, the authors study the influence of tectonic stress fields on natural gas migration and accumulation in low-permeability rocks in the Kuqa Depression and analyze the evolution of the structure and tectonuclear stress field.
TL;DR: In this article, a carbopol microgel fluid was studied and the authors showed that the total duration of the fluidization process scales as τ √ √ f √ 1/(σ − ϵ - ϵ c) √ β √ ϵ − β, where β is an exponent which only depends on the microgel properties and not on the gap width or on the boundary conditions.
Abstract: Stress-induced fluidization of a simple yield stress fluid, namely a carbopol microgel, is addressed through extensive rheological measurements coupled to simultaneous temporally and spatially resolved velocimetry. These combined measurements allow us to rule out any bulk fracture-like scenario during the fluidization process such as that suggested in [Caton {\it et al., Rheol Acta}, 2008, {\bf 47}, 601-607]. On the contrary, we observe that the transient regime from solidlike to liquidlike behaviour under a constant shear stress $\sigma$ successively involves creep deformation, total wall slip, and shear banding before a homogeneous steady state is reached. Interestingly, the total duration $\tau_f$ of this fluidization process scales as $\tau_f \propto 1/(\sigma - \sigma_c)^{\beta}$, where $\sigma_c$ stands for the yield stress of the microgel, and $\beta$ is an exponent which only depends on the microgel properties and not on the gap width or on the boundary conditions. Together with recent experiments under imposed shear rate [Divoux {\it et al., Phys. Rev. Lett.}, 2010, {\bf 104}, 208301], this scaling law suggests a route to rationalize the phenomenological Herschel-Bulkley (HB) power-law classically used to describe the steady-state rheology of simple yield stress fluids. In particular, we show that the {\it steady-state} HB exponent appears as the ratio of the two fluidization exponents extracted separately from the {\it transient} fluidization processes respectively under controlled shear rate and under controlled shear stress.
TL;DR: Nonlinear rheological properties of metallo-supramolecular networks formed by the reversible cross-linking of semi-dilute unentangled solutions of poly(4-vinylpyridine) (PVP) in dimethyl sulfoxide (DMSO) are reported.
Abstract: We report here the nonlinear rheological properties of metallo-supramolecular networks formed by the reversible cross-linking of semidilute unentangled solutions of poly(4-vinylpyridine) (PVP) in dimethyl sulfoxide (DMSO). The reversible cross-linkers are bis-Pd(II) or bis-Pt(II) complexes that coordinate to the pyridine functional groups on the PVP. Under steady shear, shear thickening is observed above a critical shear rate, and critical shear rate is experimentally correlated with the lifetime of the metal-ligand bond. The onset and magnitude of the shear thickening depend on the amount of cross-linkers added.Incontrasttothebehaviorobservedinmosttransientnetworks,thetimescaleofnetworkrelaxationis found to increase during shear thickening. The primary mechanism of shear thickening is ascribed to the shear-induced transformation of intrachain cross-linking to interchain cross-linking, rather than nonlinear high tension along polymer chains that are stretched beyond the Gaussian range.
TL;DR: In this paper, the authors derived the mathematical criteria for the formation of secondary loops in large amplitude oscillatory shear deformation and quantified the location of the apparent intersection, and furthermore suggested a qualitative physical understanding for the associated nonlinear material behavior.
Abstract: When the shear stress measured in large amplitude oscillatory shear (LAOS) deformation is represented as a 2-D Lissajous–Bowditch curve, the corresponding trajectory can appear to self-intersect and form secondary loops. This self-intersection is a general consequence of a strongly nonlinear material response to the imposed oscillatory forcing and can be observed for various material systems and constitutive models. We derive the mathematical criteria for the formation of secondary loops, quantify the location of the apparent intersection, and furthermore suggest a qualitative physical understanding for the associated nonlinear material behavior. We show that when secondary loops appear in the viscous projection of the stress response (the 2-D plot of stress vs. strain rate), they are best interpreted by understanding the corresponding elastic response (the 2-D projection of stress vs. strain). The analysis shows clearly that sufficiently strong elastic nonlinearity is required to observe secondary loops on the conjugate viscous projection. Such a strong elastic nonlinearity physically corresponds to a nonlinear viscoelastic shear stress overshoot in which existing stress is unloaded more quickly than new deformation is accumulated. This general understanding of secondary loops in LAOS flows can be applied to various molecular configurations and microstructures such as polymer solutions, polymer melts, soft glassy materials, and other structured fluids.
TL;DR: The results show that the dissected aorta is dominated by locally highly disturbed, and possibly turbulent, flow with strong recirculation, and the non-Newtonian behavior of blood into the same transitional flow model has yielded a slightly lower peak wall shear stress and higher maximum turbulence intensity without causing discernible changes to the distribution patterns.
Abstract: Aortic dissection is the most common acute catastrophic event affecting the thoracic aorta. The majority of patients presenting with an uncomplicated type B dissection are treated medically, but 25% of these patients develop subsequent aneurysmal dilatation of the thoracic aorta. This study aimed at gaining more detailed knowledge of the flow phenomena associated with this condition. Morphological features and flow patterns in a dissected aortic segment of a presurgery type B dissection patient were analyzed based on computed tomography images acquired from the patient. Computational simulations of blood flow in the patient-specific model were performed by employing a correlation-based transitional version of Menter's hybrid k-epsilon/k-omega shear stress transport turbulence model implemented in ANSYS CFX 11. Our results show that the dissected aorta is dominated by locally highly disturbed, and possibly turbulent, flow with strong recirculation. A significant proportion (about 80%) of the aortic flow enters the false lumen, which may further increase the dilatation of the aorta. High values of wall shear stress have been found around the tear on the true lumen wall, perhaps increasing the likelihood of expanding the tear. Turbulence intensity in the tear region reaches a maximum of 70% at midsystolic deceleration phase. Incorporating the non-Newtonian behavior of blood into the same transitional flow model has yielded a slightly lower peak wall shear stress and higher maximum turbulence intensity without causing discernible changes to the distribution patterns. Comparisons between the laminar and turbulent flow simulations show a qualitatively similar distribution of wall shear stress but a significantly higher magnitude with the transitional turbulence model.
TL;DR: In this article, the authors focus on the evolution of structure and stress for an experimental system of 2D photoelastic particles that is subjected to multiple cycles of pure shear.
Abstract: This work focuses on the evolution of structure and stress for an experimental system of 2D photoelastic particles that is subjected to multiple cycles of pure shear. Throughout this process, we determine the contact network and the contact forces using particle tracking and photoelastic techniques. These data yield the fabric and stress tensors and the distributions of contact forces in the normal and tangential directions. We then find that there is, to a reasonable approximation, a functional relation between the system pressure, P, and the mean contact number, Z. This relationship applies to the shear stress τ, except for the strains in the immediate vicinity of the contact network reversal. By contrast, quantities such as P, τ and Z are strongly hysteretic functions of the strain, e. We find that the distributions of normal and tangential forces, when expressed in terms of the appropriate means, are essentially independent of strain. We close by analyzing a subset of shear data in terms of strong and weak force networks.