Critical Shear Rate Research Articles

Dry polymer model for a string of pushers: Nontrivial scaling of tumbling time with shear rate

Understanding the dynamics of active polymers is an important component for modeling the dynamic response of biological cells and microorganisms when subjected to external forces. A minimal computational model of active filaments that captures all the essential features of activity-induced filament motion is required to study large systems of dense suspensions. In this paper we present a model of a dry active polymer, compare its steady-state static and dynamic properties with those of a polymer consisting of extensile stresslets, and show that the two models give very similar results. When subjected to shear flow, both models show two distinct regimes in the tumbling interval as a function of shear rate, with a diffuse crossover regime in between. At high shear rate, the active and passive polymers follow the same power law for the tumbling interval. However, the tumbling interval is smaller than that of the passive polymer for low shear rates and larger for the high shear rate. We show that activity-induced large bends increase the coupling of the polymer with the shear gradient in the low-shear-rate regime, leading to a decrease in the tumbling interval. However, in the high-shear-rate regime, the active forces on the folds at the end of the stretched polymer prevent it from tumbling, leading to a larger tumbling interval. We show that the critical shear rate, at which the tumbling interval versus the shear-rate curve of the active polymer crosses that of the passive polymer, is determined by the balance of bending and shear forces and increases with κ/N3, where κ is the bending coefficient and N is the length of the polymer. Published by the American Physical Society 2024

The effects of MWCNT-STF on the impact resistance of fibre metal laminates

Fibre metal laminates (FMLs) have been widely used in the aerospace and transportation industries, due to their excellent impact and fatigue resistance. However, it is challenging to further improve the impact resistance of FMLs. In this study, shear thickening fluids (STFs) with different ratios of silicon nanoparticles to polyethylene glycol (PEG) were first produced to obtain the STF with optimized performance. Then, multiwalled carbon nanotubes (MWCNTs) were added to the STF to produce MWCNT-STF to further improve its shear thickening behaviour. The rheological test results have shown that the MWCNT-STF with the composition ratio of 30 wt% PEG200+0.2 wt% MWCNTs+69.8 wt% SiO2 demonstrates the best shear thickening function, with the peak viscosity being enhanced by 365 % but the critical shear rate reduced by 69 % compared with the original STF. The MWCNT-STF was then impregnated to Kevlar fabric by ultrasonic impregnation process. The experimental results show that MWCNTs contribute the best adhesion to Kevlar fabric when the ratio of MWCNT-STF to alcohol is 1:2, with the optimal add-on ratio as 53 %. The yarn pulling-out tests show that the addition of MWCNTs to STF improves the friction between the fabric yarns, which in turn increases their shear resistance. Then STF and MWCNT-STF Kevlar fabrics were integrated to FML manufacturing. The quasi-static perforation tests indicate that MWCNT-STF helps improve the specific energy absorption of the MWCNT-STF FMLs by up to 16 %, but with limited impact on the peak perforation force. More effectively, the low velocity impact tests show that the 3/2 FMLs made with 1, 2, and 3 layers of MWCNT-STF Kevlar fabric have an enhancement on their peak impact force by 20, 36 and 38 %, while on specific energy absorption by 30, 40 and 42 %, respectively, compared to 3/2 STF FMLs counterparts.

Influence of physical properties and shear rate on static liquefaction of saturated loess

Flow sliding instability of saturated loess slopes is a common geological hazard in loess areas of China. Previous studies have found that the occurrence of flow-slip loess landslides is closely related to static liquefaction and is controlled by physical characteristics and load conditions. In this work, we comprehensively study these influences of physical properties (i.e. initial pore structure, gradation, dry density) and shear rate on the static liquefaction of saturated loess through a series of consolidated undrained triaxial tests, and the effect mechanism of these related factors on the static liquefaction of saturated loess are also discussed. The results present that: (1) The peak deviator stress and the maximum pore pressure of the undisturbed loess are much greater than these of the remodeled one under each level of confining pressure (except 450 kPa). Further, the calculated liquefaction potential index (LPI) of undisturbed loess is much greater, indicating that undisturbed loess is more prone to static liquefaction due to the initial pore structure. (2) The lower the relative clay/silt content ratio of the saturated remodeled loess, the stronger the potential liquefaction ability. With the increase of the relative content of clay from 0.125 to 0.698, the stress-strain curve gradually transitions from strain softening to hardening. (3) The remodeled loess show the steady-state strength tends to continuously increase with the increase of dry density from 1.38 g/cm3 to 1.56 g/cm3, while the LPI increases first and then decreases, and the largest value appears when the dry density reaches to 1.44 g/cm3. The reason is that the value of 1.44 g/cm3 is the normal consolidated condition, which essentially reflects the potential liquefaction change during the transformation from under consolidated to over consolidated state.(4) The effect of shear rate on the stress-strain curve of remolded loess is not significant, but the peak strength and ultimate pore pressure show a trend of increasing and then decreasing with the increase of shear rate. There exists a “critical shear rate “of 0.1 mm/min reflecting the liquefaction of loess is more likely to occur when reaches to this critical value. (5) Based on the comparison of static liquefaction tests, the influencing factors of static liquefaction of saturated loess are: initial pore structure> gradation > dry density > shear rate. This study can provide a systematic evaluation for understanding the influencing factors of static liquefaction capacity of saturated loess (especially remolded one), and also has a reference for explaining the loess flow-sliding failure mechanism under disturbance conditions.

On the rheological properties of Silica – Poly(ethylene-oxide) dispersions: "Shake-gels" II The effect of silica concentration and molecular weight of polymer

Using rheological techniques, the effect of silica concentration and polyethylene oxide molecular weight has been investigated to follow the gelation/shear thickening behaviour of PEO silica water mixtures, a phenomenon which has become known as "shake gels”. By monitoring the viscosity as a function of the shear rate it was observed that at a critical shear rate the viscosity increased by many orders of magnitude. The higher the silica concentration and the higher the polymer molecular weight the lower the shear rate required for the gel to form. Once formed by dropping the shear rate to a low value or by monitoring the viscoelastic moduli the relaxation of the gel back into the liquid state could be followed. It was noted that the lower the polymer molecular weight or the lower the silica concentration the faster the relaxation process was. It is postulated that the silica particles effectively cross link the gel so that the more silica particles per polymer molecule would result in slower relaxation. Conversely in the gel formation stage the more silica particles per polymer the faster the gel forms are a consequence of the ease by which the silica particles can cross link to form the gel.

Application and Optimization of Rheological Mathematical Models in Preparation and Injection Process of Polymer Flooding

The impact of shear on the properties of polymer solutions is not fully accounted for in the simulation of conventional rheological models. To optimize the application of these mathematical models, the shear rheological characteristics of polyacrylamide at various concentrations were investigated under different shear rates and shear modes. The results indicate that pseudoplastic fluid polymers exhibit two distinct rheological characteristics within their "shear thinning" rate range. The critical shear rate at this threshold signifies the point at which shear stress begins to disrupt the structure of the polymer solution, resulting in a rheological curve that transitions from high to low shear rates without reverting back to a low-to-high state. The concentration of the solution has minimal effect on the critical shear rate of the polymer, which is primarily determined by the inherent properties of the polymer itself. The application of rheological models during preparation and injection processes can elucidate the effects of shear on polymers by analyzing changes in apparent viscosity. Furthermore, the rheological model following supercritical shear rates requires modifications to the consistency coefficient (K) and flow index (n) based on shear rheological data obtained from high to low shear rates.

Shear Thickening, Star-Shaped Polymer Electrolytes for Lithium-Ion Batteries.

The safety concerns associated with current lithium-ion batteries are a significant drawback. A short-circuit within the battery's internal components, such as those caused by a car accident, can lead to ignition or even explosion. To address this issue, a polymer shear thickening electrolyte, free from flammable solvents, has been developed. It comprises a star-shaped oligomer derived from a trimethylolpropane (TMP) core and polyether chains, along with the inclusion of 20 wt.% nanosilica. Notably, the star-shaped oligomer serves a dual function as both the solvent for the lithium salt and the continuous phase of the shear thickening fluid. The obtained electrolytes exhibit an ionic conductivity of the order of 10-6 S cm-1 at 20 °C and 10-4 S cm-1 at 80 °C, with a high Li+ transference number (t+ = 0.79). A nearly thirtyfold increase in viscosity to a value of 1187 Pa s at 25 °C and a critical shear rate of 2 s-1 were achieved. During impact, this electrolyte could enhance cell safety by preventing electrode short-circuiting.

Significance of Thompson and Troian slip effects on Fe3O4-CoFe2O4 ethylene glycol-water hybrid nanofluid flow over a permeable plate

This study aims to analyze the electromagnetic, hydro-thermal characteristics of hybrid nanofluid (Fe3O4-CoFe2O4/EG-water) flow over a permeable plate considering the Thompson and Troian slip, and suction/injection effects. By employing suitable similarity transformations, the governing partial differential equations (PDEs) describing the flow are converted into a set of highly nonlinear ordinary differential equations (ODEs). These transformed equations are computationally solved using the “bvp4c technique” in MATLAB. The detailed graphs illustrate the impact of various factors for instance, the Hartmann number, electric parameter, velocity slip parameter, critical shear rate, permeability parameter, heat generation/absorption parameter and suction/blowing parameter on velocity and temperature distributions. Also, the tabular data facilitates a detailed quantitative examination of these influences. The velocity and temperature profiles exhibit opposite nature influenced by factors such as the Hartmann number, velocity slip, critical shear rate, permeability parameter, and suction/blowing effects. Furthermore, as the critical shear rate, velocity slip, permeability and suction/blowing parameters increase, so do the absolute values of the Nusselt number. Conversely, higher heat generation/absorption parameter and Eckert number cause a decrease in these absolute values.

Shear-thickening-fluid-based meta-material for adaptive impact response

Lattice and Triply-Periodic-Minimal-Surface (TPMS) structures are widely employed in different engineering disciplines, combining interesting designs with optimal mechanical properties thanks to the capabilities of Additive Manufacturing. In this framework, a technique based on filling a 3D-printed flexible structure with a shear-thickening fluid (STF) is proposed to improve the mechanical response of the resulting bi-phase composite under impact loads in the low-velocity regime up to around 3 m/s. Test data indicate that once the critical shear rate threshold for the fluid is reached, the STF-filled structure can effectively smooth the peak acceleration by 50%, decrease the penetration depth and significantly enhance the energy absorption capability by a remarkable 85%. The proposed hybrid composite paves the way to novel functional applications, exploiting the adaptive behavior of non-Newtonian fillers. To this end, a Finite-Element model is proposed to further investigate and optimize the underlying fluid-structure interaction responsible for improvement of the dynamic compressive response.

Melt-spun polylactide/ethylene vinyl alcohol copolymer fiber

Polylactide/ethylene vinyl alcohol copolymer (PLA/EVOH) blends and fibers with different weight ratios were prepared by melt blending, and two-step melt spinning, respectively. PLA and EVOH in PLA/EVOH blends were immiscible. When EVOH content was ≤60 %, EVOH with the average diameter of about 3 μm was dispersed in PLA matrix uniformly. The dual continuous phases could be observed in PLA/EVOH blend with 70 wt% EVOH. When the EVOH content was ≥80 %, the spherical PLA phase with the diameter of 0.25 to 1 μm was dispersed in EVOH matrix. The introduction of EVOH as nucleating agent could promote the crystallization of PLA. Both PLA and EVOH components in PLA/EVOH blends formed individual crystal phases. The viscosity of PLA/EVOH blend with 5 % EVOH was lower than that of neat PLA. The viscosity of PLA/EVOH blends with the EVOH content of ≥10 % was much higher than that of neat PLA, which showed obvious shear thinning behavior. With the increase of EVOH content, the shear thinning behavior became obvious and the critical shear rate decreased gradually. The drawn PLA/EVOH fibers with the tensile strength of ≥16 cN/tex exhibited good mechanical properties. In addition, the introduction of EVOH could improve the hydrophilicity of PLA fibers.

Synthesis and rheological performance of shear-thickening waterborne polyurethane

Shear-thickening fluids (STFs) are a new type of intelligent material with excellent performance whose viscosity increase sharply with the increase of shear rate or shear stress. However, the synthesis yield of dispersed phase particles is low, and the particle re-dispersion process is challenging for the industrial production of STFs. In this work, through structural design, a waterborne polyurethane (WPU) with typical shear-thickening properties was synthesized for the first time. This synthesis process is conducive to industrial production. The rheological properties of the synthesized WPU at different concentrations, temperatures, and pH were studied using a rheometer. The results showed that the WPU exhibited typical shear-thickening behavior. However, due to the special core–shell structure of the WPU particles, the shear rate has two transition responses to the shear-thickening behavior. With increasing concentration, the shear-thickening performance of the WPU is enhanced, and the critical shear rate is decreased. For the coexistence of Brownian motion and solvation, the rheological curve of the WPU exhibits a complex response to temperature increase; its shear-thickening behavior decreases with rising temperature, but the viscosity first decreases and then increases with temperature. Due to the presence of carboxyl groups on the surface of the WPU particles, its shear-thickening performance shows a strong response to pH. By appropriately adjusting the pH, the viscosity and particle size of the WPU can be increased through the ionization of carboxyl groups, thereby enhancing the shear-thickening behavior.

Thompson and Troian slip effects on ternary hybrid nanofluid flow over a permeable plate with chemical reaction

This study seeks to elucidate the electromagnetic hydro-thermal aspects of ternary hybrid nanofluid (THNF) (MoS2-SiO2-GO/water) flow over a permeable plate in the presence of Thomson and Troian slip, suction/blowing, and chemical reaction. Through the utilization of similarity transformations, the nonlinear partial differential equations (PDEs) representing the conservation of mass, momentum, energy, and concentration, are converted into ordinary differential equations (ODEs). The subsequent step involves solving these ODEs utilizing the bvp4c technique in MATLAB. The detailed graphical representations precisely elucidate the action of diverse parameters, such as Hartmann number, electric parameter, velocity slip parameter, Lewis number, critical shear rate, suction/blowing, thermophoresis, and Brownian motion parameter on the velocity, thermal, and concentration profiles. The momentum and temperature distributions respond oppositely to the parameters Hartmann number, electric parameter, and suction/blowing. Also, the absolute values of Nusselt and Sherwood numbers increase with elevated critical shear rate, velocity slip, and suction/blowing parameter but decrease with higher Lewis number and chemical reaction parameter. Moreover, adjusting the velocity slip parameter from 0.2 to 0.6 results in a 24.14% decrease in the skin friction coefficient and a 16.79% increase in the local Nusselt number.

Molecular dynamics study of the rheology of benzene-based nanofluids with metal particles

The purpose of this paper is to study the rheology of the nanofluids with ordinary spherical particles. The rheology of benzene and benzene-based nanofluids with copper and aluminium particles of 3 and 6 nm diameter been studied by the nonequilibrium molecular dynamics method. The maximum volume concentration of the nanoparticles was 6.11 %. It has been shown that all fluids considered, including benzene and benzene-based nanofluids, exhibit Newtonian behavior at low shear rates. However, with increasing shear rate, both benzene and nanofluids become pseudoplastic. The mechanisms of the rheology change of simple liquid and nanofluids have studied. This change in their rheology is a threshold phenomenon, in which the critical shear rate depends on the concentration, size, and material of the nanoparticles. The critical shear rate increases with increasing particle size and decreases with increasing particle concentration. Correlations describing the critical shear rates at which the rheology of nanofluids changes are obtained. It is shown that the rheology change is due to a change in the structure of both the base fluid and the nanofluid. In all cases, the change in rheology results from the weakening of the short-range order in these fluids. As the shear rate increases, the difference between viscosity of the base fluid and all studied nanofluids decreases monotonically.

Analytical solution of the Poiseuille flow of a De Kee viscoplastic fluid

We provide an explicit analytical solution of the planar Poiseuille flow of a viscoplastic fluid governed by the constitutive equation proposed by De Kee and Turcotte (1980). Formulae for the velocity and the flow rate are derived, making use of the Lambert W function. It is shown that a solution does not always exist because the flow curve is bounded from above and hence the rheological model can accommodate stresses only up to a certain limit. In fact, the flow curve reaches a peak at a critical shear rate, beyond which it exhibits a negative slope, giving rise to unstable solutions.

Attractive carbon black dispersions: Structural and mechanical responses to shear

The rheological behavior of colloidal dispersions is of paramount importance in a wide range of applications, including construction materials, energy storage systems, and food industry products. These dispersions consistently exhibit non-Newtonian behaviors, a consequence of intricate interplays involving colloids morphology, volume fraction, and interparticle forces. Understanding how colloids structure under flow remains a challenge, particularly in the presence of attractive forces leading to cluster formation. In this study, we adopt a synergistic approach, combining rheology with ultra small-angle x-ray scattering, to probe the flow-induced structural transformations of attractive carbon black (CB) dispersions and their effects on the viscosity. Our key findings can be summarized as follows. First, testing different CB volume fractions, in the high shear rate hydrodynamic regime, CB particles aggregate to form fractal clusters. Their size conforms to a power law of the shear rate, ξc∝γ˙−m, with m≃0.5. Second, drawing insights from the fractal structure of clusters, we compute an effective volume fraction ϕeff and find that microstructural models adeptly account for the hydrodynamic stress contributions. We identify a critical shear rate γ∗˙ and a critical volume fraction ϕeff∗, at which the clusters percolate to form a dynamical network. Third, we show that the apparent yield stress measured at low shear rates inherits its properties from the percolation point. Finally, through data scaling and the integration of Einstein’s viscosity equation, we revisit and discuss the Caggioni–Trappe–Spicer model, revealing a significant connection between its empirical parameters and the structural properties of CB dispersions under flow.

Probing the roles of surface characteristic of suspended nanoparticle in shear thickening suspensions

The surface characteristic of suspended nanoparticle is an extremely important property for the non-Newtonian behavior of shear thickening suspensions. Easy access to distinct surface characteristics and a deep understanding of the role of friction and adhesion forces remains challenging yet critical to study the suspension performance. In this work, by synthesizing mesoporous silica with different surface roughness and groups, the influence of surface characteristics on the non-Newtonian behavior of suspension is comprehensively explored. The results show that the suspensions containing particles with rough surface and hydroxyl groups on the surface have higher shear thickening power, lower critical shear rate, lower discontinuous shear thickening transition fraction and lower jamming fraction. Furthermore, the test of the first normal stress difference N1 shows that the interparticle friction and adhesion both reduce the critical volume fraction of the transition from fluid lubrication dominance to frictional contact dominance in the suspension. This finding deepens the understanding of phase transformation mechanism, and open an approach to accelerate the advanced shear thickening suspensions design for next-generation intelligent materials.

Study on the synergistic effect of additives on the processing performance of UHMWPE/HDPE blends

AbstractThe challenging melt processing of ultrahigh‐molecular‐weight polyethylene (UHMWPE) melt spinning hinders its efficiency and quality, which can be improved by processing aids that enhance its flowability. Building upon modifications of UHMWPE with high‐density polyethylene (HDPE), this article studies the effects of CaSt2, polyethylene glycol (PEG), silicone powder, and their compound additives on the processing performance of UHMWPE/HDPE blends. The modification effects and mechanisms are studied by analyzing processing torque, melt flow rate, viscoelastic activation energy, and rheological performance. The research results indicate that 1 wt% PEG significantly improves the processing flowability of UHMWPE/HDPE blends, and PEG mainly plays an internal lubrication role on the molecular chains of the blends. The combination of 0.5 wt% CaSt2 and 0.5 wt% silicone powder exhibits a synergistic effect of internal and external lubrication on the UHMWPE/HDPE blends melt processing, further improving the processing performance of UHMWPE/HDPE blends. Compared with unmodified blends, the maximum screw speed for obtaining qualified as‐spun filaments of UHMWPE/HDPE blends modified with CaSt2/silicone powder compound additives increases from 5 to 20 rpm, which means that the critical shear rate of the modified UHMWPE/HDPE blend melt processing is significantly improved. Meanwhile, the processing torque decreases by about 22%.

Aqueous dispersions of highly conductive carbon nanomaterials: Unusual aggregation mechanism

Determination of the optimal conductive composition and operational flow conditions are of profound significance for aqueous–based semisolid flow battery and supercapacitor applications. We systematically investigated the development of equilibrium microstructure and flow–induced structural transitions in aqueous dispersions of carbon black (CB) in 1 M LiNO3 at 25 °C. Simultaneous measurements of the impedance spectra and optical texture distinguished two unique percolated dispersions with distinct microstructure and aggregation mechanisms around a critical CB content: CCB = 4 wt%. A tenuous network of weakly–linked aggregates with rapid diffusion–limited aggregation mechanism (DLA) is formed at CCB < 4 wt%. A compact network of strongly fused aggregates with slow reaction–limited aggregation mechanism (RLA) dominates the dispersion microstructure above the critical CCB. The percolated dispersions commonly exhibited a three–regime (two thinning separated by thickening regime) flow curve consistent with structural breaking up (conductivity loss) and alignment of flowing units (conductivity recovery) at a critical shear rate that pertain to CCB.

Droplet coalescence in coupled shear and electric fields: A molecular dynamics study

Water droplet coalescence in coupled flow and electric fields is a phenomenon extensively observed in petrochemical, microfluidic, and chemical synthesis processes; however, the existing research in this domain lacks the necessary depth. This paper employs a molecular dynamics approach to elucidate the microscopic mechanisms governing droplet aggregation in water-in-oil emulsions. The investigation spans various shear rates and droplet angles, considering droplet morphology and molecular interactions. The study reveals an interrelation between shear rate and droplet angle effects on droplet coalescence, both seeking to understand the impact of the flow field's shear effect on the electrocoalescence of water droplets. Increasing the shear rate proves advantageous to droplet aggregation up to a certain threshold; however, complete coalescence is hindered beyond the critical shear rate, γ = 7.5 × 109 s−1. Augmenting the droplet angle amplifies the shear force exerted by the flow field on the droplets, while diminishing the radial electric field force. Consequently, an optimal droplet angle of θ = 25° facilitates comprehensive droplet merging. The analysis of the coalescence process, interaction energy, charge density and radial distribution function shows that the approximate process of droplet motion is as follows: the shear effect drives the droplets to approach, and then the directional migration of charge and electrostatic attraction between water molecules drive the coalescence. The existence of a moderate shear effect enables complete droplet coalescence. This study offers theoretical guidance for the exploration of dynamic electrocoalescence technology.

Dynamics and Structures of Amyloid Aggregates under Fluid Flows.

In this work, we investigate how fluid flows impact the aggregation mechanisms of Aβ40 proteins and Aβ16-22 peptides and mechanically perturb their (pre)fibrillar aggregates. We exploit the OPEP coarse-grained model for proteins and the Lattice Boltzmann Molecular Dynamics technique. We show that beyond a critical shear rate, amyloid aggregation speeds up in Couette flow because of the shorter collisions times between aggregates, following a transition from diffusion limited to advection dominated dynamics. We also characterize the mechanical deformation of (pre)fibrillar states due to the fluid flows (Couette and Poiseuille), confirming the capability of (pre)fibrils to form pathological loop-like structures as detected in experiments. Our findings can be of relevance for microfluidic applications and for understanding aggregation in the interstitial brain space.

Rheology of paste-like food inks for 3D printing: Effects of nutrient and water content

This research delves into understanding the effects of composition on the rheological response of multi-component food inks for 3D food printing. Accordingly, the motivation is to decouple the nutrient and water content effects on the rheology. We formulated inks by combining pea fractions with water and employing a water-holding-capacity based hydration method. Rheology is characterized by steady shear rate and oscillatory strain amplitude sweeps. Strain sweep curves infer that the deformation response of all inks follows a similar trend, and samples sharing the same macronutrient formulation are mapped to a master curve after scaling with the elastic plateau modulus. Samples sharing the same macronutrient formulation mapped to a master curve after scaling with the elastic modulus. Shear rate testing showed that the inks were shear thinning yield stress materials. Shear rate sweeps also collapsed on a master curve scaled by the yield stress and critical shear rate on the y and x axes. The yield stress and the plateau modulus appeared to be controlled by the water content, while the shear and strain thinning exponents were independent of the formulations, inferring that the rheology is scaled by the water content while preserving the shear thinning response. Observing the independence of the rheological properties from the nutrient composition and scalability of the rheology by the water content provided a step forward in developing formulations with various nutrient content at desired ow properties, which promises personalized nutrition. Furthermore, the study shows the applicability of various rheological techniques, which are expected to contribute to the literature on the rheology of granular pastes.