Transient Shear Rheology Research Articles

Constitutive modeling of dilute wormlike micelle solutions: Shear-induced structure and transient dynamics

We present a reformulation of the 'reactive rod model' (RRM) of Dutta and Graham [Dutta, Sarit and Graham, Michael D., JNNFM 251 (2018)], a constitutive model for describing the behavior of dilute wormlike micelle solutions. The RRM treats wormlike micelle solutions as dilute suspensions of rigid Brownian rods undergoing reversible scission and growth in flow. Evolution equations for micelle orientation and stress contribution are coupled to a kinetic reaction equation for a collective micelle length, producing dynamic variations in the length and rotational diffusivity of the rods. This model has previously shown success in capturing many critical steady-state rheological features of dilute wormlike micelle solutions, particularly shear-thickening and -thinning, non-zero normal stress differences, and a reentrant shear stress-shear rate curve, and could fit a variety of steady state experimental data. The present work improves on this framework, which showed difficulty in capturing transient dynamics and high-shear behavior, by reformulating the kinetic equation for micelle growth on a more microstructural (though still highly idealized) basis. In particular, we allow for micelle growth associated with strong alignment of rods and breakage due to tensile stresses along the micelles. This new formulation captures both steady and transient shear rheology in good agreement with experiments. We also find good agreement with available steady state extensional rheology.

Evaluating rheological models for human blood using steady state, transient, and oscillatory shear predictions

The rheological characterization of human blood, through modeling and analysis of steady state, transient, and oscillatory shear flows, has made tremendous progress recently. Due to the aggregation of red blood cells at low shear rates, many recent models for blood rheology include a structural, thixotropic component with one of the most recent attempts unifying this approach with a viscoelastic formulation. We will show how these models, along with proposed modifications to another recent structural, kinetic thixotropy model, can improve modeling predictions. Results are compared to the Maxwell-like Bautista-Manero-Puig model, the Oldroyd-8 inspired viscoelastic Anand-Kwack-Masud model, a viscoelastic-thixotropic model from Blackwell and Ewoldt, and the Herschel-Bulkley model. We explore the weaknesses of the legacy blood models and then demonstrate the efficacy of the newly improved models for modeling human blood steady state and transient shear rheology to predict oscillatory shear flow.

The transient shear rheology of a thermotropic liquid crystalline polymer in the super-cooled state

Abstract To determine the processing temperature of a representative thermotropic liquid crystalline polymer (TLCP) in the super-cooled state, small amplitude oscillatory shear (SAOS), the startup of shear flow and differential scanning calorimetry (DSC) measurements were performed. Good agreement on the onset crystallization temperature was obtained from the SAOS and the startup of shear flow. Compared with DSC, the rheological measurements were more sensitive for detecting crystallization in the super-cooled state. The rheological study suggested that crystallization did not occur unless the TLCP was cooled 30 °C below its melting point, and its viscosity was significantly increased as the material was super cooled. Additionally, super cooling the TLCP did not significantly affect the relaxation of shear stress after preshearing. However, the recovery of transient shear stress in the interrupted shear measurements was suppressed to a great extent in the super-cooled state.

Viscoelastic properties of solutions of polystyrene melts and carbon dioxide: Analysis of a transient shear rheology approach

In this work, the viscoelastic, thermal, and diffusion properties of solutions of polystyrene melts and carbon dioxide (CO2) were analyzed using plate–plate rheometry in the transient mode. The objective of this study was to evaluate a transient shear rheology approach for high viscosity polymer melts, to verify superposition principles for polystyrene/CO2 solutions, and to measure the glass transition temperature as a function of pressure and CO2 concentration. Two different procedures of saturating polystyrene with carbon dioxide were applied, i.e., loading with the blowing agent below the glass transition temperature of polystyrene and at the measurement temperature. Stress-growth experiments in shear were performed in order to measure the transient viscosity of polystyrene/CO2 solutions in the linear regime. A shift of the transient viscosity data to a mastercurve was applied in order to determine the shift factor of the viscosity and the average relaxation time. Our data indicate that the steady-state viscosity and the average relaxation time are proportional to the temperature–pressure–concentration shift factor aT,p,c within experimental scatter, and consequently a time–temperature–pressure–concentration superposition principle holds for polystyrene/CO2 solutions. Whereas the viscosity of polystyrene strongly depends on the applied temperature and pressure, the elastic equilibrium compliance depicts only a weak dependence on temperature and pressure.

Quantitative mesoscale modeling of the oscillatory and transient shear rheology and the extensional rheology of pressure sensitive adhesives

The development of novel pressure sensitive adhesives, formed by the drying of a polymer latex emulsion, is hampered by a lack of understanding of the relation between microscopic details and the large-scale rheology. In a previous paper [Soft Matter, 2011, 7, 5036] we introduced a coarse-grained computer simulation model that aims to provide such a link. To reach sufficiently large time and length scales each latex particle is represented by just six degrees of freedom, and transient forces are introduced to capture the effect of slow changes in the degree of chain intermixing and in the number of sticker groups shared between pairs of latex particles. In this paper we show that this model can nearly quantitatively predict the shear and extensional nonlinear rheology by careful tuning of only a few parameters to the linear rheology. We find a complex transient viscosity with multiple inflection points and maxima under shear flow, as well as a strong strain hardening under extensional flow, all in agreement with experimental observations. We investigate the influence of each of the model's parameters on the linear and nonlinear rheology.

Rheological characterization in shear of a model dumbbell polymer concentrated solution

We investigate the linear and non-linear rheological behavior in shear of a concentrated solution of “dumbbell” polystyrene with long linear backbone and dense short brushes at both ends and compare it with corresponding linear polymers. This type of dumbbells has never been rheologically characterized before. In linear viscoelasticity, the dumbbell polymers show significant differences with conventional linear polymers. In particular, the reptation relaxation of the dumbbell is strongly slowed down. Furthermore, the addition of the side chains increases the friction so that the dumbbell lies above the η 0 vs. number of entanglements relation of linear samples. Transient shear rheology experiments on weakly entangled solutions show a retardation of the chain stretch relaxation of the dumbbell by a factor 2.5 vs. a linear polymer with the same Rouse time. Additionally, a second peak in the transient viscosity is observed at high shear rates.

Transient shear rheology of carbon nanofiber/polystyrene melt composites

We characterize the transient shear rheology of polystyrene/carbon nanofiber composites. Our experimental measurements of the composites show increasing stress overshoot responses to transient shear as the carbon nanofiber concentration increases. We also find the steady state viscosity reached at long times during application of a constant shear rate increases with increasing carbon nanofiber concentration. Flow reversal experiments show the effects of nanofiber orientation and structural evolution on the composite's rheological response. We present a microstructurally based constitutive model where all but two parameters are determined by rheological characterization of the pure polymer and the shape and concentration of the nanoparticles. The Folgar-Tucker constant, C I , is treated as a fitting parameter, while several definitions for the shape factors A, B, C and F are evaluated. We make note of the effects each parameter has on the model's predictions. We find that the constitutive model is in agreement with our experimentally measured transient shear rheology of the PS/CNF melt composites for the CNF concentrations and shear rates presented.

Dynamic properties of shear thickening colloidal suspensions

The transient shear rheology (i.e., frequency and strain dependence) is compared to the steady rheology for a model colloidal dispersion through the shear thickening transition. Reversible shear thickening is observed and the transition stress compares well to theoretical predictions. Steady and transient shear thickening are observed to occur at the same value of the average stress. The critical strain for shear thickening is found to depend inversely on the frequency at fixed applied stress for low frequencies (high strains), but is limited to an apparent minimum critical strain at higher frequencies. This minimum critical strain is shown to be an artifact of slip. Lissajous plots illustrate the transition in material properties through the shear thickening transition, and the energy dissipated by a shear thickening suspension is analyzed as a function of strain amplitude.