Rheologica Acta最新文献

Concentrated emulsions and foams and microgels are comprised of deformable particles making these materials display complex rheological behavior that includes a yielding transition from an elastic solid to viscous fluid. Most studies of this class of soft matter involve shear flows and only a few report both shear and normal stresses. Here, we report measurements of the shear stress and two normal stress differences for a Carbobol microgel, which is usually classified as simple yield stress fluid, subjected to constant shear rate flows. Similar to our previous study, the shear stress evolves through the yield point in a manner indicative of simple yield stress fluid behavior while the normal stress differences evolve in a reproducibly chaotic manner. We also find that the evolution of the stresses is dependent on the whether the microgel has been in a state of relaxation or recovery prior to the measurement.

The mechanisms of flow in suspensions of soft particles above the glass-transition volume fraction and in the jammed state were probed using orthogonal superposition rheometry (OSR). A small amplitude oscillatory shear flow is superimposed orthogonally onto a steady shear flow, which allows monitoring the viscoelastic spectra of sheared jammed core–shell microgels during flow. The characteristic crossover frequency ω_c, deduced from the viscoelastic spectrum, provides information about the shear-induced structural relaxation time, which is connected to the microscopic yielding mechanism of cage breaking. The shear rate evolution of the crossover frequency is used to achieve a superposition of all spectra and get a better insight of the flow mechanism. Despite their inherent softness, the hybrid core–shell microgels exhibit similarities with hard sphere-like flow behavior, with the main difference that for the microgels, the transition from a glassy to a jammed state introduces a volume fraction dependence of the scaling of ω_c with shear rate. We further check the application of the Kramers–Kronig relations on the experimental low strain amplitude OSR data finding a good agreement. Finally, the low frequency response at high strain rates was investigated with open bottom cell geometry, and instrumental limits were identified. Based on these limits, we discuss previous OSR data and findings in repulsive and attractive colloidal glasses and compare them with the current soft particle gels.

We report the linear and nonlinear rheological properties of human serum albumin (HSA) hydrogels with and without urea using small (SAOS) and large (LAOS) amplitude oscillatory shear tests. In SAOS tests, pure HSA and HSA-urea hydrogels exhibited a predominantly solid-like behavior (G' ≫ G''), with viscoelastic properties proportional to HSA concentrations. As urea concentration increased, the viscoelastic properties of the hydrogels decreased and the frequency dependence declined, indicating a sparser cross-linking network. Under LAOS flow, both pure HSA and HSA-urea hydrogels exhibited intra- and inter-cycle strain-stiffening, which became more pronounced with increasing urea concentration. The presence of urea delayed the onset of the nonlinear behavior in HSA-urea hydrogels, in proportion to the urea concentration, decreasing the hydrogel strength. The degree of nonlinearity, quantified by the intrinsic nonlinear parameter Q₀ from Fourier-transform rheology, decreased with increasing urea concentration. Additionally, critical strain amplitudes obtained from LAOS tests indicated that the yield strain of the HSA-urea hydrogels increased with urea concentration. The intracycle behavior was analyzed by sequence of physical processes methods. Molecular dynamics simulations were performed to analyze the hydrogels behavior at the atomic level. Significant changes in the hydrogel network were attributed to the efficient insertion of urea into the HSA hydrogen bond network, which forms the cross-linking network. Thus, the hydrogen bonding between urea and HSA, as well as urea’s contribution to HSA denaturation, affects the gelation, linear, and nonlinear viscoelastic properties of the HSA hydrogels.

By considering the rotationality of shear flow, we distinguish between tube segments created by reptation before the inception of shear flow and those created during flow. Tube segments created before inception of shear flow experience both stretch and orientation, while tube segments created after inception of flow are not stretched, but are only aligned in the flow direction. Based on this idea, the Rotation Zero Stretch (RZS) model allows for a quantitative description of the start-up of shear flow and stress relaxation after step-shear strain experiments, in agreement with data of polystyrene long/short blends and corresponding polystyrene 3-arm star polymers investigated by Liu et al. (Polymer 2023, 281:126125), as well as the shear viscosity data of poly(propylene carbonate) melts reported by Yang et al. (Nihon Reoroji Gakkaishi 2022, 50:127–135). In the limit of steady-state shear flow, the RZS model converges to the Doi-Edwards IA model, which quantitatively describes the steady-state shear viscosity of linear polymer melts and long/short blends. The assumption of “non-stretching” of tube segments created during rotational flow is therefore in agreement with the available experimental evidence. Three-arm star polymers behave in a similar way as corresponding blends of long and short polymers confirming the solution effect of the short arm in asymmetric stars. The analysis of step-shear strain experiments reveals that stress relaxation is at first dominated by stretch relaxation, followed at times larger than the Rouse stretch relaxation time by relaxation of orientation as described by the damping function of the Doi-Edwards IA model. The RZS model does not require any nonlinear-viscoelastic parameter, but relies solely on the linear-viscoelastic relaxation modulus and the Rouse stretch relaxation time.

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This research investigates the interactions between Carbopol® and zinc microparticles and their role in the stabilization of zinc within the framework of zinc slurry–air redox flow batteries (Zn-air RFBs). The study explores the potential of yield stress fluids, particularly PAA (Carbopol®) microgels, as stabilizers for zinc particles during battery operation, and regarding sedimentation in particular. Two effects that can limit the effectiveness of PAA yield stress in stabilizing the suspension are examined in this study. First is the ionic strength and pH that can evolve during the slurry formulation. The second effect is associated to zinc polymer interactions that can develop during the suspension preparation. Using methodologies based on rheometry and UV–Vis spectroscopy, the research identifies that the primary stabilization challenge is the adsorption of Carbopol® microgels onto zinc surfaces, which significantly influences the gel’s yield stress. These insights contribute to an enhanced understanding of the physical chemistry of the suspending fluid, facilitating the development of more efficient and stable Zn-Air RFBs.

The flow through a 4:1 planar contraction has  been investigated using different rheological models having the same shear viscosity, namely, the inelastic Carreau-Yasuda model (CY), the enhanced Bautista-Manero-Puig (eBMP), and the exponential version of the Phan-Thien/Tanner (PTT). Noticeable discrepancies were observed with the CY model and the eBMP in terms of the velocity profiles along the centerline and in the exit channel (near the end of the geometry) normal to the flow direction. Transient planar extensional viscosity shows a large effect on vortex dynamics although the effect of transient and steady elongation on pressure drop seems negligible. Simulation results allowed gathering that pressure drop is largely influenced by the shear-thinning behavior of the fluid, noticeably affected by elasticity, and less by extensional viscosity.

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The transient rheological behaviour of semi-dilute kaolinite clay suspensions is investigated. Specifically, the flow curve hysteresis and step shear rate tests are used to investigate the shear behaviour of kaolinite suspensions. We found that there is a coupling effect between thixotropy and slip that dominates this transient rheological behaviour. It appears that the onset of solid-like slip in the system is a function of wall roughness and interfacial phenomena between the particles of the suspension and the wall. The coupled phenomena of slip and thixotropy are investigated using the application of sandpaper, varying the gap, and using different geometries. To illustrate the importance of slip in this system, we propose a Coupled Thixotropy and Slip (CTS) model that couples a thixotropic Structure Parameter Model (SPM) to an existing slip model.

We have found that the unique particle properties of fly ash can be applied to the modification of shear thickening fluid. In this paper, rheological properties and microscopic thickening mechanism of fly ash/silicon-based shear thickening fluid (subsequently abbreviated as FA/SiO₂-STF) are studied. Ultrasonic technology and mechanical stirring method were used to prepare FA/SiO₂-STF with different mass fractions of fly ash, and then rheometer was used to carry out steady-state rheological testing for FA/SiO₂-STF, and 4%FA/SiO₂-STF dynamic rheological test and temperature sensitivity testing, respectively. The thickening mechanism of FA/SiO₂-STF was analyzed by scanning electron microscope. The rheological test results show that the FA/SiO₂-STF with 4% fly ash content exhibits remarkable shear thickening effect. Finally, the relationship between the viscosity and shear rate of FA/SiO₂-STF is numerically described by a mathematical model, which can accurately reflect the viscosity thickening effect.