Rheologica Acta最新文献

The long-wave theory is used to model the thin film flow of a generalized second-grade fluid (GSGF) down a tilted plate with a bump topography. The derived single non-linear partial differential equation for the film thickness describes the surface wave generated by the bump, which disturbs the uniform flow. The model involves the non-Newtonian and geometrical parameters that investigate the wave’s shape and amplitude. The model equation is strongly non-linear due to the GSGF’s constitutive equations, and it is solved numerically using the finite volume method, where the flux function is approximated implicitly using the upwind scheme. The simulation reveals that the bump creates the surface wave, it splits and propagates, and its shape and size are influenced by the bump’s height and the non-Newtonian fluid properties.

With the growing demand of mineral consumption, the management of the mining waste is crucial. Cemented paste backfill (CPB) is one of the techniques developed by the mining industry to fill the voids generated by the excavation of underground spaces. The CPB process is the subject of various studies aimed at optimizing its implementation in the field. In this article, we focus on the modelling of the backfill phase where it has been shown in Vigneaux et al. (Cem. Concr. Res. 164:107038, 2023) that a viscoplastic lubrication model can be used to describe CPB experiments. The aim here is to propose an accelerated method for performing the parameters’ estimation of the properties of the paste (typically its rheological properties), with an inverse problem procedure based on observed height profiles of the paste. The inversion procedure is based on a metamodel built from an initial partial differential equation model, thanks to a polynomial chaos expansion coupled with a principal component analysis.

The influence of a mechanical or a thermal pretreatment of a linear (L-PP) and a long-chain branched polypropylene (LCB-PP) in the molten state was studied. The molar mass distributions and the branching structure were determined by high-temperature gel permeation chromatography HT-GPC coupled with laser-light scattering. The samples were extruded through long or short capillaries of various geometries corresponding to a predominant shear or elongational deformation. As a rheological probe, the extrudate swell at low stresses was measured for the differently pretreated samples. For the L-PP, neither molecular nor rheological changes were observed. However, the extrudate swell of the LCB-PP was found to decrease with increasing volume throughput. It was more strongly affected by shear in the capillary than by molecule stretching in the entry region. The smaller extrudate swell was accompanied by a decrease of the high molar mass tail of the LCB-PP, which could be the reason for the decay of swell, in principle. However, a comparable degradation of the high molar mass tail was obtained by a pure thermal treatment that was shown to leave the extrudate swell unchanged. This result and the unaffected branching structures found by high-temperature gel permeation chromatography (HT-GPC) support the hypothesis of a change of the branching topography by the mechanical pretreatment.

We derive explicit analytical expressions for the recurrence relations using the analytical matrix method for frequency response and the Bautista-Manero-Puig model for complex fluids. The BMP model is derived from the Extended Irreversible Thermodynamics formalism and has been shown to be useful in predicting the complex rheological behavior of self-associative systems. All harmonics of the alternating normal and shear stresses in oscillatory shear with various amplitude oscillatory regimes (AOS) can be calculated analytically, i.e., small amplitude oscillatory shear (SAOS), medium amplitude oscillatory shear (MAOS), and large amplitude oscillatory shear (LAOS). We show that incorporating the effects of the first and second normal stress differences for all AOS regimes leads to the emergence of higher harmonics. We establish the limits between the different AOS regimes based on criteria suggested by the analytical method. For some typical systems, such as CTAB-NaSal, we found a satisfactory quantitative agreement with the measured behavior of AOS.

The steady pipe flow of thixotropic yield stress fluids has been investigated theoretically based on a modified isotropic kinematic hardening (mIKH) model. Analytical solution is derived for a specific case (m = n = 1) and a general semi-analytical solution is put forward as well. The effect of thixotropic yield stress on shear rate and velocity profiles is illustrated by comparing to other well-known solutions. Moreover, the influences of model parameters are examined. It is worth noting that shear banding may occur at the yielded surface in case of a sufficiently large Bingham number, thixotropic number, and flow index, but a sufficient small value of structure-related exponent.

Microrheology with optical tweezers (MOT) is an all-optical technique that allows the user to investigate a materials’ viscoelastic properties at microscopic scales, and is particularly useful for those materials that feature complex microstructures, such as biological samples. MOT is increasingly being employed alongside 3D imaging systems and particle tracking methods to generate maps showing not only how properties may vary between different points in a sample but also how at a single point the viscoelastic properties may vary with direction. However, due to the diffraction limited shape of focussed beams, optical traps are inherently anisotropic in 3D. This can result in a significant overestimation of the fluids’ viscosity in certain directions. As such, the rheological properties can only be accurately probed along directions parallel or perpendicular to the axis of trap beam propagation. In this work, a new analytical method is demonstrated to overcome this potential artefact. This is achieved by performing principal component analysis on 3D MOT data to characterise the trap, and then identify the frequency range over which trap anisotropy influences the data. This approach is initially applied to simulated data for a Newtonian fluid where the trap anisotropy induced maximum error in viscosity is reduced from ~ 150% to less than 6%. The effectiveness of the method is corroborated by experimental MOT measurements performed with water and gelatine solutions, thus confirming that the microrheology of a fluid can be extracted reliably across a wide frequency range and in any arbitrary direction. This work opens the door to fully spatially and angularly resolved 3D mapping of the rheological properties of soft materials over a broad frequency range.

Dissolution of cellulose is crucial for its regeneration and chemical modification, such as homogeneous transesterification, for example. The cellulose dissolution in ionic liquid (IL) media is suggested as a prospective environmentally friendly alternative to conventional solvents. In this study, novel distillable ionic liquid 5-methyl-1,5,7-triaza-bicyclo-[4.3.0]non-6-enium acetate, [mTBNH][OAc] was used for cellulose dissolution. This IL has high dissolving power towards cellulose and durability for recycling. However, the disadvantage of ILs is their high viscosity, which limits the supreme cellulose concentration in IL solutions, and their high cost, hindering their commercialization. The addition of low-viscous, low-cost, and naturally derived co-solvents can reduce the overall viscosity and cost. In this study, rheology experiments were conducted to investigate the flow behavior of cellulose in [mTBNH][OAc] ionic liquid mixed with the green co-solvents such as γ-Valerolactone (GVL), dimethyl isosorbide (DMI), and N,N′-dimethylpropyleneurea (DMPU). A study of the rheology showed that the viscosity reduces at low doses of co-solvent (≤ 50 wt%) but causes the structuring of the cellulose solution and its gelation (or phase separation) at high doses (≥ 50 wt%). The rheological study also indicated that the flow activation energy of cellulose in IL/co-solvent systems is lower than that in pure IL and decays in the order of DMPU > DMI > GVL > DMSO.

Graphical abstract

This study explores the effect of alcohol on the rheological behavior of SiO₂ nanoparticle (NP) in cetyltrimethylammonium bromide (CTAB)/potassium hydrogen phthalate (PPA) wormlike micelles (WLMs). No previous literature explored the effect of chemical stresses on NP-mediated WLMs. The addition of 0.05 wt% NPs increased the zero-shear viscosity of 30 mM CTAB/20 mM PPA WLMs by 30%. A further increase in SiO₂ NPs disrupted the micellar structure, resulting in reduced viscosity. The optimum SiO₂ NP content increased from ~ 0.05 wt% to ~ 0.4 wt% as the CTAB and PPA concentrations increased from 30 mM CTAB/20 mM PPA to 90 mM CTAB/60 mM PPA. The effect of ethanol and 1-hexanol on WLMs composed of 0.05 wt% SiO₂ NP/30 mM CTAB/20 mM PPA was assessed. The NPs enhanced the shear viscosity of ethanol/WLM systems. Notably, at ethanol concentrations of 0.14 wt%, 0.60 wt% and 1.40 wt%, the zero-shear viscosity increased by approximately 60%, 100%, and 25%, respectively. SiO₂ NPs acted as junction points, facilitating the crosslinking among shorter micelles and improving micellar entanglement. Conversely, SiO₂ NPs had little impact on 1-hexanol/WLM systems owing to the strong interaction between WLMs and 1-hexanol at the micellar interface.