Rheologica Acta最新文献

Despite many attempts, the molecular mechanism of the nonlinear viscoelastic response of polymeric liquids in fast shear flows has not yet been fully elucidated. In this study, we examined the viscosity growth curves for a few well-characterized, nearly monodisperse, densely branched pom-pom polystyrene melts. The viscosity growth curves exhibit double peaks rather than the widely reported (for most polymer melts) single peak. To investigate the underlying molecular mechanism responsible for the observed behavior, we conducted primitive chain network (multi-chain sliplink) simulations, and found that the first and second peaks correspond to arm orientation and backbone stretch, respectively. We further observed that backbone stretch is reduced by coherent molecular tumbling at the flow start-up, and hence the second peak intensity comes out comparable to that of the orientation-induced first peak. In our sample, the number of backbone entanglements is small, so the mechanism of branchpoint withdrawal does not play a significant role.

Viscoelastic fluid flows are widely described by elastic dumbbell models such as Oldroyd-B and FENE-P. However, these constitutive equations can yield significantly different predictions, which may contradict experimental observations. In this work, we analyze the fully developed flow of a viscoelastic fluid in a straight channel and present a theory based on the lubrication approximation for calculating the elastic stresses and flow rate for four elastic dumbbell models, including various microstructurally inspired terms. We compare the predictions of different models and elucidate the impact of (i) the finite extensibility, (ii) conformation-dependent friction coefficient, and (iii) conformation-dependent non-affine deformation on the polymer stresses, velocity, and flow rate. We demonstrate that including all three microstructurally inspired terms in a constitutive equation can significantly affect the response of a viscoelastic fluid even in a fully developed flow, thus highlighting their potential necessity for accurate modeling of viscoelastic channel flows with mixed kinematics.

Once surface active colloids, such as microgels, adsorb to an interface, they modify the viscoelastic properties of the interface. In contrast to what happens in bulk, the elastic properties have a non-monotonic dependence on the generalized area fraction. In this study, we describe this phenomena by performing molecular dynamic simulations of effective pair potentials in two dimensions. Our potential model is constructed by taking available interfacial rheology experimental results as a starting point. From this knowledge, we need to take into account, for very dense monolayers, that the interaction between adsorbed microgels has to consider multiple factors, including an increase in microgel stiffness when compressed and the presence of a dense inner core region. To account for these properties, we adopt a square-shoulder multi-Hertzian model, which predicts equilibrium structures and rheological properties in qualitatively agreement with experiments. Crucially, this model avoids the reentrant liquid behavior commonly observed with soft Hertzian-based pair potentials at high concentrations. Building on these insights, we thus explore the parameter space of this potential, providing novel predictions for dense monolayers composed of differently crosslinked microgels, awaiting for experimental investigation in the near future.

Nonlinear shear and elongational start-up data of three entangled PS solutions consisting of the same weight fraction of a linear long-chain polystyrene PS-600 k and three different styrene oligomeric solvents were recently reported by Cui et al. (Rheol Acta 64:97-105, 2025). The solvents are two linear styrene oligomers of different molecular weights as well as a star styrene oligomer. We show that start-up of shear viscosity and apparent normal stress difference as well as start-up of elongational viscosity can consistently be described by the Rotation Zero Stretch (RZS) model (Rheol Acta 63:573, 2024; Phys Fluids 36:093124, 2024), which is based on the tube model and a flow-strength sensitive evolution equation of stretch. In extensional flows, the RZS model reduces to the Enhanced Relaxation of Stretch (ERS) model (J Rheol. 65:1413, 2021). The modeling is based exclusively on the linear-viscoelastic characterization of the solutions and a consistent set of Rouse stretch relaxation times for PS-600 k.

Graphical Abstract

Dense suspensions with high solid volume fractions ((phi ge 0.5)) are prevalent in nature and throughout industry. These highly loaded suspensions can exhibit complex rheological behaviors, including both shear thinning and subsequent thickening with increasing shear rate. Understanding the mechanisms behind these rheological behaviors can improve the design of materials systems to behave as predicted and desired under process flows. Here, we present a study on an industrially relevant, dense ((phi =0.55)), colloidal alumina suspension and show how the addition and loading of a non-adsorbing polyvinylpyrrolidone (PVP) at different molecular weights can be used to tune and control its rheological properties. PVP was added at varying concentrations spanning the dilute and semi-dilute non-entangled regimes for each molecular weight. The addition of PVP at concentrations in the dilute regime was shown to increase the viscosity of the suspension and induce discontinuous shear thickening (DST). However, further increases in PVP loading particularly within the semi-dilute non-entangled regime and at higher PVP molecular weights also increased the dynamic yield stress of the material, made the suspension more shear thinning, and delayed the onset of DST. Rheological measurements were coupled with insights on the relevant particle and polymer length scales from small angle neutron scattering (SANS), including Rheo-SANS measurements, to inform on the mechanisms by which non-adsorbing polymers influence suspension rheology across multiple flow regimes.

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Clay-polymer mixtures, characterised by high mechanical stiffness, are widely utilised in strengthening soft and easy-to-break materials. Here, we present microrheological results on shake-gels made of natural (montmorillonite) and synthetic (Laponite) disk-shaped clay particles in combination with poly(ethyleneoxide). These clay-polymer suspensions represent shear-thickening fluids that display a large increase in viscosity upon large-amplitude shaking. By performing both bulk and microrheology experiments, we probe the phase behaviour and mechanical response of these nanocomposites. Slight tuning of either the particle or polymer concentration leads to dramatic changes in the macroscopic appearance of the mixture, transitioning from a low-viscosity fluid to a stiff gel, capable of sustaining its own weight. We relate these observations to microscopic measurements, providing insight into the local reorganisation of the constituting building blocks and the time-evolution of each phase.

Enhanced oil recovery (EOR) in heavy and extra-heavy oil reservoirs primarily relies on thermal methods like cyclic steam stimulation and steam flooding. However, reservoir heterogeneities often lead to poor sweep efficiency, unswept oil pockets, and early water breakthrough. To mitigate these issues, a thermoreversible hydroxypropyl methylcellulose (HPMC) hydrogel was evaluated for channeling control. This study investigates the effects of carbon nanospheres, aluminum (Al₂O₃), silicon (SiO₂), magnesium (MgO), and chromium oxide (Cr₂O₃) nanoparticles on the viscoelastic and thermal properties of a low-concentration HPMC hydrogel. Nanoparticles improved gel performance, with MgO and Al₂O₃ increasing the resistance to flow of the hydrogel under high-pressure conditions, lowering gelation temperature by up to 10 °C, and reducing syneresis up to 13%. These results highlight the potential of nanoparticle-reinforced HPMC gels to enhance EOR efficiency in heterogeneous reservoirs.

Graphical Abstract