Showing papers in "Rheologica Acta in 2023"

Two-dimensional glass transition–like behavior of Janus particle–laden interface

Abstract: Abstract Understanding the interactive behavior of Janus particles (JPs) is a growing field of research. The enhancement in binding energy, in comparison to homogenous particles, and the dual characteristic of JPs open up new possibilities for novel applications. In many such applications, interfacial materials become subjected to flows that produce dilational and shear stresses. Therefore, it is important to understand the impact that the Janus character brings to interfaces. In this work, we study the microstructure of two-dimensional (2D) JP monolayers formed at the air–water interface and examine the shear viscoelasticity with an interface rheometer that was adapted for in situ surface pressure control via a Langmuir trough. We extend concepts from bulk rheology to data obtained from interfacial rheology as a tool to understand and predict the monolayer’s viscoelastic behavior. Finally, by calculating the time relaxation spectrum from the measured 2D dynamic moduli, we conclude that a phenomenon similar to glass transition is taking place by analogy.

Rheological transient effects on steady-state contraction flows

Abstract: Abstract It may be assumed that the steady-state kinematics of viscoelastic contraction flows depends on the time-independent rheological properties only. This idea is supported by the large number of references explaining steady simulation results by considering only steady-state material functions. Even with numerical simulations, it would be difficult to prove such a statement wrong. However, using the Bautista-Manero-Puig class of models allows to obtain the same steady rheological response but with different transient evolution. Here, we considered two fluids, one displaying a monotonic trend towards the steady-state and the other with at least one visible overshoot in the material functions. Our results show that for the transient evolution with the overshoot fluid, a significant increase in the steady pressure drop is gathered. In addition, vortex response is quite different for the two fluids. This research gives evidence that the transient evolution in rheometrical functions has great impact on steady-state flow behavior.

Modeling elongational viscosity and brittle fracture of 10 polystyrene Pom-Poms by the hierarchical molecular stress function model

Abstract: Abstract A Pom-Pom polymer with q a side chains of molecular weight M w,a at both ends of a backbone chain of molecular weight M w,b is the simplest branched polymer topology. Ten nearly monodisperse polystyrene Pom-Pom systems synthesized via an optimized anionic polymerization and a grafting-onto method with M w,b of 100 to 400 kg/mol, M w,a of 9 to 50 kg/mol, and q a between 9 and 22 are considered. We analyze the elongational rheology of the Pom-Poms by use of the hierarchical multi-mode molecular stress function (HMMSF) model, which has been shown to predict the elongational viscosity of linear and long-chain branched (LCB) polymer melts based exclusively on the linear-viscoelastic characterization and a single material parameter, the so-called dilution modulus G D . For the Pom-Poms considered here, we show that G D can be identified with the plateau modulus $${G}_{N}^{0}={G}_{D}$$ G N 0 = G D , and the modeling of the elongational viscosity of the Pom-Poms does therefore not require any fitting parameter but is fully determined by the linear-viscoelastic characterization of the melts. Due to the high strain hardening of the Pom-Poms, brittle fracture is observed at higher strains and strain rates, which is well described by the entropic fracture criterion. Graphical abstract

SLE3S-water system: a linear rheological characterisation

Abstract: Abstract Surfactant-water mixtures display a complex rheological behaviour, with changes in parameters such as viscosity and moduli of several orders of magnitude as a consequence of phase changes, depending on their concentration and temperature: this criticism heavily affects different industrial processes. In our work, linear rheological behaviour of aqueous mixtures of a commercial anionic surfactant, sodium lauryl ether sulphate, is investigated in a range of temperature (30–60 °C) and surfactant concentration (20–72%wt) of technological relevance. Four phases with different texture are identified by polarised light microscopy: micellar, hexagonal, cubic and lamellar, all showing a shear-thinning behaviour. Rheological parameters of cubic phase show a net jump in a relatively narrow temperature range, suggesting a temperature-induced phase change. The systematic analysis of the rheological behaviour of this widely used surfactant system, reported here for the first time, can be of fundamental support for many industrial applications. Graphical Abstract

Fracture in elongational flow of two low-density polyethylene melts

Abstract: Abstract Samples of two commercial low-density polyethylene melts were investigated with respect to their fracture behavior in controlled uniaxial extensional flow at constant strain rate in a filament stretching rheometer. In order to assess the possible influence of grain boundaries on fracture, the samples were prepared by three different types of pre-treatment: by compression molding of (1) virgin pellets used as received, (2) pellets homogenized in a twin-screw extruder, and (3) pellets that were milled into powder by cryogenic grinding under liquid nitrogen. The elongational stress growth data were analyzed by the Extended Hierarchical Multi-mode Molecular Stress Function (EHMMSF) model developed by Wagner et al. (Rheol. Acta 61, 281-298 (2022)) for long-chain branched (LCB) polymer melts. The EHMMSF model quantifies the elongational stress growth including the maximum in the elongational viscosity of LDPE melts based solely on the linear-viscoelastic relaxation spectrum and two nonlinear material parameters, the dilution modulus G D and a characteristic stretch parameter $${\overline{\lambda}}_m$$ λ ¯ m . Within experimental accuracy, model predictions are in excellent agreement with the elongational stress growth data of the two LDPE melts, independent of the preparation method used. At sufficiently high strain rates, the fracture of the polymer filaments was observed and is in general accordance with the entropic fracture criterion implemented in the EHMMSF model. High-speed videography reveals that fracture is preceded by parabolic crack opening, which is characteristic for elastic fracture and which has been observed earlier in filament stretching of monodisperse polystyrene solutions. Here, for the first time, we demonstrate the appearance of a parabolic crack opening in the fracture process of polydisperse long-chain branched polyethylene melts.

Bayesian coarsening: rapid tuning of polymer model parameters

Abstract: Abstract A protocol based on Bayesian optimization is demonstrated for determining model parameters in a coarse-grained polymer simulation. This process takes as input the microscopic distribution functions and temperature-dependent density for a targeted polymer system. The process then iteratively considers coarse-grained simulations to sample the space of model parameters, aiming to minimize the discrepancy between the new simulations and the target. Successive samples are chosen using Bayesian optimization. Such a protocol can be employed to systematically coarse-grained expensive high-resolution simulations to extend accessible length and time scales to make contact with rheological experiments. The Bayesian coarsening protocol is compared to a previous machine-learned parameterization technique which required a high volume of training data. The Bayesian coarsening process is found to precisely and efficiently discover appropriate model parameters, in spite of rough and noisy fitness landscapes, due to the natural balance of exploration and exploitation in Bayesian optimization.

Experimental and simulative determination and correction of the effective gap extension in structured coaxial measuring systems

Abstract: Abstract The use of structured measuring systems to prevent wall slip is a common approach to obtain absolute rheological values. Typically, only the minimum distance between the measuring surfaces is used for further calculation, implying that no flow occurs between the structural elements. But this assumption is misleading, and a gap correction is necessary. To determine the radius correction $$\Delta r$$ Δ r for specific geometries, we conducted investigations on three Newtonian fluids (two silicon oils and one suspension considered to be Newtonian in the relevant shear rate range). The results show that $$\Delta r$$ Δ r is not only shear- and material-independent, but geometry-dependent, providing a Newtonian flow behaviour in a similar viscosity range. Therefore, a correction value can be determined with only minute deviations in different Newtonian fluids. As the conducted laboratory measurements are very time-consuming and expensive, a CFD-approach with only very small deviations was additionally developed and compared for validation purposes. Therefore, simulation is an effective and resource-efficient alternative to the presented laboratory measurements to determine $$\Delta r$$ Δ r for the correction of structured coaxial geometries even for non-Newtonian fluids in the future.