Steady-state Rheological Measurements Research Articles

Investigating the impact of zinc ion substitution on the rheological properties and hyperthermia potential of cobalt ferrite-based ferrofluids

Zn-substituted cobalt ferrite-based ferrofluids (FFs) were successfully synthesized using a modified co-precipitation method. In this study, we aimed to investigate the heating performance of the synthesized FFs using a resonance coil-based magnetic hyperthermia setup and characterize their viscoelastic behaviour using a magneto rheometer. The crystalline spinel phase with Fd3m (227) symmetry was confirmed through Powder X-ray Diffraction (PXRD. Additionally, Vibrating Sample Magnetometer (VSM) measurements allowed us to analyze the effect of Zn substitution on the room temperature magnetic properties of the FFs. To gain insights into the viscoelastic properties, steady-state rheological measurements were performed. The FFs exhibited non-Newtonian behaviour at low shear rates (<100 s−1) and transitioned to Newtonian fluid behaviour at higher shear rates. Furthermore, we investigated the magneto-viscoelastic behaviour of the FFs across a range of magnetic field strengths (0 to 1 T) using three different fluids. It was observed that the viscosity increased as the field strength increased, which can be attributed to the formation of chain-like structures. The heating efficiency of the FFs was evaluated in terms of Specific Absorption Rate (SAR) using the Initial Slope Method (ISM) and Box Lucas Method (BLM). The SAR values for the FFs exhibited a significant increase with Zn substitution, with the maximum values recorded as 188 ± 8 W/g (ISM) and 212 ± 9 W/g (BLM) for Zn0.4Co0.6Fe2O4, surpassing the SAR value of CoFe2O4 (72 ± 8 W/g, ISM; 92 ± 5 W/g, BLM). To assess biocompatibility, we conducted a cell viability assay using HEK-293 cell lines. The results demonstrated high cell viability (>75%) for Zn0.4Co0.6Fe2O4 at a concentration of 100 µg/ml after a 48-hour incubation period.

Signature of jamming under steady shear in dense particulate suspensions

Under an increasing applied shear stress (), the viscosity of many dense particulate suspensions increases drastically beyond a stress onset (), a phenomenon known as discontinuous shear-thickening. Recent studies point out that some suspensions can transform into a stress induced solid-like shear jammed (SJ) state at high particle volume fraction (). SJ state develops a finite yield stress and hence is distinct from a shear-thickened state. Here, we study the steady state shear-thickening behaviour of dense suspensions formed by dispersing colloidal polystyrene particles (PS) in polyethylene glycol (PEG). We find that for small values the viscosity of the suspensions as a function of can be well described by Krieger–Dougherty (KD) relation. However, for higher values of (), KD relation systematically overestimates the measured viscosity, particularly for higher values. This systematic deviation can be rationalized by the weakening of the sample due to flow induced failures of the solid-like SJ state. Using Wyart–Cates model, we propose a method to predict the SJ onset from the steady state rheology measurements. Our results are further supported by in situ optical imaging of the sample boundary under shear.

Effect of SiO2-based nanofluids in the reduction of naphtha consumption for heavy and extra-heavy oils transport: Economic impacts on the Colombian market

The main objective of this study is twofold. First, we evaluate the effect of nanofluids as additives to optimize the transport of heavy crude oil using steady-state rheological measurements and dynamic flow tests. The use of these additives produces viscosity reduction for heavy (HO) and extra heavy (EHO) crude oils. Second, we show how the addition of nanofluids can lead to significant economic and environmental impacts in Colombia as a country and possibly the region through optimization in the use of naphtha for transportation. Four nanofluids were prepared using different solvents and fumed silica nanoparticles. Mixtures of nanofluids with naphtha were prepared to reduce the viscosity of the crude oil as to attain mobility targets (400–500cP at 311K). To this end, 27% and 63% v/v of naphtha were required for heavy and extra-heavy crude oils, respectively. Through steady-state rheological measurements and dynamic flow tests in a pipeline, we found that the best performing material turned out to be the nanofluid composed of 1000mg/L of SiO2 and biodiesel without surfactant. This nanofluid yielded a reduction in the consumption of naphtha of approximately 50% v/v. Our results show that the best-performing nanofluid can serve as an optimizing agent for the transport conditions of heavy and extra-heavy crude. The proposed technology was evaluated through a preliminary economic analysis that shows a potential reduction of dilution costs of almost 50%, equivalent to more than USD $2.5million per day, as well as a decreased energy consumption associated with pumping of 37%, able of generating savings of more than USD $2million per year. Additionally, a discussion on environmental impacts are presented, and the advantages of reducing naphtha consumption are discussed. A nanofluid-based technology could generate savings in both energy and fuel consumptions for the oil and gas industry and resolve national or regional issues constrained by limited access to diluent.

Interactions between dodecylammonium chloride and carrageenans in the semidilute regime

Interactions between dodecylammonium chloride (DDACl) and κ-, ι- and λ-carrageenans (abbreviated as KC, IC and LC) in the semidilute regime have been studied using dynamic light scattering, zeta potential and rheological measurements. Addition of oppositely charged surfactant monomers to carrageenan solutions leads to the formation of various soluble or insoluble complexes (PSC). An increase in the carrageenan linear charge density shifts the appearance of mixture turbidity and phase separation toward higher DDACl concentration. A decrease in PSC size and charge as well as in solution viscosity with increasing DDACl concentration indicates collapsing of carrageenan chains. At higher surfactant concentrations, light scattering shows the existence of larger PSC that remain negatively charged close to phase separation. Steady-state rheological measurements showed that all samples exhibit shear thinning behavior, well fitted by the power law equation, indicating that they mainly maintain carrageenan rheological properties. The change of flow behavior parameters with increasing DDACl concentration indicates progressive transition from shear thinning to nearly Newtonian behavior. The effect of DDACl concentration on flow behavior parameters increases from KC to LC, suggesting that PSC structural properties are mainly determined by the conformation and charge of carrageenan chains. Small-amplitude oscillatory shear measurements reveals viscoelastic behavior of all single carrageenan solutions. Upon the addition of DDACl the viscous contribution to viscoelastic deformation increases with increasing DDACl concentration indicating that the elasticity of carrageenan solutions was slightly reduced for all KC mixtures and for IC and LC mixtures at lower DDACl concentrations. The loss of carrageenan solution viscoelastic properties strongly depends on the carrageenan type and DDACl concentration.

Rheology of draining steady-state foams

Aqueous foams continuously age due to fluid drainage and bubble coarsening, which makes it difficult to perform steady-state rheological measurements. Consequently we have developed the foam drainage rheology technique, where perfusion counteracts fluid drainage and bubble replenishment counteracts bubble coarsening during measurement of the shear stresses by a rheometer. We evaluate published power-law and Herschel-Bulkley models and find that parameters derived from emulsion experiments cannot describe our results. We propose a hybrid model, which combines our earlier film-shearing model, where the film thickness depends on liquid volume fraction, with a Herschel-Bulkley shear-rate dependence.

Modeling the Effect of Cell-Associated Polymeric Fluid Layers on Force Spectroscopy Measurements. Part II: Experimental Results and Comparison with Model Predictions

In this paper, experimentally obtained force curves on Staphylococcus aureus are compared with a previously developed model that incorporates hydrodynamic effects of extracellular polysaccharides together with the elastic response of the bacterium and cantilever. Force-displacement curves were predicted without any adjustable parameters. It is demonstrated that experimental results can be accurately described by our model, especially if viscoelastic effects of the extracellular polysaccharide layer are taken into account. Polysaccharide layer viscoelasticity was treated by means of a multimode Phan-Thien/Tanner (PTT) constitutive equation. Typical maximum relaxation times range from 0.2 to 2 s, whereas the corresponding zero-shear-rate viscosities are 6-9 Pa.s, based on published, steady-state rheological measurements on Staphylococcus aureus polysaccharide extracted from its native environment. The bacterial elastic constant is found to be in the range 0.02-0.4 N/m, corresponding to bacterial wall Young's moduli in the range of a few hundred MPa. Repeatability of measurements performed on different bacteria is found to be only fair, due to large individuum variability, whereas repetitions of measurements on the same bacterium showed high reproducibility. Improved force-indentation curve predictions are expected if transient rheological characterization of extracellular polysaccharides is available. More desirable however is the direct, in vivo rheological characterization of the extracellular polysaccharide. A model-based analysis of experimental force-indentation curves shows that appreciable further experimental improvements are still necessary to achieve this goal.

Correlations between the Viscoelastic Behavior of Pyrene-Labeled Associative Polymers and the Associations of Their Fluorescent Hydrophobes

The viscoelastic behavior of a series of three pyrene-labeled hydrophobically modified alkali swellable emulsion copolymers (Py-HASEs) was investigated. All Py-HASEs thickened the aqueous solutions with viscosities orders of magnitude larger than that of a HASE control which displayed no pyrene hydrophobe. This fact demonstrated that the pyrene molecule is a good hydrophobe for associative thickeners such as HASEs. The Py-HASE solutions exhibited shear thinning, whose magnitude was found to increase with increasing pyrene content. A large shear-thinning effect indicates that a large fraction of the elastically active cross-links has been severed. Fluorescence measurements on the Py-HASEs confirmed that the smaller the pyrene content of the Py-HASE, the more intermolecular associations it formed, in agreement with the results obtained by rheology. Above the overlap concentration of the polymers, the zero-shear viscosity of the Py-HASE solutions increased steeply with increasing polymer concentration. The onset concentration where viscosity increases matches the onset concentration where intermolecular associations are being formed, as probed by fluorescence. Oscillatory rheological measurements were carried out to determine the terminal relaxation time, Td, and the storage modulus at the infinite time limit, G0, of the Py-HASE network. G0 was found to increase with decreasing pyrene contents, indicating that Py-HASEs with lower pyrene contents exhibited a higher density of elastically active chains. This result is in agreement with the trends obtained by the fluorescence and steady-state rheology measurements. A model is suggested that accounts for the fluorescence and rheology results.

Effect of Coal Surface Wettability on Aggregation of Fine Coal Particles

Fine coal aggregation in concentrated coal-water suspensions was studied through steady-state rheological measurements. The wettability of coal particles was modified by dry oxidation of a bituminous coal. The degree of hydrophobicity was monitored using film flotation. The rheological measurements revealed that hydrophobic coal particles aggregate in aqueous suspensions over a wide pH range, and produce slurries with high yield stresses. As the degree of coal oxidation increases, the coal particles become more hydrophilic and such suspensions exhibit lower yield stress values. The yield stress values for suspensions prepared from oxidized/hydrophilic coal become a strong function of pH and correlate well with the surface charge as determined through electroacoustic measurements. For the suspensions prepared from the samples with intermediate degree of oxidation the effect of coal surface heterogeneity becomes very important. These results indicate that the coal particle aggregation, and thus the yield stress of aqueous suspensions of coal, is strongly affected by wettability of coal particles.

Dynamic and steady‐state rheological measurements on polymer melts

AbstractMany attempts to correlate steady‐state and dynamic rheological data have been reported. Thus far, the correlations have not been convincing, particularly for polymer melts that exhibit highly non‐Newtonian behavior. The purpose of this work was to provide a means for obtaining a useful correlation. The Maxwell viscoelastic model was used to correlate data from four different modes of shear—two involving steady‐state deformation and two involving dynamic deformation. As certain defined requirements were met, excellent agreement was obtained for polymer melts that represent a wide range of viscoelastic behavior. One of these requirements was that the polymer molécule should have a certain degree of flexibility.