Rheology Of Suspensions Research Articles

On the Rheology of Thixotropic and Rheopexic Suspensions

On the Rheology of Thixotropic and Rheopexic Suspensions

Effects of a spherical slip cavity filled with micropolar fluid on a spherical micropolar droplet

In this work, a two-fluid phase flow problem involving an axisymmetrical quasi-steady motion of a spherical micropolar droplet translating at a concentric point in a second non-mixable micropolar fluid within a spherical impermeable cavity with a slip surface is analysed under low Reynolds numbers. The two fluid phases that have a microstructure (micropolar fluid) are the case that is being focused on. The Stokes equations are solved inside and outside the droplet for the velocity fields. In addition, based on the concentric position, general solutions in terms of spherical coordinates are obtained. In this case, tangential couple stress and continuity of microrotation are used. For different cases, the normalised drag forces acting on the droplet are represented via graphs for different values of relative viscosity, droplet-to-cavity radii ratio, and the parameter that connects the tangential couple stress with microrotation. The normalised drag force is found to be a monotonically increasing function of the drop-to-cavity radii ratio. It is found that when the droplet-to-cavity radii ratio approaches zero, there is a very strong interaction between the droplet and the cavity. When comparing a solid sphere to a gas bubble, the normalised drag force is larger. Additionally, the results showed that permitting spin and slip at the cavity’s interior surface improved the wall correction factor influencing the droplet. The present study is important in the fields of natural, industrial, and biomedical processes such as raindrop formation, liquid–liquid extraction, suspension rheology, sedimentation, coagulation, and the motion of blood cells in an artery or vein.

Yield stress analysis of cellulose nanocrystals (CNCs) in hyaluronic acid suspensions

Due to their biodegradability features, cellulose nanocrystals (CNCs) and hyaluronic acid (HA) have been simultaneously used in the matrix of hydrogels for biomedical applications, such as corneal transplantation, and skin regeneration. Although rheology of these hydrogels may provide useful information for their applications, little to no attention has been paid to rheological characterization. In this study, we analyzed the rheology of HA-CNC suspensions and more specifically their yielding behavior. Through different rheological experiments, known as stress ramp, shear rate ramp and amplitude sweep; it was observed that HA-CNC gels possessed two yield points. Reproducible magnitudes of yield stress were obtained by optimizing the experimental conditions. The rheo-optics characterizations confirmed that the first and second yield points could be attributed to the bond and cage breakage phenomena. Studying the effect of concentration, the second yield stress increased linearly by CNC concentration, whereas the first yield point manifested a power-law dependence on concentration (exponent of 0.5). This power-law relationship was further justified by the evolution of average distance between the CNC individual particles (d), calculated through SAXS analysis.

Rheology of Suspensions of Flat Elastic Particles.

We consider a suspension of noninteracting flat elastic particles in a Newtonian fluid. We model a flat shape as three beads, carried along by the flow according to Stokes law, and connected by nonlinear springs, chosen such that the energy is quadratic in the area. In analogy with common dumbbell models involving two beads connected by linear springs, we solve the stochastic equations of motion exactly to compute the constitutive law for the stress tensor of a flat elastic particle suspension. A lower convected time derivative naturally arises as part of the constitutive law, but surprisingly the rheological response in strong extensional and strong contracting flows is similar to that of the classical Oldroyd-B model associated with dumbbell suspensions.

Progress and challenges in suspension rheology

Progress and challenges in suspension rheology

On the Rheology of Thixotropic and Rheopexic Suspensions

Semi-empirical equations are derived that describe the dependence of shear stress on shear rate during the flow of a one-component suspension. The suspension is considered as consisting of two fractions: single grains of the solid phase and their dimers, between which a reversible dimerization reaction occurs. In this case, the dimerization of single grains is considered as a reaction with an invariable rate constant, and the dissociation of dimers is considered as an inverse reaction with a rate constant that increases linearly with the shear rate. The equations are based on the Krieger−Doherty formula, generalized to the case of a multicomponent suspension.

Rheology of cellulose nanocrystal and nanofibril suspensions

Nanocellulose is a sustainable nanomaterial and a versatile green platform that has attracted increasing attention. Although the wide applications of its aqueous suspensions are closely related to rheology, comprehensive studies of their rheological behavior, especially the yielding behavior, are still limited. Herein, to investigate the relationship between structure and rheological properties, the viscoelasticity, thixotropy and yielding behavior of two commonly used nanocelluloses, rod-shaped cellulose nanocrystals (CNCs) and filamentous cellulose nanofibrils (CNFs), were systematically investigated. The viscosity, viscoelasticity and thixotropic behavior of the suspensions were analyzed by steady-state shear, frequency sweep, creep-recovery, hysteresis loop, and three-interval thixotropic recovery tests. The yielding behaviors were evaluated through creep, steady-state shear, step shear rate, stress ramps, amplitude sweep, and large amplitude oscillatory shear tests. The rheological properties of the two typical suspensions showed a strong dependence on concentration and time. However, compared to CNC suspensions, CNF suspensions exhibited stronger thixotropy and higher yield stress due to the higher aspect ratio of CNF and the stronger structural skeleton of the suspensions as supported by Simha's equation and micromorphology analysis. This work provides a theoretical rheology basis for the practical applications of nanocellulose suspensions in various fields.

The effect of Navier slip on the rheology of a dilute two-dimensional suspension of plate-like particles

Through boundary integral simulations and asymptotic analysis, we investigate the effect of a finite Navier slip length on the rheological proprieties of a dilute two-dimensional suspension of plate-like particles in the creeping flow limit. Specifically, we study the effects of Navier slip, particle thickness and Péclet number on the effective shear viscosity and average normal stress difference of an isolated two-dimensional plate-like particle in an unbounded shear flow field. We find that Navier slip induces a significant reduction in the effective viscosity and increases the average normal stress difference. The effect of slip becomes more enhanced as the thickness of the particle decreases and as the Péclet number increases. Remarkably, the analysis suggests that it is theoretically possible for a dilute suspension of slip plate-like particles at high Péclet numbers to have a shear viscosity smaller than that of the suspending fluid.

Improving Rheology, Slurryability and Stability of Iron Ore Suspension Employing Variances in Size Distribution

Improving Rheology, Slurryability and Stability of Iron Ore Suspension Employing Variances in Size Distribution

Numerical simulation of the effects of non-Newtonian property of the solvent on particle suspension rheology

To investigate the effects of the non-Newtonian properties of solvents on particle suspension rheology (non-Newtonian property and viscosity), we analyzed suspension flows with different solvent characteristics. We used the power-law model to express the non-Newtonian properties of solvents. The power-law index of the suspension was denoted as nsus. and that of the solvent as [Formula: see text] to distinguish between their non-Newtonian properties. Besides, to evaluate the non-Newtonian property of the particle suspension, we estimated its power-law index [Formula: see text] by comparing its particle velocity with the velocity profile of a power-law fluid. As a result, regardless of the different area fraction, the power-law index of the particle suspension was lower than that of the solvent for [Formula: see text], 0.8, and 1.0. On the other hand, for [Formula: see text] and [Formula: see text], the power-law index of the particle suspension was higher than that of the solvent. Regarding the particle position, particles for [Formula: see text] were more concentrated on the wall side than those of the suspensions with [Formula: see text], 0.8, and 1.0. Thus, this study confirms that a change in the non-Newtonian property of a suspension due to interactions between particles and power-law fluids is an essential factor when discussing the rheology of suspensions.

Hidden hierarchy in the rheology of dense suspensions

Hidden hierarchy in the rheology of dense suspensions

Modeling and Analysis of Suspension-Based Electrolytes in Redox Flow Cells

Electrochemical systems with flowable suspension electrolytes (FSEs) have potential to help address growing demands for large-scale energy storage in order to bridge the gap between energy demand and renewable energy generation. Redox flow batteries with FSEs, for example, have several advantages in addition to long-duration energy storage and independent scaling of power and energy offered by the traditional flow batteries.1 FSEs containing solid energy storage materials enable higher charge storage capacities and hence higher energy densities, as well as new operating modes such as dissolution from and precipitation onto suspended particles.2-3 However, these emerging electrolyte compositions challenge cell design and operation as new performance trade-offs arising from the complex interplay of non-Newtonian rheology, flow-dependent transport properties, and electrochemical phenomena, must be considered.4 In this presentation, we will introduce a one-dimensional model to study the performance of a representative FSE-containing redox flow cell as a function of various material properties and operating conditions. The model integrates porous electrode theory with non-Newtonian suspension rheology and flow-dependent charge and species transport to investigate connections between rheological and electrochemical phenomena. We expound key dimensionless groups that arise from non-dimensionalizing the governing equations and describe the relative magnitudes of interrelated processes. We discuss model predictions of the cell performance in the multivariable space of the dimensionless groups using computed cell overpotentials and power relative to the pumping power, and identify specific operating regimes. Ultimately, the analysis will offer design considerations for electrochemical systems with FSEs, allowing more informed materials selection, cell engineering, and system architectures. Acknowledgments This work was funded by the Skoltech – MIT Next Generation Program. B.J.N gratefully acknowledge the NSF Graduate Research Fellowship Program under Grant Number 1122374. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. References M. L. Perry and A. Z. Weber, J. Electrochem. Soc., 163, A5064–A5067 (2016).H. Parant et al., Carbon, 119, 10–20 (2017).X. Wang, J. Chai, and J. “Jimmy” Jiang, Nano Materials Science, 3, 17–24 (2021).N. C. Hoyt, R. F. Savinell, and J. S. Wainright, Chemical Engineering Science, 144, 288–297 (2016).

A review of the shear rheology of carbon black suspensions

The microstructural link to the rheology of carbon black suspensions has recently become clear as a result of advances in computational and experimental methods. This understanding reveals the important role of the restructuring, build-up, and break-up of carbon black agglomerates in simple shear, rationalized by a dimensionless balance of the hydrodynamic forces acting to break the agglomerates apart against the cohesive forces holding them together (i.e., the Mason number). The Mason number not only can predict the origin of reversible thixotropy seen in carbon black suspensions observed at higher shear intensities, but can also be used to rationalize the evolution of microstructure at lower shear intensities. This review focuses on carbon black suspension behavior, but the insights derived from carbon black suspensions are broadly applicable to a diverse class of soft matter including colloidal gels relevant to a variety of applications.

A fully resolved smoothed particle hydrodynamics-discrete element method study of the rheology of suspensions: The role of inertia and grain shape

This paper presents a numerical study on suspensions of monodisperse non-Brownian grains in a Couette flow. The fully resolved coupled smoothed particle hydrodynamics and discrete element method is employed to model the motion of arbitrarily shaped grains in a viscous fluid. The numerical method is benchmarked against its capability in accurately handling grain–fluid hydrodynamics and inter-grain collisions. It is then used to simulate suspension flows of varying particle Reynolds and Bagnold numbers subjected to different shear rates, solid concentrations, and solid-to-fluid density ratios. A special focus is placed on the effect of grain shape with different aspect ratios and convexities on the flow behavior. Both the inertia and the grain shape are found to affect the grain–fluid and inter-grain interactions and uniquely contribute to the overall shear stress and the rheology of the suspension. The local profiles of solid concentration suggest the presence of grain layering near the boundary walls, which becomes more pronounced with higher solid concentration and inertia, and increased non-circularity in grain shape. A further examination of the pair distribution function and average particle rotation reveals a strong correlation between suspension viscosity and grain microstructure and kinematics.

Rheology of Suspensions of Solid Particles in Liquids Thickened by Starch Nanoparticles

The rheology of suspensions of solid particles in aqueous matrix liquids thickened by starch nanoparticles (SNP) was investigated. The SNP concentration varied from 9.89 to 34.60 wt% based on the aqueous matrix phase. The solids concentration of suspensions varied from 0 to 47 wt% (0 to 56 vol%). The suspensions at any given SNP concentration were generally Newtonian at low solids concentrations. At high solids concentrations, the suspensions were non-Newtonian shear-thinning. With the increase in the SNP concentration, the suspensions become non-Newtonian at a lower solids concentration. The rheological behavior of non-Newtonian suspensions could be described adequately with a power-law model. The consistency index of the suspension increased with the increase in solids concentration of the suspension at any given SNP concentration. The flow behavior index of suspensions was well below unity at high solids concentrations, indicating non-Newtonian shear-thinning behavior. The value of the flow behavior index decreased with the increase in solids concentration indicating an enhancement of shear-thinning in suspensions. The experimental viscosity and consistency data for Newtonian and non-Newtonian suspensions showed good agreement with the predictions of the Pal viscosity model for suspensions.

Electrosteric Control of the Aggregation and Yielding Behavior of Concentrated Portlandite Suspensions.

Portlandite (calcium hydroxide: CH: Ca(OH)2) suspensions aggregate spontaneously and form percolated fractal aggregate networks when dispersed in water. Consequently, the viscosity and yield stress of portlandite suspensions diverge at low particle loadings, adversely affecting their processability. Even though polycarboxylate ether (PCE)-based comb polyelectrolytes are routinely used to alter the particle dispersion state, water demand, and rheology of similar suspensions (e.g., ordinary portland cement suspensions) that feature a high pH and high ionic strength, their use to control portlandite suspension rheology has not been elucidated. This study combines adsorption isotherms and rheological measurements to elucidate the role of PCE composition (i.e., charge density, side chain length, and grafting density) in controlling the extent of PCE adsorption, particle flocculation, suspension yield stress, and thermal response of portlandite suspensions. We show that longer side-chain PCEs are more effective in affecting suspension viscosity and yield stress, in spite of their lower adsorption saturation limit and fractional adsorption. The superior steric hindrance induced by the longer side chain PCEs results in better efficacy in mitigating particle aggregation even at low dosages. However, when dosed at optimal dosages (i.e., a dosage that induces a dynamically equilibrated dispersion state of particle aggregates), different PCE-dosed portlandite suspensions exhibit identical fractal structuring and rheological behavior regardless of the side chain length. Furthermore, it is shown that the unusual evolution of the rheological response of portlandite suspensions with temperature can be tailored by adjusting the PCE dosage. The ability of PCEs to modulate the rheology of aggregating charged particle suspensions can be generally extended to any colloidal suspension with a strong screening of repulsive electrostatic interactions.

Novel PDMS-based transparent suspensions suitable for fluid flow characterization by optical techniques

Optical measurement techniques, such as µPIV and LDV are widely used in experimental fluid dynamics to investigate pure fluids or suspension flows. Optical access is a major concern in suspension experimentation. The suspended particle material must have a refractive-index matching those of the base fluid and wall set-up. This work reports a novel refractive-index-matched transparent suspension. The particulate matter is comprised of sub-10 µm spherical PDMS particles. PMDS is a widely used material to manufacture microchannels due to its low-refractive-indexand non-toxicity. PDMS microparticles were produced and suspended in a refractive-index matching base fluid (DMSO aqueous solution) and two transparent suspension concentrations were formulated [∼ 10% and ∼ 18% (w/w)]. Suspension rheology was characterized, andoptical properties analysed. μPIV successfully measured the velocity flow of the transparent suspensions in a circular-section microchannel, and the velocity profiles agreed well with the analytical solution.

Rheological state variables: A framework for viscosity parametrization in crystal-rich magmas

Igneous processes involve multiphase magmas, and the rate at which these processes operate is tightly controlled by multiphase rheology. The rheology of magmatic suspensions containing irregular-shaped crystals is complex, and this complexity is illustrated by the strong non-linear response to deformation and the lack of apparent self-similarity when comparing the results of experiments conducted under different shear conditions. Structural and empirical models have been developed to describe the experimental data. However, they are not predictive due to presence of several tuning parameters and their strong dependence on the flow variables and suspension characteristics. The current study proposes a state variable framework that restores self-similarity in the rheology of crystal-rich magmas. This framework only uses two rheological state variables and defines a universal rheological model to explain the relative viscosity of magmatic suspensions while considering microstructural changes such as crystals’ orientation, deformation, and comminution as well as shear-induced heterogeneities. We show that the viscosity of crystal-rich magmas as function of crystal content obeys the well-known monotonic and divergent behavior documented for suspensions of solid particles. This is achieved when the appropriate state of crystal dispersion is obtained using a unique product of the two rheological state variables. This theoretical approach, compared to empirical models, is as powerful for fitting, and can be also extrapolated beyond experimental data used in the fitting procedure. Hence, the state variable framework can be generalized to form a unified law with predictive power. At low shear rates, this framework is fully predictive as we can directly constrain both variables and their unique product. At high shear rates, one of the variables must be computed and measured independently. It is also shown that the proposed model can be used to detect the presence of anomalies, e.g., crystal breakage and shear-induced heterogeneities, in experimental data.

Orientational dynamics and rheology of active suspensions in weakly viscoelastic flows

Microswimmer suspensions in Newtonian fluids exhibit unusual macroscale properties, such as a superfluidic behavior, which can be harnessed to perform work at microscopic scales. Since most biological fluids are non-Newtonian, here we study the rheology of a microswimmer suspension in a weakly viscoelastic shear flow. At the individual level, we find that the viscoelastic stresses generated by activity substantially modify the Jeffery orbits well-known from Newtonian fluids. The orientational dynamics depends on the swimmer type; especially pushers can resist flow-induced rotation and align at an angle with the flow. To analyze its impact on bulk rheology, we study a dilute microswimmer suspension in the presence of random tumbling and rotational diffusion. Strikingly, swimmer activity and its elastic response in polymeric fluids alter the orientational distribution and substantially amplify the swimmer-induced viscosity. This suggests that pusher suspensions reach the superfluidic regime at lower volume fractions compared to a Newtonian fluid with identical viscosity.

Investigation of the Physicochemical Interactions and Modification Effects of Polyurethane on Asphalt Binder

Polyurethane (PU) is a synthetic polymeric material with excellent mechanical properties and chemical designability. An in-depth understanding of the synergetic effect between polyurethane and asphalt binder is needed to produce durable PU-modified asphalt paving materials. This study focuses on the physicochemical changes, the states of in situ synthesized PU, and the modification effects of PU after PU prepolymers and chain extenders are added into asphalt binder. Investigation methods involve the Fourier transform infrared (FTIR) spectroscopy, partial Corbett separation, confocal fluorescence microscopy, particle size distribution test, rheological test, and theoretical calculation. It was found that the incorporation of PU has no obvious chemical influence on the asphalt binder and the main working principle of PU modification is its physical dispersion in asphalt binder. In situ synthesized PU is uniformly dispersed in asphalt binder and the size distribution of PU particles follows a log-normal distribution. As PU content exceeds 15%, the formed PU particles in asphalt binder are noticeably enlarged. Moreover, PU modification leads to significant improvements in asphalt binder properties, including high-temperature rutting resistance, elastic recovery, and deformation resistance. Rheological experiments and theoretical calculations under the framework of suspension rheology indicate that PU-modified asphalt binder can be reasonably modeled as a bimodal solid particle suspension, and the dynamic viscosity of PU-modified asphalt binder is predominantly determined by the dynamic viscosity of the maltenes and the volume fractions of PU and asphaltenes.