Session T26: Suspensions: Rheology

On the nonlocality of dense frictionless suspensions: Experimental evidence and proposed theory

We have recently characterized the rheology of viscous and inertial suspensions of frictionless particles across multiple lengths from nm to m by coupling advanced force microscopy, conventional rheometry, and lab-scale experiments. The well-resolved experimental results demonstrate that the transition from the viscous to inertial regime occurs at a surprisingly small value of the particle Reynolds number suggesting that a larger length scale may play a role. Therefore, the current local rheological laws based on binary collisions and transfer of momentum are inadequate in explaining the behavior of dense suspensions and their transition. These experiments suggest a hypothesis that the interplay of colloidal and hydrodynamic forces leads to the formation of clusters of particles which dominate the transmission of momentum across the material. In this talk, we discuss our hypothesis and put forward a proposed theory to address the nonlocality of dense frictionless suspensions accounting for the viscous stresses and inertial impacts acting on the clusters. We provide support for our hypothesis and proposed theory by presenting our preliminary experimental results of cluster formation in channel flows of dense suspensions.

    

Numerical simulations of sheared dense granular suspensions in transition between the Newtonian and Bagnoldian regime.

Dense suspensions constituting non-colloidal particles and a viscous carrying fluid are ubiquitous in natural processes and are also encountered in industrial applications. Rheology of these dense granular suspensions is not fully comprehended and a unified theoretical framework to describe suspension flows across various flow regimes is still lacking. Recent experiments have identified that dense granular suspensions exhibit a transition in rheology from Newtonian to Bagnoldian at a Stokes number around 10 irrespective of the volume fraction of the suspension. In the present work, we perform fully coupled, grain-resolved numerical simulations of volume-imposed and pressure-imposed rheometry using the Immersed Boundary Method which allow us to determine the stress balance in the streamwise and vertical directions of the suspension. These simulations also ascertain the stress exchange between the fluid and particle phases. We explore the aforementioned transition for a wide range of Stokes numbers as the ratio of competing inertial and viscous effects. The data will also be used to evaluate the individual contribution of contact and hydrodynamic stresses to the total stress.   

Microstructure and rheology of dense frictionless non-Brownian suspensions

Discrete element simulations are used to characterize the microstructure and rheology of concentrated non-Brownian suspensions of spheres in which short-range repulsive forces prevent the formation of direct frictional contacts and limit the strength of lubrication interactions.  Ball and Melrose (1994 & 1995) showed that sheared suspensions of smooth spheres with purely hydrodynamic interactions form particle clusters with increasingly smaller interparticle distances and stronger lubrication forces eventually arresting the motion of the suspension for any finite imposed stress.  This paper will examine the finite particle cluster sizes and the resulting particle velocity correlation functions that arise in the presence of short-range repulsive forces.  We will characterize the mechanisms by which clusters form and evolve, providing insight into the flow-microstructure relationship.  We will also consider how stresses are transmitted across a cluster through normal repulsive forces and both normal and tangential lubrication forces contributing to the bulk rheological response and its dependence on volume fraction and the range of the interparticle repulsion.

References:

Ball and Melrose, Proc. of INDO U.K. forum (1994).

Melrose and Ball, Europhys. Lett., 32 (6), pp. 535-540 (1995)

    

On the rheology and microstructure of wizarding quidditch ball suspensions

This study focuses on dense frictional non-Brownian suspensions, and it is motivated by one central question: "What is the role of particle shape in dense frictional non-Brownian suspensions?". Answering this question has so far remained elusive due to the difficulties that arise, such as characterizing the shape, detecting the contact points among particles and calculating forces among particles of arbitrary shape. In this talk, we present our experimental results on the shear-reversal experiments and rheological characterization of dense frictional suspension. More specifically, our system of study includes suspensions of crushed and spherical particles of the same materials in the same solvent. We shear the suspensions for a wide range of stresses. We benefit from the measurements of the advanced force microscopy and characterization of the rheology to carefully examine the role of adhesion forces at small shear stresses and the role of particle shape disentangled from adhesion forces at large imposed shear stresses. Our experiments shed light on the pure role of particle shape independent of the surface forces on the hydrodynamic interactions, sliding and rolling friction and consequently the microstructure and rheological behavior of the non-Brownian suspensions.   

Suspensions of plate-like particles with slip can display negative intrinsic viscosity

It is generally accepted in suspension rheology that the effective viscosity of a suspension of rigid particles increases with solid loading. Challenging this assumption by designing solute particles that reduce the viscosity of the solvent, i.e. negative intrinsic viscosity suspensions, would enable to add useful properties to a fluid without the penalty of increased friction and viscous loss. Through a combination of MD simulations and micro-hydrodynamic theory, we have demonstrated a new class of viscosity-reducing suspensions, obtained by a mere change of particle shape and a judicious choice of surface hydrodynamic slip properties and aspect ratio of the particles. Key features are a plate-like particle shape and a hydrodynamic slip length larger than the thickness of the particle. The effective viscosity reduction, consequence of the change in rotational dynamics [1] (Kamal et al,., Nat. Comm., 11(1), 2020; Kamal et al., J. Fluid Mech. (19, 2021), has been analyzed in our group via i) numerical calculation of the (dilute limit) particle-stress for plate-like particles with Navier slip at their surface, ii) molecular dynamics simulations of few-layer graphene nanoparticles in water and other solvents, and iii) dynamic multi-particle Boundary Integral simulations for increasing solid fractions. The results indicate that there is a practically realizable geometric and concentration range in which a suspension of plate-like particles can have a viscosity significantly smaller than that of the suspending fluid, and that this range may be realizable with suspensions of 2D nanomaterials such as graphene.   

Shear-thickening rheology of concentrated nanoparticle suspensions containing non-adsorbing polymer

The additive manufacturing technique of direct-ink-writing can be used to fabricate custom and precise structures from ceramic nanoparticle suspensions for applications in electronics and biomedical industries, with polymers often added as viscosity modifiers. Understanding how polymer addition affects concentrated suspension rheology, and in particular the onset of shear-thickening, is crucial for the design of 3D-printable polymer-ceramic suspensions. Here, we examine the rheology of aqueous alumina nanoparticle suspensions with and without addition of non-adsorbing polyvinylpyrrolidone (PVP). For PVP-free suspensions, the viscosity and yield stress both increase as the alumina loading increases from 55 to 57.5 vol%. Discontinuous shear-thickening is observed at high shear rates and the onset stress of shear-thickening decreases slightly with increasing alumina loading. Addition of 0.5-5 vol% PVP to a 55 vol% alumina suspension also causes large increases in both viscosity and yield stress. Interestingly, in this case the onset stress of shear-thickening increases with polymer addition. We relate the overall suspension rheology to the polymer conformation in the suspending medium, demonstrating how polymer addition can extend the processing window during extrusion.

    

Designing a model bio-compatible yield stress fluid composed of hairy nanoparticles in aqueous solvents

Despite the importance of mucus in clinical detection of trapped bacteria and viruses, a systematic study of mucus has been hindered by sample limitations such as rheological variance between species and the diseases it carries. We present an opportunity to study bacterial motility in a carefully tailored artificial mucus made from a set of bio-compatible elastoviscoplastic fluids of varying yield stresses and microstructures. Covalently tethered poly(ethylene glycol) methyl ether (mPEG) on nanometer-sized silica particles are well-dispersed in aqueous interstitial fluids in order to mimic the rheological behavior of mucus. Self-suspended hairy nanoparticles (HNPs) are known to exhibit soft glassy rheology, with static yield stress coming from interactions of tethered mPEG. The HNPs retain this yielding behavior in suspensions, and their yield stress value decreases gradually with the addition of solvent. We additionally find that these suspensions show a thixotropic behavior, enabling us to probe the dynamic yield stress in the presence of background shear flows similar to those produced by bacteria motion in mucociliary clearance as well as mucus secretions in gastrointestinal and respiratory tracts. The microstructure of the fluids is probed through small angle X-ray scattering (SAXS), which shows a homogeneous distribution of HNPs in the suspension. The microstructural length scale characterization allows us to understand the geometrical constraints, structure, and continua that bacteria encounter while moving their bodies and flagella through the model elastoviscoplastic fluids.   

The Measurement of Low Yield Stress of Dilute Clay Suspensions using a Novel In-Line Microfluidic Rheometer

The yield stress (τ_y) is a rheological parameter of great significance in particle suspensions as it depicts the inherent structural strength of the liquid-spanning network made up of particles. The τ_y is a function of the weight fraction of the solid particles, and hence, the former could be a surrogate measure of the latter, which is a useful process parameter in various applications such as the recovery of Athabasca bitumen oil from middlings in the Clark’s hot water extraction process, and the transportation of oil tailings and other minerals to the tailing ponds. These dilute particle suspensions possess a yield stress as low as a few mPa to a few Pa. The existing rheometers that possess the ability to detect low yield stress cannot be employed in-line, and the ones that can be, do not measure low yield stress. Thus, we propose using an inexpensive, easy-to-use, and easy-to-clean technique using an in-house microfluidic platform for in-line rapid measurement of low τ_y. The τ_y of the suspension of water and cloisite clay of various weight fractions ranging between 3 to 6% was measured to be between 0.065 to 8 Pa. The τ_y showed a cubic dependence on the weight fraction, and the relationship between them was used for the evaluation of the unknown weight fraction through the measurement of τ_y. We also evaluated a τ_y as low as 0.001 Pa and as high as 20 Pa using our technique for other examples such as whole blood, mucin suspensions, kaolinite suspensions, wastewater sludge, and carbopol gel.   

Rheology of mobile sediment beds in laminar shear flow: effects of creep and polydispersity

Classical scaling relationships for rheological quantities of monodisperse particles have become increasingly popular for closures of two-phase flow modelling. We extend these considerations to more realistic sediment compositions. We investigate the rheological behavior of sheared sediment beds composed of polydisperse spherical particles with a diameter size ratio of up to 10 in a laminar Couette-type flow. The data were generated by fully coupled, grain resolved direct numerical simulations and yield depth-resolved profiles of the relevant rheological quantities. A comparison against experimental data shows excellent agreement for the monodisperse case. We improve upon the parameterization of the μ(J)-rheology by expressing its empirically derived parameters as a function of the maximum particle volume fraction. Furthermore, we extend these considerations by exploring the creeping regime for very low viscous numbers. Considering the low shear rates of our data, we found that the friction coefficient governing the quasi-static state tends to a finite value for vanishing shear, which decreases the critical friction coefficient by a factor of three for all cases investigated.

    

Viscosity reduction of cementitious suspension under an ultrasonic effect

The application of ultrasound to particle suspensions has been widely studied to improve the properties of suspensions. Many studies have focused on the dispersive effect of ultrasound by cavitation in suspensions with low or medium volume fractions of solids. Although it was reported that the viscosity of particle suspensions can be reduced with increased dispersion, the role of ultrasound in decreasing the viscosity of dense particle suspensions has been elusive thus far. Here we quantitatively studied the effect of ultrasound on the viscosity of dense cementitious suspensions. The change in viscosity depends on the application time and the power of an externally imposed ultrasound. Aggregates and agglomerates in the suspension can be degraded by cavitation effects even with a high volume fraction of solid. In addition, it was observed that the effective viscosity of MTA (Mineral Trioxide Aggregate), a cementitious material for endodontic use, decreased while flowing in the microchannel, which can possibly improve the dental treatment. These results suggest that ultrasound can offer an attractive means to control the flow characteristics of cementitious materials for various applications.   

Breakage of Jammed Suspensions using DC Electric Field

Application of DC electric fields (EFs) to networked suspensions of non-polar particles dispersed in polar liquid induces network breakage. The particles are soluble at high temperature and upon cooling, nucleate, grow and form interparticle connections. For such materials, we demonstrate that application of EF breaks the network converting the solid-like material to a low viscosity fluid with no yield stress. Maxwell stresses in contact zone between two particles are compressive and must be large enough to cause breakage of particle-particle contacts within the network.

Experiments are performed with model suspensions containing network of low ε particles in high ε continuous phase. Upon application of EF, a reduction of more than one order in viscosity and yield stress has been observed and corresponding breakage and consolidation of microstructure has also been recorded. The extent and rate of breakage of network and corresponding rheological signatures are studied as a function of wax concentration, shear rate, and dielectric constants of the solvent phase. These are correlated to the estimated Maxwells stresses acting on particles to break the network in presence of EF and Mason’s number indicating the dominance of viscous versus electric forces during reduction of viscosity.   

Shear rheology of magnetorheological fluids under triaxial fields

Magnetorheological (MR) fluids are suspensions of magnetizable microparticles in a Newtonian fluid. In an external magnetic field, these particles polarize and the resulting forces cause structure to form on short time scales. The microstructure translates to a sharp change in the rheological properties of the sample. In particular, the MR fluids show a pronounced shear-thinning behavior.

Traditionally, MR is studied under steady uniaxial DC fields. In this scenario, the microstructure consists of (1D) chain-like aggregates aligned with the field. However, it has been shown that more complex structures, e.g. 3D networks of sheets, can be generated by simply applying AC triaxial fields of high frequency [1].

In this work, the rheological response of MR fluids under shear flow and triaxial fields is evaluated through particle level simulations. Several field configurations and shear rates are tested to ascertain the interplay among them, focusing on their effect on the microstructure and viscosity. Finally, we seek to collapse those results in terms of the Mason number (ratio between disrupting shear and magnetic attractive forces).

References

[1] Martin et al. (2004). Physical Review E, 69(2), 021508