Non-colloidal Suspensions Research Articles

Contact stress decomposition in large amplitude oscillatory shear of concentrated noncolloidal suspensions

The concentrated noncolloidal suspensions show complex rheological behavior, which is related to the existence of contact stress. However, determining the contact stress in time-varying flow like oscillatory shear is challenging. Herein, we propose a contact stress decomposition method to decompose the total stress directly into contact stress and hydrodynamic stress in large amplitude oscillatory shear (LAOS). The results of hydrodynamic stress and contact stress are consistent with those determined by the shear reversal experiment. The contact stress decomposition also explains the failure of the Cox–Merz rule in noncolloidal suspensions because the particle contacts exist in steady shear but are absent in small amplitude oscillatory shear. The intracycle and intercycle of contact stress are further analyzed through the general geometric average method. The intracycle behaviors exhibit strain hardening, strain softening, and shear thickening. The intercycle behaviors show bifurcations in stress-strain and stress-strain rate relations, where the transition strains at different concentrations define the state boundaries between the discrete particle contacts, the growing of particle contacts, and the saturated contacts. We also established a phenomenological constitutive model using a structural parameter to describe the shear effect on the buildup and breakdown of particle contacts. The contact stress of noncolloidal suspensions with wide ranges of particle concentrations and strain amplitudes under LAOS can be well described by the model.

Porous structures impact on particle dynamics of non-Brownian and noncolloidal suspensions

This study aims to provide valuable insights into the impact of porous structures on particle dynamics in non-Brownian, non-colloidal suspension flows at very low Reynolds numbers. Two experimental approaches, Particle Image Velocimetry (PIV) with refractive index matching and Optical Flow Tracking Velocimetry (OFTV) were employed to analyze very dilute suspensions over various porous media models. The study considered three different porous structures with permeabilities ranging from 0.7 to 0.9 and three different thicknesses ranging from 0.2 cm to 0.5 cm, while the suspension bulk volume fraction was maintained at 3 %. In the PIV analysis, we observed that decreasing the porous permeability resulted in the maximum velocity location within the free flow region moving closer towards the interface between the flow and the porous media. We further quantified the effect of the porous structure on the suspension by characterizing interface properties, such as dimensionless slip velocity, shear rate, and slip length. These interface properties were found to be influenced by both the thickness and permeability of the porous media. Next, we analyzed particle migration due to the presence of porous structures using OFTV for very dilute suspensions of 1 %, 2 %, and 3 %, considering a porous medium with known physical properties and thickness. The study revealed two local concentration maxima: one within the free flow region on top of the rod arrays used to create the porous structure and a second along the rods' centerline inside the porous media model.

Newtonian coalescence in colloidal and noncolloidal suspensions

Newtonian coalescence in colloidal and noncolloidal suspensions

Limits for safe viscosity measurement of non-colloidal suspensions in rotational rheometry—a numerical simulation-based approach

Limits for safe viscosity measurement of non-colloidal suspensions in rotational rheometry—a numerical simulation-based approach

Hysteresis and ribbons in Taylor-Couette flow of a semidilute noncolloidal suspension

Hysteresis and ribbons in Taylor-Couette flow of a semidilute noncolloidal suspension

Numerical simulations of suspensions of rigid spheres in shear-thinning viscoelastic fluids

In multiphase flows, accurately modeling the interaction between the liquid phase of complex fluids and a porous medium of solid spheres poses a fundamental challenge. The dynamics of moderately dense non-colloidal suspensions constituted by static random arrays of mono-disperse spherical particles in non-linear viscoelastic fluids is studied numerically. This numerical study consists of about 9000 different systems, in which the volume fraction ϕ (0.04≤ϕ≤0.2) of the dispersed solid phase, the Reynolds number Re(5≤Re≤50), the solvent viscosity ratio β(0.05≤β≤0.9), the Weissenberg number Wi(0.5≤Wi≤4), and the mobility parameter of the Giesekus model α (0.1≤α≤0.5) were varied to understand the particle's interactions with the viscoelastic suspending fluid. We aim to investigate the relationship between the volume fraction of the dispersed solid phase and the non-linear rheology of shear-thinning viscoelastic fluids with the normalized average drag force ⟨F⟩. In addition, by assessing the flow patterns predicted numerically, we were able to provide a characterization of the velocity and stress fields as a function of the simulation parameters.

The second shear-thinning and strain-stiffening behaviors of bidisperse non-colloidal suspensions

Experimental research is done to determine the shear-thinning behavior of a bidisperse non-colloidal suspension under steady-state shear and the strain-stiffening behavior under oscillatory shear. The second shear-thinning behavior is displayed when the volume fraction of the bidisperse particle suspension is between medium and high. It exhibits with an increase in shear rate, the viscosity drops by approximately three orders of magnitude. At low shear rates, a strong particle size dependence of viscosity is observed, while at high shear rates, the particle size dependence is almost non-existent. To further understand the behavior of the second shear thinning, three sets of oscillatory shear tests and steady-state shear tests (constant shear rate) are carried out at three stopping points in parallel using a unique experimental technique we have developed. The values of modulus and viscosity at the third position are significantly smaller than those at the first and second positions. In the oscillatory shear test, the storage and loss moduli of the bidisperse suspension first decrease and then increase as the strain amplitude increases. The particle volume fraction, not the particle size, is closely related to the bidisperse critical strain amplitude. Compared with monodisperse suspension, bidisperse suspension has low viscosity and larger modulus well under steady-state shear and oscillatory shear operations, which is beneficial to the development of new materials and processes. Moreover, by adding a surfactant of a specific concentration to the bidisperse sample, it is proved that the change in the microstructure of the suspension particle is responsible for the rheological properties of the suspension.

Single-bubble sonoluminescence of aqueous suspensions of ZnS or Tb(acac)3·H2O nanoparticles

Aqueous suspensions of nanoparticles (below 50 nm in size) of ZnS or Tb(acac)3·H2O crystals were prepared by dispersion under multibubble sonolysis. Single-bubble sonoluminescence (SL) of these colloidal suspensions in an ultrasonic standing wave was found. For a ZnS suspension, the SL spectrum of a bubble moving at acoustic pressure pa = 1.29 bar, recorded with 12 nm resolution, exhibited a ZnS band with a maximum at 470 nm against the background of a wide continuum of bubble luminescence (200–700 nm). At a resolution of 1.2 nm, numerous lines of atomic (Zn, S) and ionic (S+, S2+, S3+) emitters were present in the 270–670 nm range. For a Tb(acac)3 suspension at 12 nm resolution, Tb3+ quasilines were detected at 488, 544, 585, and 618 nm (5D4 – 7Fj transitions, j = 6,5,4,3). At 1.2 nm resolution, no terbium atomic lines were found. The ZnS band and Tb3+ quasilines were present, although with a lower intensity (at a constant continuum intensity), in the SL spectra of suspensions for a stationary (pa = 1.07 bar) bubble. The atomic emitter lines for a stationary bubble were absent in the spectra of both suspensions. The ZnS band and Tb3+ quasilines in the SL spectra were similar to those in the photoluminescence (PL) spectra. However, the spectrum of a moving bubble in a Tb(acac)3 suspension differed from the PL spectrum in the relative intensities of the quasilines for some transitions. It was concluded that a non-equilibrium plasma was ignited in a stationary or moving bubble in suspensions under periodic compression of its gas content. This was the main source of luminescence during sonolysis, which was observed as thermal emission bursts in a plasma with a spectral continuum. In addition, sonophotoluminescence was generated as re-emission of the continuum luminescence partially absorbed by nanoparticles in suspensions; this occurred as flashes of ZnS or Tb3+ characteristic luminescence. Additionally, for a moving bubble, there was luminescence of these emitters associated with the entry of nanoparticles into the bubble and the collisional excitation of emitters in the plasma. In the case of a ZnS suspension, there was luminescence of atomic emitters (Zn, S) that formed upon decomposition and excitation of fragments of the initial emitter (ZnS) in a bubble plasma. The absence of a similar component in the SL spectrum of the Tb(acac)3 suspension may be attributable to low efficiency of the collisional excitation of terbium atoms, which were undoubtedly formed upon decomposition of nanoparticles in a bubble plasma. The emitters and mechanisms of the studied single-bubble sonoluminescence in colloidal suspensions were compared with those for the previously considered (J. Luminescence. 252 (2022) 119389) multibubble sonotriboluminescence in non-colloidal suspensions of ZnS or Tb(acac)3·H2O crystals.

Transitions in Taylor-Couette flow of concentrated non-colloidal suspensions.

Taylor-Couette flow of concentrated non-colloidal suspensions with a rotating inner cylinder and a stationary outer one is numerically investigated. We consider suspensions of the bulk particle volume fraction ϕb = 0.2, 0.3 with the ratio of annular gap to the particle radius ε = 60 confined in a cylindrical annulus of the radius ratio (i.e. ratio of inner and outer radii) η = 0.877. Numerical simulations are performed by applying suspension-balance model and rheological constitutive laws. To observe flow patterns caused by suspended particles, the Reynolds number of the suspension, based on the bulk particle volume fraction and the rotating velocity of the inner cylinder, is varied up to 180. At high Reynolds number, modulated patterns undiscovered in the flow of a semi-dilute suspension emerge beyond a wavy vortex flow. Thus, a transition occurs from the circular Couette flow via ribbons, spiral vortex flow, wavy spiral vortex flow, wavy vortex flow and modulated wavy vortex flow for the concentrated suspensions. Moreover, friction and torque coefficients for suspensions are estimated. It turns out that suspended particles significantly enhance the torque on the inner cylinder while reducing friction coefficient and the pseudo-Nusselt number. In particular, the coefficients are reduced in the flow of more dense suspensions. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (Part 2)'.

Unified torque scaling in counter-rotating suspension Taylor-Couette flow.

Torque measurements are reported for the Taylor-Couette flow of a neutrally buoyant non-colloidal suspension in the counter-rotation regime up to a particle volume fraction of [Formula: see text]. A unified scaling relation for the dimensionless torque (the pseudo-Nusselt number) of the form [Formula: see text] is shown to hold over a range of Taylor numbers [Formula: see text] that covers primary, secondary and tertiary bifurcating states; here, [Formula: see text] is the reduced Taylor number, [Formula: see text] is the critical Taylor number at primary bifurcation and [Formula: see text] is the relative viscosity of the suspension. Possible effects of flow transitions and inhomogeneous distribution of particles on torque scaling are discussed. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (part 1)'.

Particle migration of suspensions in a pressure-driven flow over and through a porous structure

Laboratory experiments were conducted to study particle migration and flow properties of non-Brownian, noncolloidal suspensions ranging from 10% to 40% particle volume fraction in a pressure-driven flow over and through a porous structure at a low Reynolds number. Particle concentration maps, velocity maps, and corresponding profiles were acquired using a magnetic resonance imaging technique. The model porous medium consists of square arrays of circular rods oriented across the flow in a rectangular microchannel. It was observed that the square arrays of the circular rods modify the velocity profiles and result in heterogeneous concentration fields for various suspensions. As the bulk particle volume fraction of the suspension increases, particles tend to concentrate in the free channel relative to the porous medium while the centerline velocity profile along the lateral direction becomes increasingly blunted. Within the porous structure, concentrated suspensions exhibit smaller periodic axial velocity variations due to the geometry compared to semidilute suspensions (bulk volume fraction ranges from 10% to 20%) and show periodic concentration variations, where the average particle concentration is slightly greater between the rods than on top of the rods. For concentrated systems, high particle concentration pathways aligned with the flow direction are observed in regions that correspond to gaps between rods within the porous medium.

Role of confinement in the shear banding and shear jamming in noncolloidal fiber suspensions.

The jamming effect is critical in processing short fiber-reinforced thermoplastics (FRTs). Fiber jamming can induce discontinuous shear thickening (DST) in simple shear and result in fiber-matrix separation in more complex flows such as injection molding and compression molding of FRTs. The confinement effect commonly induces local jams and strongly enhances fiber jamming. However, the transient evolution of local fiber jams under confinement and its correlation with the tumbling of fibers are still elusive. In this study, we adopted rheo-PIV (particle image velocity) techniques to study this effect for glass fiber-reinforced thermoplastics (FRTs). The translational and tumbling motion of fiber were determined during rheological measurements, and the distribution of fiber orientation was determined by X-ray CT. Three shear banding regions appeared after the viscosity overshoot under high shear stress in suspensions with high fiber content, which was associated with the three regions of fiber orientation across the gap due to confinement. Shear banding was ascribed to the different tumbling speeds across the gap because of the different initial orientations and different wall confinements near and far from the wall. The local shear thickening and jamming behavior became most significant under intermediate confinement, and were affected by shear strain, shear stress, and fiber contents. 3D state diagrams were constructed to show the confinement effect on the evolution of shear banding and jamming.

Reversing strains revisited

• Non-colloidal suspensions were tested in large amplitude sinusoidal strains • A reversing flow disengages contact friction in a strain of only about 10 −4 • This sudden disengagement explains the very small linear range of response . We consider strain reversing flow patterns for inelastic non- colloidal suspensions. A main result of this study is that the unlocking of the suspension structure takes place in a very small shear strain (O (10 −4 )) after a strain-rate reversal. This implies that for any computer simulation the motion would have to be continually scanned everywhere to find areas of strain reversal where sudden changes in stress take place We suggest that in oscillatory flow the difficult task of finding the range of linear G’ and G” behaviour of suspensions is also due to the sudden drops in stress upon strain-rate reversal; we expect the linear region of strain to be of the order of magnitude of half the sphere roughness ratio (0.5 of average roughness / sphere radius), which is here about 0.00075.So finding meaningful linear G’ and G” values is always going to be difficult.

Anisotropic Oldroyd-type models for non-colloidal suspensions of viscoelastic particles in Newtonian and yield-stress fluids via homogenization

<h2>Abstract</h2> This talk presents new rheological models, derived by means of homogenization techniques, for non-colloidal suspensions of particles undergoing finite deformations and rotations under Stokes flow conditions. The particles can be nonlinear viscoelastic, or elasto-viscoplastic, while the suspending fluid can be Newtonian or viscoplastic. The microstructure is described by two microstructural ellipsoids, which are allowed to evolve with the flow, characterizing the shape and orientation of the inclusions and the angular dependence of their distribution in space. The models account for particle interactions through the two-point correlation function for the particle centers, in a way that reproduces exactly classical and more recent estimates for dilute suspensions. While special cases of the models have been considered in earlier publications, novel features of this work include the possibility of multiple inclusion families, physically motivated evolutions laws for the particle distributions and more general rheological behaviors for the matrix and inclusions. The resulting models can be viewed as anisotropic generalizations of Oldroyd's invariant models accounting for statistical measures of the microstructure and their evolution.

Rheology of periodically sheared suspensions undergoing reversible-irreversible transition.

The rheology of noncolloidal suspensions under cyclic shear is studied numerically. The main findings are a strain amplitude (γ_{0}) dependent response in the shear stress and second normal stress difference (N_{2}). Specifically, we find a reduced viscosity, an enhanced intracycle shear thinning, the onset of a finite N_{2}, and its frequency doubling, all near a critical strain amplitude γ_{c} that scales with the volume fraction ϕ as γ_{c}∼ϕ^{-2}. These rheological changes also signify a reversible-irreversible transition (RIT), dividing stroboscopic particle dynamics into a reversible absorbing phase (for γ_{0}<γ_{c}) and a persistently diffusing phase (for γ_{0}>γ_{c}). We explain the results based on two flow-induced mechanisms and elucidate their connection in the context of RIT through the underlying microstructure, which tends toward hyperuniformity near γ_{0}=γ_{c}. Overall, we expect this correspondence between rheology and emergent dynamics to hold in a wide range of settings where structural organizations are dominated by volume exclusions.

Turbulent Rayleigh–Bénard convection in non-colloidal suspensions

This study presents direct numerical simulations of turbulent Rayleigh–Bénard convection in non-colloidal suspensions, with special focus on the heat transfer modifications in the flow. Adopting a Rayleigh number of $10^8$ and Prandtl number of 7, parametric investigations of the particle volume fraction $0\leq \varPhi \leq 40\,\%$ and particle diameter $1/20\leq d^*_p\leq 1/10$ with respect to the cavity height, are carried out. The particles are neutrally buoyant, rigid spheres with physical properties that match the fluid phase. Up to $\varPhi =25\,\%$ , the Nusselt number increases weakly but steadily, mainly due to the increased thermal agitation that overcomes the decreased kinetic energy of the flow. Beyond $\varPhi =30\,\%$ , the Nusselt number exhibits a substantial drop, down to approximately 1/3 of the single-phase value. This decrease is attributed to the dense particle layering in the near-wall region, confirmed by the time-averaged local volume fraction. The dense particle layer reduces the convection in the near-wall region and negates the formation of any coherent structures within one particle diameter from the wall. Significant differences between $\varPhi \leq 30\,\%$ and 40 % are observed in all statistical quantities, including heat transfer and turbulent kinetic energy budgets, and two-point correlations. Special attention is also given to the role of particle rotation, which is shown to contribute to maintaining high heat transfer rates in moderate volume fractions. Furthermore, decreasing the particle size promotes the particle layering next to the wall, inducing a similar heat transfer reduction as in the highest particle volume fraction case.

An experimental approach to investigate the viscoelastic and rheological behavior of polydisperse glass suspensions

Polydispersity is an essential factor in characterizing suspensions, specifically in industrial processes. In the present work, we report the viscosity of non-colloidal polydisperse glass suspensions for the first time and study the predictability of log-transformed quadratic viscosity equations over a wide range of blending ratios at a constant total volume fraction of solid particles. The viscosity and viscoelasticity measurement of multimodal suspensions was carried out by mixing three monomodally distributed solid particle suspensions with a relatively narrow size distribution to prepare bidisperse and tridisperse suspensions. The results showed a good agreement between experimental and predicted viscosity data for multimodal suspensions, making the generalized model a good alternative to describe the viscosity of polydisperse mixtures. The diameter ratio of large-to-small particles and the volume fraction of small particles in bidisperse systems strongly affected the viscoelastic behavior of multimodal suspensions. In addition, the dynamic rheology measurement showed that the storage modulus was independent of frequency, while a linear frequency dependence of loss modulus was observed for multimodal suspensions. In the case of polydisperse suspensions, this yield stress and dimensionless elastic modulus increased by declining the particle size ratio and increasing the volume fraction of small particles in bidisperse subsystems.

Multiscale nature of electric-field-induced structural formations in non-colloidal suspensions.

Non-colloidal suspensions undergoing dipolar interactions in an electric field have been extensively studied and are also known as smart materials as they share similarities with electrorheological (ER) fluids. Although the macroscopic responses are well-documented, the multiscale nature of such suspensions is still lacking. In this study, a large-scale Stokesian dynamics simulation is used to investigate the structural formation of such suspensions in an electric field up to highly concentrated regimes across different length scales: from particle-level (microscale) to particle cluster-level (mesoscale) and stress response-level (macroscale). It is observed that at a volume fraction of ϕ ≈ 30%, the steady-state structures are the most isotropic at the microscale, but at the macroscale, their normal stress fields are the most anisotropic. Interestingly, these structures are also the most heterogeneous at both the microscale and mesoscale. Furthermore, the effects of confinement on the multiscale responses are explored, revealing that there could be a strong link between the mesoscale and macroscale. This multiscale nature can offer the potential for precisely controlling or designing ER fluids in practical applications.

An analysis of non-colloid suspended particles in a Newtonian fluid over porous media

Transport of suspension particles over porous media occurs in many industrial and environmental processes such as filtering and sedimentation. Among these applications, flow of red blood cells over EGL has received considerable attention in the recent years. In this paper, by considering EGL as a porous mesh, we present the fundamental physics of suspensions of particles at various suspension concentrations over porous layers. We quantitatively examine the flow of mono-disperse non-colloidal suspensions over a porous medium by developing an analytical framework to model the flow in a channel where the lower surface was replaced by various porous media. The model validated by comparing the predictions of the flow at low suspension concentrations with the flow of pure Newtonian fluid over a porous layer and a good agreement was found. We showed that the volumetric flow rate induced by the presence of porous media depends on the values of the permeability parameter α=LK, the thickness ratio δ=HL, and the concentration of the suspension flow ϕ where L is the half-thickness of the free flow region, H is the thickness of the porous layer, and K is the permeability of the porous layer. Furthermore, the velocity and shear stress profiles for the flow over the permeable layer were examined for the limiting cases of α→0 and α→∞.

Numerical simulation of non-colloidal suspension flows in a parallel-plate geometry

Numerical simulation of non-colloidal suspension flows in a parallel-plate geometry