Shear thickening is a widespread phenomenon in suspension flow that, despite sustained study, is still the subject of much debate. The longstanding view that shear thickening is due to hydrodynamic clusters has been challenged by recent theory and simulations suggesting that contact forces dominate, not only in discontinuous, but also in continuous shear thickening. Here, we settle this dispute using shear reversal experiments on micron-sized silica and latex particles to measure directly the hydrodynamic and contact force contributions to shear thickening. We find that contact forces dominate even continuous shear thickening. Computer simulations show that these forces most likely arise from frictional interactions.
The rheology of suspensions of Brownian, or colloidal, particles (diameter d ≲ 1 µm) differs markedly from that of larger grains (d ≳ 50 µm). Each of these two regimes has been separately studied, but the flow of suspensions with intermediate particle sizes (1 µm ≲ d ≲ 50 µm), which occur ubiquitously in applications, remains poorly understood. By measuring the rheology of suspensions of hard spheres with a wide range of sizes, we show experimentally that shear thickening drives the transition from colloidal to granular flow across the intermediate size regime. This insight makes possible a unified description of the (non-inertial) rheology of hard spheres over the full size spectrum. Moreover, we are able to test a new theory of friction-induced shear thickening, showing that our data can be well fitted using expressions derived from it.Complex fluids, polymers, colloids and surfactant solutions find wide applications, partly because of their highly tuneable behavior under deformation and in flow. The success of the mean-field 'tube' model for polymers [1], which describes how each chain is constrained by thousands of neighbours, means it has long been possible to predict ab initio their linear and non-linear rheology from the molecular topology with very few free parameters. In particular, a scaling description is available of the dependence of rheology on molecular weight.However, progress in suspension rheology has been more difficult [2]. The small number of nearest neighbours (order 10) rules our any mean-field description: local details matter. It is now possible to predict the lowshear viscosity of a suspension of Brownian hard spheres (HS, diameter d ≲ 1 µm) up to volume fractions of φ ≲ 0.6, and the rheology of granular HS (d ≳ 50 µm) is increasingly being studied. Surprisingly, however, how the rheology of HS changes over the whole size spectrum remains unknown, because the behavior in the industriallyubiquitous intermediate size regime, 1 ≲ d ≲ 50 µm, has not been systematically explored. We offer such an exploration in this Letter, and show that the physics bridging the colloidal and the granular regimes is shear thickening.The rheology of colloidal HS is well known [3][4][5]: the viscosity is determined by the particle volume fraction, φ, and the dimensionless shear rate, or Péclet number, Pe (= τ Bγ , the shear rateγ non-dimensionalised by the Brownian time, τ B , needed for a free particle to diffuse its own radius). At Pe ≪ 1 the flow is Newtonian; the viscosity becomes immeasurably large at φ g ≈ 0.58 [5,6]. Shear thinning starts at Pe ≲ 1, reaching a second Newtonian regime at Pe ≫ 1 with a viscosity that diverges at random close packing [2], φ RCP ≈ 0.64, the densest amorphous packing for lubricated (frictionless) HS.Since τ B scales as d 3 , granular HS inhabit the Pe ≫ 1 regime at all practical shear rates. Extrapolating naïvely from the above description of colloidal flow, one expects Newtonian behaviour with a viscosity diverging at φ RCP . Experiments do find a Newtonian viscosity, but i...
We present experimental results on dense corn-starch suspensions as examples of non-Brownian, nearly-hard particles that undergo continuous and discontinuous shear thickening (CST and DST) at intermediate and high densities respectively. Our results offer strong support for recent theories involving a stress-dependent effective contact friction among particles. We show however that in the DST regime, where theory might lead one to expect steady-state shear bands oriented layerwise along the vorticity axis, the real flow is unsteady. To explain this, we argue that steady-state banding is generically ruled out by the requirement that, for hard non-Brownian particles, the solvent pressure and the normal-normal component of the particle stress must balance separately across the interface between bands. (Otherwise there is an unbalanced migration flux.) However, long-lived transient shear-bands remain possible
This corrects the article DOI: 10.1103/PhysRevLett.115.088304.
We present a phenomenological model for granular suspension rheology in which particle interactions enter as constraints to relative particle motion. By considering constraints that are formed and released by stress respectively, we derive a range of experimental flow curves in a single treatment and predict singularities in viscosity and yield stress consistent with literature data. Fundamentally, we offer a generic description of suspension flow that is independent of bespoke microphysics.Concentrated particulate dispersions are ubiquitous in industry. When the particle size is in the granular (i.e., non-Brownian) regime (radius R ≳ 1 µm), their flow is notoriously difficult to predict and control [1, 2]. Paradoxically, a suspension of non-Brownian hard particles has no intrinsic time or stress scale and so should have a viscosity η that is independent of shear stress σ and ratė γ [2, 3]. In reality, three classes of flow curve η(σ) are observed, none of which is Newtonian. Some granular suspensions shear thin (dη dσ < 0, class 1) [4,5], others shear thicken (dη dσ > 0, class 2) [6-8] while others show a varied combination of thinning and thickening (class 3): thinning then thickening (class 3a) [9, 10], thickening then thinning (class 3b) [11][12][13] or more complex behavior [10,14,15] (class 3c). In each class, the suspensions can become solid-like [16] or flow unstably [17,18].Such behavior likely stems from details of the particle interactions [2] set by, e.g., surface chemistry [19] or roughness [20]. Most models incorporate such interactions in a bespoke manner. Notably, a phenomenological model by Wyart and Cates (WC) [21] predicts thickening (class 2) due to a transition from frictionless (static friction coefficient µ ≈ 0) to frictional (µ > 0) particle contacts above a critical "onset stress". Atomic force microscopy confirms this picture for several systems [15,22] and the WC model fits a number of experimental flow curves [7,8,18]; although, quantitative discrepancies with microscopic simulations remain [23].To recast the WC model within a more general framework, recall that frictional contacts constrain interparticle sliding. Crucially, the WC model is agnostic to the exact mechanism by which sliding is constrained, so that disparate microphysics, e.g., stress-induced interlocking of asperities [20,24], hydrogen bonding [25] or 'traditional' Coulomb friction can all give rise to the same macroscopic, shear-thickening phenomenology.In this broader framework, the WC model deals with a single type of constraint: sliding. Rolling (rotations about axes perpendicular to the line of centres) and twisting (rotations about the line of centres) degrees of freedom remain unconstrained. By assuming that sliding constraints are formed at increasing stress, the WC model accounts for class 2 behavior, which, however, is rare in practice. Real systems are typically class 1 or 3, for which current explanations involve the ad hoc "bolting together" of different kinds of bespoke physics [10].Here, we generalize the ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.