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Register a new userA unified description of the rheology of hard-particle suspensions
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The rheology of suspensions of Brownian, or colloidal, particles (diameter $d lesssim 1$ $mu$m) differs markedly from that of larger grains ($d gtrsim 50$ $mu$m). Each of these two regimes has been separately studied, but the flow of suspensions with intermediate particle sizes (1 $mutextrm{m} lesssim d lesssim 50$ $mu$m), 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.
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We explore the rheology predicted by a recently proposed constitutive model for jammed suspensions of soft elastic particles derived from microscopic dynamics [Cuny et al., arXiv:2102.05938]. Our model predicts that the orientation of the anisotropy of the microstructure, governed by an interplay between flow vorticity and contact elasticity, plays a key role at yielding and in flow. It generates normal stress differences contributing significantly to the yield criterion and Trouton ratio. It gives rise to non-trivial transients such as stress overshoots in step increases of shear rates, residual stresses after flow cessation and power law decay of the shear rate in creep. Finally, it explains the collapse of storage modulus as measured in parallel superposition for a yielded suspension.
Mixing a small amount of liquid into a powder can give rise to dry-looking granules; increasing the amount of liquid eventually produces a flowing suspension. We perform experiments on these phenomena using Spheriglass, an industrially-realistic model powder. Drawing on recent advances in understanding friction-induced shear thickening and jamming in suspensions, we offer a unified description of granulation and suspension rheology. A liquid incorporation phase diagram explains the existence of permanent and transient granules and the increase of granule size with liquid content. Our results point to rheology-based design principles for industrial granulation.
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Microcapsules are commonly used in applications ranging from therapeutics to personal care products due to their ability to deliver encapsulated species through their porous shells. Here, we demonstrate a simple and scalable approach to fabricate microcapsules with porous shells by interfacial complexation of cellulose nanofibrils and oleylamine, and investigate the rheological properties of suspensions of the resulting microcapsules. The suspensions of neat capsules are viscous liquids whose viscosity increases with volume fraction according to a modified Kreiger-Dougherty relation with a maximum packing fraction of 0.73 and an intrinsic viscosity of 4. When polyacrylic acid (PAA) is added to the internal phase of the microcapsule, however, the suspensions become elastic and display yield stresses with power-law dependencies on capsule volume fraction and PAA concentration. The elasticity appears to originate from associative interactions between microcapsules induced by PAA that resides within the microcapsule shells. These results demonstrate that it is possible to tune the rheological properties of microcapsule suspensions by changing only the composition of the internal phase, thereby providing a novel method to tailor complex fluid rheology.
We develop a statistical framework for the rheology of dense, non-Brownian suspensions, based on correlations in a space representing forces, which is dual to position space. Working with the ensemble of steady state configurations obtained from simulations of suspensions in two dimensions, we find that the anisotropy of the pair correlation function in force space changes with confining shear stress ($sigma_{xy}$) and packing fraction ($phi$). Using these microscopic correlations, we build a statistical theory for the macroscopic friction coefficient: the anisotropy of the stress tensor, $mu = sigma_{xy}/P$. We find that $mu$ decreases (i) as $phi$ is increased and (ii) as $sigma_{xy}$ is increased. Using a new constitutive relation between $mu$ and viscosity for dense suspensions that generalizes the rate-independent one, we show that our theory predicts a Discontinuous Shear Thickening (DST) flow diagram that is in good agreement with numerical simulations, and the qualitative features of $mu$ that lead to the generic flow diagram of a DST fluid observed in experiments.
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