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.

Particle-based simulations of discontinuous shear thickening (DST) and shear jamming (SJ) suspensions are used to study the role of stress-activated constraints, with an emphasis on resistance to gear-like rolling. Rolling friction decreases the volume fraction required for DST and SJ, in quantitative agreement with real-life suspensions with adhesive surface chemistries and "rough" particle shapes. It sets a distinct structure of the frictional force network compared to only sliding friction, and from a dynamical perspective leads to an increase in the velocity correlation length, in part responsible for the increased viscosity. The physics of rolling friction is thus a key element in achieving a comprehensive understanding of strongly shear-thickening materials.

We propose a unifying rheological framework for dense suspensions of non-Brownian spheres, predicting the onsets of particle friction and particle inertia as distinct shear thickening mechanisms, while capturing quasistatic and soft particle rheology at high volume fractions and shear rates respectively. Discrete element method simulations that take suitable account of hydrodynamic and particle-contact interactions corroborate the model predictions, demonstrating both mechanisms of shear thickening, and showing that they can occur concurrently with carefully selected particle surface properties under certain flow conditions. Microstructural transitions associated with frictional shear thickening are presented. We find very distinctive divergences of both microstructural and dynamic variables with respect to volume fraction in the thickened and non-thickened states.

We study the viscoelastic response of amorphous polymers using theory and simulations. By accounting for internal stresses and considering instantaneous normal modes (INMs) within athermal non-affine theory, we make parameter-free predictions of the dynamic viscoelastic moduli obtained in coarse-grained simulations of polymer glasses at non-zero temperatures. The theoretical results show very good correspondence with rheology data collected from molecular dynamics simulations over five orders of magnitude in frequency, with some instabilities that accumulate in the low-frequency part on approach to the glass transition. These results provide evidence that the mechanical glass transition itself is continuous and thus represents a crossover rather than a true phase transition. The relatively sharp drop of the low-frequency storage modulus across the glass transition temperature can be explained mechanistically within the proposed theory: the proliferation of low-eigenfrequency vibrational excitations (boson peak and nearly-zero energy excitations) is directly responsible for the rapid growth of a negative non-affine contribution to the storage modulus.

Shear flow of dense, non-Brownian suspensions is simulated using the discrete element method, taking particle contact and hydrodynamic lubrication into account. The resulting flow regimes are mapped in the parametric space of solid volume fraction, shear rate, fluid viscosity and particle stiffness. Below a critical volume fraction φc, the rheology is governed by the Stokes number, which distinguishes between viscous and inertial flow regimes. Above φc, a quasistatic regime exists for low and moderate shear rates. At very high shear rates, the φ dependence is lost and soft particle rheology is explored. The transitions between rheological regimes are associated with the evolving contribution of lubrication to the suspension stress. Transitions in microscopic phenomena such as inter-particle force distribution, fabric and correlation length are found to correspond to those in the macroscopic flow. Motivated by the bulk rheology, a constitutive model is proposed combining a viscous pressure term with a dry granular model presented by Chialvo, Sun and Sundaresan [Phys. Rev. E. 85, 021305 (2012)]. The model is shown to successfully capture the flow regime transitions.

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Shear thickening, an increase of viscosity with shear rate, is a ubiquitous phenomenon in suspended materials that has implications for broad technological applications. Controlling this thickening behavior remains a major challenge and has led to empirical strategies ranging from altering the particle surfaces and shape to modifying the solvent properties. However, none of these methods allows for tuning of flow properties during shear itself. Here, we demonstrate that by strategic imposition of a high-frequency and low-amplitude shear perturbation orthogonal to the primary shearing flow, we can largely eradicate shear thickening. The orthogonal shear effectively becomes a regulator for controlling thickening in the suspension, allowing the viscosity to be reduced by up to 2 decades on demand. In a separate setup, we show that such effects can be induced by simply agitating the sample transversely to the primary shear direction. Overall, the ability of in situ manipulation of shear thickening paves a route toward creating materials whose mechanical properties can be controlled.T he viscosity of a densely packed suspension of particles can increase radically when sheared beyond a critical stress (1, 2). This thickening behavior has been exploited in technological applications ranging from vehicle traction control to flexible spacesuits that protect astronauts from micrometeorite impacts (3-5). It may also lead to flow problems, such as pipe blockage during industrial extrusion processes (6). Shear thickening has generally been considered an inherent material property (7), rather than as a response that can be tuned. As a consequence, suspension process design is often constrained within tight bounds to avoid thickening (8), whereas the applications of such flow behavior are limited by a lack of tunability.To design our control method, we take advantage of the underlying shear-thickening mechanisms that have been revealed recently. Experiments and simulations have shown that when the stress applied to a suspension of micrometer-sized particles exceeds a critical value, the particle-particle interaction switches from lubricated to frictional, enhancing resistance to flow (6, 9-13). The stress is transmitted through shear-induced force chains, which arise from frictional particle contacts (6, 13-15), aligned along the compressive axis. Such chains are fragile (16,17) and are constantly broken and rebuilt during steady shear.This fragility paradigm asserts that these stress-transmitting chains are themselves a product of the stress, with a finite chainassembly time required following startup or perturbations to the flow direction (18,19). These insights suggest a strategy for controlling thickening. For perturbations slower than chain assembly, contact rearrangement is sufficiently fast that force chains remain aligned with the instantaneous net compressive axis. Conversely, for perturbations faster than the assembly time, chains cannot reach compatibility with the instantaneous net compressive axis, but occupy a pa...

Driven tuning dramatically enhances flowability in a broad class of suspended granular materials.