Shear thickening in dense particulate suspensions was recently proposed to be driven by the activation of friction above an onset stress needed to overcome repulsive forces between particles. Testing this scenario represents a major challenge because classical rheological approaches do not provide access to the frictional properties of suspensions. Here we adopt a different strategy inspired by pressure-imposed configurations in granular flows that specifically gives access to this information. By investigating the quasi-static avalanche angle, compaction, and dilatancy effects in different nonbuoyant suspensions flowing under gravity, we demonstrate that particles in shear-thickening suspensions are frictionless under low confining pressure. Moreover, we show that tuning the range of the repulsive force below the particle roughness suppresses the frictionless state and also the shearthickening behavior of the suspension. These results, which link microscopic contact physics to the suspension macroscopic rheology, provide direct evidence that the recent frictional transition scenario applies in real suspensions.soft matter | shear thickening | dense suspensions | friction D iscontinuous shear thickening occurs in suspensions whose viscosity dramatically increases, sometimes by several orders of magnitude, when the imposed shear rate exceeds a critical value (1). The archetype of such suspensions is cornstarch immersed in water. When sheared vigorously or under impact, these fluids suddenly turn into solids (2). Such remarkable properties play a key role in the flowing behavior of modern concrete (3) and have motivated applications ranging from soft-body protections to sports equipment (4). They also offer promising perspectives for the design of smart fluids with tunable rheology (5). However, the potential realm of development and applications remains largely underexplored due to the lack of understanding of this transition (6).This situation has improved very recently due to new theoretical and numerical works (7,8). Because non-Brownian suspensions of hard frictional particles immersed in a viscous fluid are Newtonian, as imposed by dimensional analysis (8-10), the key idea of these studies is to add a short-range repulsive force between particles in addition to hydrodynamics and contact forces. This repulsive force can, for instance, stem from electrostatic charges or from a specific coating of polymers on the surface of the particle (11). At small shear rate (or small stress), the repulsive force prevents the grains from coming into contact; the suspension thus flows easily as if particles were frictionless. In the remainder of this paper, this state is referred to as frictionless. The viscosity of such a frictionless suspension would diverge at random close packing, whose volume fraction is φrcp = 0.64 for monodisperse spheres. Conversely, at large shear rate (or large stress), the repulsive force is overcome by the hydrodynamic forces and particles are therefore pressed into frictional contacts. The visco...
Hysteresis is a major feature of the solid-liquid transition in granular materials. This property, by allowing metastable states, can potentially yield catastrophic phenomena such as earthquakes or aerial landslides. The origin of hysteresis in granular flows is still debated. However, most mechanisms put forward so far rely on the presence of inertia at the particle level. In this paper, we study the avalanche dynamics of non-Brownian suspensions in slowly rotating drums and reveal large hysteresis of the avalanche angle even in the absence of inertia. By using micro-silica particles whose interparticle friction coefficient can be turned off, we show that microscopic friction, conversely to inertia, is key to triggering hysteresis in granular suspensions. To understand this link between friction and hysteresis, we use the rotating drum as a rheometer to extract the suspension rheology close to the flow onset for both frictional and frictionless suspensions. This analysis shows that the flow rule for frictionless particles is monotonous and follows a power law of exponent α = 0.37 ± 0.05, in close agreement with the previous theoretical prediction, α = 0.35. By contrast, the flow rule for frictional particles suggests a velocity-weakening behavior, thereby explaining the flow instability and the emergence of hysteresis. These findings show that hysteresis can also occur in particulate media without inertia, questioning the intimate nature of this phenomenon. By highlighting the role of microscopic friction, our results may be of interest in the geophysical context to understand the failure mechanism at the origin of undersea landslides. arXiv:1904.03918v1 [cond-mat.soft]
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