Shear thickening in highly viscous granular suspensions

We experimentally investigate shear thickening in dense granular suspensions under oscillatory shear. Directly imaging the suspension-air interface, we observe dilation beyond a critical strain γc and the end of shear thickening as the maximum confining stress is reached and the contact line moves. Analyzing the shear profile, we extract the viscosity contributions due to hydrodynamics ηh, dilation ηc and sedimentation ηg . While ηg governs the shear thinning regime, ηh and ηc together determine the shear thickening behavior. As the suspending liquid’s viscosity varies from 10 to 1000 cSt, ηh is found to compete with ηc and soften the discontinuous nature of shear thickening. Copyright c © EPLA, 2014 Introduction. – Dense suspensions can increase their viscosity under rapid shear; i.e., they exhibit shear thickening (ST) [1–11]. To understand the origin of this ST transition, several mechanisms have been proposed. A hydro-cluster picture ascribes mild, continuous thickening to particle groups formed by viscous hydrodynamic interactions [12–14]. Dense granular suspensions can exhibit a much stronger shear thickening, so that the viscosity can increase discontinuously with the shear rate as a critical packing fraction is approached [15–17]. Recent works have related this to frictional particle interactions and dilation [2,3,18,19], similar to dry granular materials. In this scenario, the Laplace pressure due to surface tension at the suspension-air interface keeps the suspension contained. Since granular systems prefer to dilate when made to flow, the normal stress and, subsequently, the friction between suspension and shear plate dramatically increases beyond a certain applied stress. Thus, the measured flow resistance shoots up as long as dilation is counteracted by confinement. In this granular mechanism, the frictional stress between solid particles is the dominant contributor to ST. The suspending liquid mainly acts as a boundary constraint to prevent expansion. Nevertheless, viscous hydrodynamic interactions, as another dissipation mechanism, still exist in the bulk [4,8,9], and lubrication and viscous drag could become significant when the suspending liquid is highly viscous. So far, however, an experimental characterization of how the hydrodynamics couples with dilation and affects the shear thickening of nearly jammed granular suspensions has been lacking. In this letter, we address this issue by investigating dense granular suspensions across a wide range of suspending liquid viscosities. The suspended particles are chosen to be sedimenting so that the friction between particles can provide a known scaling for the onset stress of shear thickening [5]. To finely control the relative displacement between particles, oscillatory shear with controlled amplitude is applied to the samples. Measuring both global rheology and local shear profile, we quantitatively extract the contributions from viscous hydrodynamics, confinement (“frustrated dilation”) and sedimentation to the measured flow resistance. Experimental setup and protocol. – Dense granular suspensions were prepared by adding ZrO2 particles (ρZrO2 = 3.92 g/ml, 200 ± 10μm) and glass beads (ρglass = 2.55 g/ml, 22±5μm) to silicone oils, with packing fraction φ ≈ 54–55%. The particles were too large to exhibit measurable Brownian motion. Rheological measurements were performed in an Anton Paar rheometer with a 25mm diameter parallel-plate geometry allowing us to conveniently access the global normal force. The gap size d was varied from 1 to 2mm. The top plate was set to apply a sinusoidal strain, γ = γ0 sin(ωt), to the sample. We fit the measured shear stress to an oscillating waveform