Abstract
The process by which sheared suspensions go through a dramatic change in viscosity is known as discontinuous shear thickening. Although well-characterized on the macroscale, the microscopic mechanisms at play in this transition are still poorly understood. Here, by developing new experimental procedures based on quartz-tuning fork atomic force microscopy, we measure the pairwise frictional profile between approaching pairs of polyvinyl chloride and cornstarch particles in solvent. We report a clear transition from a low-friction regime, where pairs of particles support a finite normal load, while interacting purely hydrodynamically, to a high-friction regime characterized by hard repulsive contact between the particles and sliding friction. Critically, we show that the normal stress needed to enter the frictional regime at nanoscale matches the critical stress at which shear thickening occurs for macroscopic suspensions. Our experiments bridge nano and macroscales and provide long needed demonstration of the role of frictional forces in discontinuous shear thickening.
Highlights
The process by which sheared suspensions go through a dramatic change in viscosity is known as discontinuous shear thickening
We study two well-known shear-thickening systems: polyvinyl chloride (PVC) suspended in various solvents[19,20] and cornstarch particles suspended in water[21,22]
B, we measured using standard rheometry the flow curves for various solutions of PVC particles in mixtures of Dinch and mineral oil[19,20], and cornflour particles in water[21,22] at various solid fractions. All these suspensions exhibit a discontinuous shear thickening transition above a critical shear stress sC
Summary
The process by which sheared suspensions go through a dramatic change in viscosity is known as discontinuous shear thickening. Despite an extensive characterization of discontinuous shear-thickening transitions at the macroscale, there is still no clear understanding of the microscopic mechanisms at play in this transition, principally owing to the challenges associated with quantitative frictional measurements at the nanoscale[10], especially for pairs of particle[11]. When normal elastohydrodynamic contact forces are taken into account (without solid friction), these simulations capture continuous shear thickening and large increase in suspension viscosity[13]. This model predicts shear rate-independent rheology for non-Brownian systems and broader transition that observed experimentally. Repulsive forces are overcome, leading to frictional contacts and shear thickening
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