Shear thickening in densely packed suspensions of spheres and rods confined to few layers

TL;DR

This study explores how confinement in thin layers affects shear thickening in dense suspensions of spheres and rods, revealing oscillatory stress behavior and the transition to jamming or Newtonian flow depending on particle shape and confinement.

Contribution

It demonstrates how confinement and particle shape influence shear thickening and jamming, highlighting oscillatory stress patterns and the transition to ordered states.

Findings

Stress oscillations depend on sample thickness and particle layering.

Shear thickening persists down to two particle diameters for spheres.

Rods show stress increase and transition to Newtonian flow when confined below a particle length.

Abstract

We investigate confined shear thickening suspensions for which the sample thickness is comparable to the particle dimensions. Rheometry measurements are presented for densely packed suspensions of spheres and rods with aspect ratios 6 and 9. By varying the suspension thickness in the direction of the shear gradient at constant shear rate, we find pronounced oscillations in the stress. These oscillations become stronger as the gap size is decreased, and the stress is minimized when the sample thickness becomes commensurate with an integer number of particle layers. Despite this confinement-induced effect, viscosity curves show shear thickening that retains bulk behavior down to samples as thin as two particle diameters for spheres, below which the suspension is jammed. Rods exhibit similar behavior commensurate with the particle width, but they show additional effects when the thickness is reduced below about a particle length as they are forced to align; the stress increases for decreasing gap size at fixed shear rate while the shear thickening regime gradually transitions to a Newtonian scaling regime. This weakening of shear thickening as an ordered configuration is approached contrasts with the strengthening of shear thickening when the packing fraction is increased in the disordered bulk limit, despite the fact that both types of confinement eventually lead to jamming.