Boundary conditions alter density and stress fluctuations in shear-thickening suspensions

TL;DR

This study investigates how boundary conditions influence the formation of density inhomogeneities and stress fluctuations in shear-thickening suspensions, revealing that boundary constraints significantly affect the spatiotemporal dynamics of these phenomena.

Contribution

It demonstrates that boundary conditions critically modify the development and characteristics of inhomogeneities and stress fluctuations in shear-thickening suspensions, a novel insight into their rheological behavior.

Findings

Boundary conditions significantly alter fluctuation patterns.

Normal stress fluctuations correlate with high-density regions.

Boundary stiffness affects burst frequency and intensity.

Abstract

Discontinuous shear thickening (DST) in dense suspensions is accompanied by significant fluctuations in stress at a fixed shear rate. In this work, normal stress fluctuations are shown to have a one-to-one relationship with the formation and dissolution of local high-density regions. Namely, a burst in the force response corresponds to the spontaneous appearance of inhomogeneity. We observe that boundary conditions can significantly alter the spatiotemporal scale of these fluctuations, from short-lived to more sustained and enduring patterns. We estimate the occurrence frequency R and the average intensity Q of individual bursts/inhomogeneity events. The growth of R with the shear rate is the most rapid for the rigid boundary, whereas Q is nonmonotonic with confinement stiffness. Our results indicate that boundary conditions alter the development of inhomogeneity and thus the stress response under shear.