Irreversibility and rate dependence in sheared adhesive suspensions

Zhouyang Ge,Luca Brandt,Mario Minale,Raffaella Martone

doi:10.1103/physrevfluids.6.l101301

Abstract

Experiments and simulations of a non-Brownian suspension of particles demonstrate that weak van der Waals (adhesive) interactions induce rate dependence of the rheological response in oscillatory shear flow, with enhanced particle diffusivities and cluster formations below a critical shear rate, even though the steady shear behavior remains rate-independent. Phase diagrams showing the influence of volume fraction, strain amplitude, and oscillation frequency, for a given Hamaker constant, highlight the connection between irreversibility and suspension rheology.

Highlights

Recent experiments report that slowly sheared noncolloidal particle suspensions unexpectedly exhibit rate(ω)-dependent complex viscosities in oscillatory shear, despite a constant relative viscosity in steady shear
Rheology in oscillatory shear (OS) in the absence of any rate dependence in steady shear (SS) [8,9,10]. These authors demonstrate that, in OS, the suspension viscosity only depends on the maximum shear rate (γ0ω) and that data taken at different volume fractions (φ) can be rescaled on a single master curve, so as to highlight a universal behavior of these materials. This rheological observation questions the physics of self-organization in the simplest driven noncolloidal suspension: it was assumed that suspensions undergo transitions from reversible absorbing states to irreversible chaotic states if γ0 exceeds a φ-dependent critical amplitude γ0c(φ) independent of the driving frequency [1,2,11]; the experiments in [10] imply that ω may affect irreversibility
Below a critical shear rate in OS, ηR∗ exhibits power-law reductions with shear, ηR∗ ∼ Sr−α, with α a φ-dependent positive exponent largest at φ = 40%. This nonmonotonic dependence can be understood by considering the limits of very dilute and highly packed suspensions: at vanishing volume fractions, collisions and van der Waals (vdW) attractions are negligible, and the system is dominated by hydrodynamic forces and nearly rate independent; close to packing, lubrication and contact forces dominate but, because both are proportional to the shear rate, the system again exhibits weak rate dependence

Summary

Introduction

Recent experiments report that slowly sheared noncolloidal particle suspensions unexpectedly exhibit rate(ω)-dependent complex viscosities in oscillatory shear, despite a constant relative viscosity in steady shear. At volume fractions φ = 20–50%, the complex viscosities in both experiments and simulations display power-law reductions in shear, with a φ-dependent exponent maximum at φ = 40%, resulting from the interplay between hydrodynamic, collision, and adhesive interactions.

Results

Conclusion