Rheopexy and tunable yield stress of carbon black suspensions

TL;DR

This paper introduces a third class of yield stress fluids exemplified by carbon black suspensions, which exhibit rheopectic behavior and tunable yield stress influenced by flow history, supported by a predictive rheological model.

Contribution

It identifies and characterizes a new class of yield stress fluids with rheopectic behavior and flow history-dependent properties, and proposes a simple model to predict their rheological behavior.

Findings

Viscosity bifurcation with flow history dependence

Rheopectic behavior with flow acceleration after preshear

Tunable yield stress through flow history manipulation

Abstract

We show that besides simple or thixotropic yield stress fluids there exists a third class of yield stress fluids. This is illustrated through the rheological behavior of a carbon black suspension, which is shown to exhibit a viscosity bifurcation effect around a critical stress along with rheopectic trends, i.e., after a preshear at a given stress the fluid tends to accelerate when it is submitted to a lower stress. Viscosity bifurcation displays here original features: the yield stress and the critical shear rate depend on the previous flow history. The most spectacular property due to these specificities is that the material structure can be adjusted at will through an appropriate flow history. In particular it is possible to tune the material yield stress to arbitrary low values. A simple model assuming that the stress is the sum of one component due to structure deformation and one component due to hydrodynamic interactions predicts all rheological trends observed and appears to well represent quantitatively the data.