Introduction to Yield Stress Measurements

Abstract

The yield stress characteristic is a property associated with numerous types of complex fluids - whereby the material does not flow unless the applied stress exceeds a certain value. This is evident in everyday tasks such as squeezing toothpaste from a tube or dispensing ketchup from a bottle, but is important across a whole range of industries and applications. The determination of a yield stress as a true material constant can be difficult as the measured value can be very much dependent on the measurement technique employed and the conditions of the test, of which there are many. Consequently, there is no universal method for determining yield stress and there exist a number of approaches, which find favour across different industries and establishments. This White Paper discusses the various approaches available to measure yield stress, and aspects of the practical measurement set-up and test parameters that need to be considered to obtain relevant, robust and reliable yield stress data using a rotational rheometer.

Introduction

Many complex fluids, such as network forming polymers, surfactant mesophases, emulsions etc do not flow until the applied stress exceeds a certain critical value, known as the yield stress. Materials exhibiting this behavior are said to be exhibiting yield flow behavior. The yield stress is therefore defined as the stress that must be applied to the sample before it starts to flow. Below the yield stress the sample will deform elastically (like stretching a spring), above the yield stress the sample will flow like a liquid [1].

Most fluids exhibiting a yield stress can be thought of as having a structural skeleton extending throughout the entire volume of the system. The strength of the skeleton is governed by the structure of the dispersed phase and its interactions. Normally the continuous phase is low in viscosity, however, high volume fractions of a dispersed phase and/or strong interactions between components can increase the viscosity by a thousand times or more and induce solid like behavior at rest [1, 2].

When a solid-like complex fluid is sheared at low shear rates and below its critical strain the system is subjected to strain hardening. This is characteristic of solidlike behavior and results from elastic elements being stretched in the shear field. When such elastic elements approach their critical strain the structure begins to break down causing shear thinning (strain softening) and consequent flow. The stress at which this catastrophic breakdown of the structural skeleton occurs is the yield stress and the associated strain the yield strain. This process can be depicted using mechanical analogues as shown in Figure 1 using a spring in parallel with a dashpot (or damper) for a viscoelastic solid, and a spring and dashpot in the case of a gel. In both cases the material cannot deform plastically (or flow) because it is restricted by the spring, which must first be broken. In the case of a viscoelastic solid, the yielded material will behave like a Newtonian liquid, while for the gel, yielding will result in a viscoelastic liquid showing shear thinning behavior. These are simple analogues and often more complex spring/dashpot combinations are required to describe real materials.

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